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There are thousands of VOC species in ambient air, while existing techniques can only detect a small part of them (~ several hundred). The large number of unmeasured VOCs prevents us from understanding the photochemistry of ozone and aerosols in the atmosphere. The major sources and photochemical effects of these unmeasured VOCs in urban areas remain unclear. The missing VOC reactivity, which is defined as the total OH reactivity of the unmeasured VOCs, is a good indicator to constrain the photochemical effect of unmeasured VOCs. Here, we identified the dominant role of anthropogenic emission sources in the missing VOC reactivity (accounting for up to 70%) by measuring missing VOC reactivity and tracer-based source analysis in a typical megacity in China. Omitting the missing VOC reactivity from anthropogenic emissions in model simulations will remarkably affect the diagnosis of sensitivity regimes for ozone formation, overestimating the degree of VOC-limited regime by up to 46%. Therefore, a thorough quantification of missing VOC reactivity from various anthropogenic emission sources is urgently needed for constraints of air quality models and the development of effective ozone control strategies. [ABSTRACT FROM AUTHOR]

PM2.5 pollution is a major air quality issue deteriorating human health, and numerous studies focus on PM2.5 pollution in major regions such as North China Plain (NCP) and Yangtze River Delta (YRD). However, the characteristics of PM2.5 concentrations and the associated formation mechanism in the transport corridor (referred to as SWLY) between NCP and YRD are largely ignored. Based on observational data, we find the number of PM2.5 pollution events in SWLY is comparable to that in NCP, far exceeding those in YRD, indicative of the severity of air pollution over this area. Utilizing a regional climate and air quality model, we isolate the effect of seesaw transport events, e.g., transport between NCP and YRD, as well as the atmospheric stagnation on the accumulation of PM2.5 over SWLY. Specifically, seesaw events and stagnation, comparable to each other, collectively account for an average of 67 % pollution days with PM2.5 exceeding 75 µg/m³, and this fraction (85 %) is even larger for severe haze events with PM2.5 exceeding 150 µg/m³. Furthermore, the connection between seesaw transport and large-scale circulation is examined. The trans-regional transport of pollutants from NCP to YRD (YRD to NCP) is likely stimulated by positive (negative) to negative (positive) geopotential height anomaly at 500 hPa located in northern China. The health effect due to short-term PM2.5 exposure induced by the trans-regional transport and stagnation is investigated, yielding a total of 8,634 (95 % CI: 6,023–11,223) and 9,496 (95 % CI: 6,552–12,413) premature deaths respectively in SWLY during winter 2014–2019, as high as 9 % of the total premature deaths in China although the area coverage of SWLY is within 1 %. While atmospheric stagnation is in general projected to occur more frequently under a warming climate, this study indicates the importance of regional emission control to alleviate PM2.5 pollution from seesaw transport and stagnation. [ABSTRACT FROM AUTHOR]

The Amazon Basin is frequently influenced by the trans-Atlantic transport of African dust plumes during its wet season (January - April), which not only interrupts the near-pristine atmospheric condition in that season, but also provides nutrient inputs into the Amazon rainforest associated with dust deposition. The factors controlling the long-range transport (LRT) of African dust towards the Amazon Basin and consequently the overall impact of African dust over the Amazon Basin are not yet well understood. In this study, we use the chemical transport model GEOS-Chem to investigate the impact of the export of African mineral dust upon the Amazon Basin during the period of 2013 - 2017, constrained by multiple datasets obtained from AERONET, MODIS, as well as Cayenne site and the Amazon Tall Tower Observatory (ATTO) site in the Amazon Basin. With optimized particle mass size distribution (PMSD), the model well captures observed AOD regarding both the mean value as well as the decline rate of the logarithm of AOD over the Atlantic Ocean along the transport path (AOaTP), implying the consistence with observed export efficiency of African dust along the trans-Atlantic transport. With an annual emission of 0.73 ± 0.12 Pg a-1, African dust entering the Amazon Basin has surface concentrations of 5.7 ± 1.3 µg m-3 (up to 15 µg m-3 in the northeast corner) during the wet season, accounting for 47% ± 5.0% (up to 70%) of mass concentrations of total aerosols. The frequency of dust events in the Amazon Basin (defined as when surface dust concentrations reach the threshold of 9 µg m-3 on daily basis) in the wet season is around 18% averaged over the basin, with maxima over 60% at the northeast coast. During the dust events, AOD over most of the Amazon Basin is dominated by dust. Observed dust peaks over the Amazon Basin are generally associated with relatively higher African dust emissions (including Sahara and Sahel) and longer lifetime of dust along the trans-Atlantic transport, namely higher export efficiency of African dust across the Atlantic Ocean. Associated with dust deposition, we further estimate annual inputs of 52 ± 8.7, 0.97 ± 0.16 and 21 ± 3.6 mg m-2 a-1 for iron, phosphorus and magnesium deposited into the Amazon rainforest, respectively, which may well compensate the hydrologic losses of nutrients in the forest ecosystem. [ABSTRACT FROM AUTHOR]

Biomass burning is one of the key sources of urban aerosols in the North China Plain, especially in winter when the impact of secondary organic aerosols (SOA) formed from biogenic volatile organic compounds (BVOCs) is generally considered to be minor. However, little is known about the influence of biogenic SOA loading on the molecular composition of wintertime organic aerosols. Here, we investigated the water-soluble organic compounds in fine particles (PM2.5) from urban Tianjin by ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Our results show that most of the CHO and CHON compounds were derived from biomass burning; they contain O-poor and highly unsaturated compounds with aromatic rings, which are sensitive to photochemical reactions, and some of which probably contribute to light-absorbing chromophores. Under moderate to high SOA loading conditions, the nocturnal chemistry is more efficient than photooxidation to generate secondary CHO and CHON compounds with high oxygen content. Under low SOA-loading, secondary CHO and CHON compounds with low oxygen content are mainly formed by photochemistry. Secondary CHO compounds are mainly derived from oxidation of monoterpenes. But nocturnal chemistry may be more productive to sesquiterpene-derived CHON compounds. In contrast, the number- and intensity-weight of S-containing groups (CHOS and CHONS) increased significantly with the increase of biogenic SOA-loading, which agrees with the fact that a majority of the S-containing groups are identified as organosulfates and nitrooxy-organosulfates that are derived from the oxidation of BVOCs. Terpenes may be potential major contributors to the chemical diversity of organosulfates and nitrooxy-organosulfates under photo-oxidation. While the nocturnal chemistry is more beneficial to the formation of organosulfates and nitrooxy-organosulfates under low SOA-loading. The SOA-loading is an important factor associating with the oxidation degree, nitrate group content and chemodiversity of nitrooxy-organosulfates. Furthermore, our study suggests that the hydrolysis of nitrooxy-organosulfates is a possible pathway for the formation of organosulfates. [ABSTRACT FROM AUTHOR]

The interaction between water vapor and aerosol nanoparticles is of great significance in atmospheric processes. However, current knowledge of hygroscopicity of sub-10 nm organic nanoparticles and their concentration-dependent thermodynamic properties (e.g., water activity) in the highly supersaturated concentration range is scarcely available. In this study, we investigate the size dependence of hygroscopicity of organics (i.e., levoglucosan, D-glucose) in size down to 6 nm using a nano-hygroscopicity tandem differential mobility analyzer (nano-HTDMA). There is a weak size dependence of the hygroscopic growth factor observed for levoglucosan and D-glucose nanoparticles with diameters down to 20 nm. However, a clear size-dependent hygroscopic growth factor is observed for D-glucose nanoparticles down to 6 nm in size. A reduction in diameters of sub-20 nm levoglucosan is observed at the dry RHs, which is explained by partial levoglucosan evaporation into gas phase, indicting high impact of volatility of sub-20 nm levoglucosan aerosol nanoparticles. However, this also means 24 that the hygroscopic growth factors of levoglucosan nanoparticles with diameters below 20 nm are not possible to be determined. The use of water activity parameterization models proposed by Kreidenweis et al. (2005) (KD, Köhler), the Extend-Aerosol Inorganic Model (E-AIM (standard UNIFAC), and Differential Köhler Analysis (DKA) method is to determine thermodynamic properties (e.g., water activity) of levoglucosan and D glucose nanodroplets as a function of solute concentration, respectively. Predicated water activity for these aqueous organic solutions (i.e., levoglucosan, D-glucose) from the different methods are similar to observations from references in the low solute concentration (< 20 mol kg-1), while a quite difference is found in the high solute concentration (> 20 mol kg-1). In addition, we compare hygroscopicity measurements for levoglucosan and D-glucose nanoparticles with the E-AIM (standard UNIFAC), the ideal solution theory, and DKA predictions, respectively. The ideal solution theory describes well the measured hygroscopic growth factors of levoglucosan with diameters down to 20 nm and D-glucose nanoparticles with diameters higher than 60 nm, respectively, while the E-AIM (standard UNIFAC) model can successfully predict the growth factors of levoglucosan with diameters from 100 down to 6 nm at RH above 88-40 % (e.g., at RH above 88 % for 100 nm D-glucose, at RH above 40 % for 6 nm D-glucose). The use of the DKA method leads to a good agreement with measured hygroscopic growth factors of D-glucose aerosol nanoparticles with diameters from 100 down to 6 nm. [ABSTRACT FROM AUTHOR]

Nucleation and condensation associated with biogenic volatile organic compounds (BVOCs) are important aerosol formation pathways, yet their contribution to the upper tropospheric aerosols remains inconclusive, hindering the understanding of aerosol climate effects. Here, we develop new schemes describing these organic aerosol formation processes in the WRF-Chem model and investigate their impact on the abundance of cloud condensation nuclei (CCN) in the upper troposphere (UT) over the Amazon Basin. We find that the new schemes significantly increase the simulated CCN number concentrations in the UT (e.g., up to ~400 cm-3 at 0.52 % supersaturation) and greatly improve the agreement with the aircraft observations. Organic condensation enhances the simulated CCN concentration by 90 % through promoting particle growth, while organic nucleation, by replenishing new particles, contributes an additional 14 %. Deep convection determines the rate of these organic aerosol formation processes in the UT through controlling the upward transport of biogenic precursors (i.e., BVOCs). This finding emphasizes the importance of the biosphere-atmosphere coupling in regulating upper tropospheric aerosol concentrations over the tropical forest and calls for attention to its potential role in anthropogenic climate change. [ABSTRACT FROM AUTHOR]

To better understand the characteristics of aerosol activation ability and optical properties, a comprehensive airborne campaign was implemented over the North China Plain (NCP) from May to June 11, 2016. Vertical profiles of cloud condensation nuclei (CCN) number concentration NCCN) and aerosol optical properties were measured simultaneously. Seventy-two-hour air mass back trajectories show that during the campaign the measurement region is mainly influenced by air masses in northwest and southeast. Air mass sources, temperature structure, anthropogenic emissions, and terrain distribution are factors influencing NCCN profiles. CCN spectra suggest that the ability of aerosol activation into CCN is stronger in southeast air masses than in northwest air masses and stronger in the free atmosphere than near the surface. Vertical distributions of aerosol scattering Ångström exponent (SAE) indicate that aerosols near the surface mainly originate from primary emissions consisting of more fine particles. The combined effect of aerosol lifting aloft and long-distance transport increase SAE and make it vary more in the free troposphere than near the surface. For parameterizing NCCN, the equation NCCN=10ß ·s-is used to fit the relationship between NCCN and the aerosol scattering coefficient (s) at 450 nm. The fitting parameters ß and - have linear relationships with the SAE. Empirical estimates of NCCN at 0.7% water vapor supersaturation (ss) from aerosol optical properties are thus retrieved for the two air masses: NCCN=10-0.22·SAE+2.39 ·s0.30·SAE+0.29 for northwest air masses and NCCN=10-0.07·SAE+2.29 ·s0.14·SAE+0.28 for southeast air masses. The estimated NCCN at 0.7% ss agrees with that measured, although the performance differs between low and high concentrations in the two air masses. The results highlight the important impact of aerosol sources on the empirical estimate of NCCN from aerosol optical properties. [ABSTRACT FROM AUTHOR]

In the past decade, ozone (O3) pollution has become a severe environmental problem in major cities in China. Here, based on available observational records, we investigated the long-term trend of ozone pollution in China during 2014–2020. Ozone concentrations were higher in urban areas than in non-urban areas. During these seven years, the highest ozone concentrations primarily occurred in summer in northern China, and in autumn or spring in southern China. Although ozone precursors, including nitrogen oxides (NOx) and carbon monoxide (CO), continuously decreased throughout the seven years, four ozone metrics that were used to characterize ozone exposure levels increased from 2014 to 2017 and reached a plateau after 2017. The long-term trend of ozone concentrations differed across seasons; especially from 2019 to 2020 when ozone concentrations decreased in summer and increased in winter. To analyze the causes of this observed trend, a photochemical box model was used to investigate the change in ozone sensitivity regime in two representative cities – Beijing and Shanghai. Our model simulations suggest that the summertime ozone sensitivity regime in urban areas of China has changed from a VOC-limited regime to a transition regime during 2014–2020; by 2020, the urban photochemistry is in a transition regime in summer but in a VOC-limited regime in winter. This study helps to understand the distinct trends of ozone in China and provides insights into efficient future ozone control strategies in different regions and seasons. [ABSTRACT FROM AUTHOR]

Atmospheric processes, including both primary emissions and secondary formation, may exert complex effects on aerosol hygroscopicity, which is of significant importance in understanding and quantifying the effect of aerosols on climate and human health. In order to explore the influence of local emissions and secondary formation processes on aerosol hygroscopicity, we investigated the hygroscopic properties of submicron aerosol particles at a rural site in the North China Plain (NCP) in winter 2018. This was conducted by simultaneous measurements of aerosol hygroscopicity and chemical composition, using a self-assembled hygroscopic tandem differential mobility analyzer (HTDMA) and a capture-vaporizer time-of-flight aerosol chemical speciation monitor (CV-ToF-ACSM). The hygroscopicity results showed that the particles during the entire campaign were mainly externally mixed, with a more hygroscopic (MH) mode and a less hygroscopic (LH) particles mode. The mean hygroscopicity parameter values (κmean) derived from hygroscopicity measurements for particles at 60, 100, 150, and 200 nm were 0.16, 0.18, 0.16, and 0.15, respectively. During this study, we classified two distinct episodes with different RH/T conditions, indicative of different primary emissions and secondary formation processes. It was observed that aerosols at all measured sizes were more hygroscopic under the high RH (HRH) episode than those under the low RH (LRH) episode. During the LRH, ? decreased with increasing particle size, which may be explained by the enhanced domestic heating at low temperature, causing large emissions of non- or less-hygroscopic primary aerosols. This is particularly obvious for 200 nm particles, with a dominant number fraction (> 50 %) of LH mode particles. Using O : C-dependent hygroscopic parameters of secondary organic compounds (κSOA), closure analysis between the HTDMA_measured κ and the ACSM_derived ? was carried out. The results showed that κSOA under the LRH episode was less sensitive to the changes in organic oxidation level, while κSOA under the HRH had a relatively stronger dependency on the organic O : C. This feature suggests that the different sources and aerosol evolution processes, partly resulting from the variation in atmospheric RH/T conditions, may lead to significant changes in aerosol chemical composition, which will further influence their corresponding physical properties. [ABSTRACT FROM AUTHOR]

Volatile organic compounds (VOCs) regulate atmospheric oxidation capacity, and the reactions of VOCs are key in understanding ozone formation and its mitigation strategies. When evaluating its impact, most previous studies did not fully consider the role of oxygenated VOCs due to limitations of measurement technology. By using a proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) combined with gas chromatography mass spectrometer (GC-MS) technology, we are able to quantify a large number of oxygenated VOCs in a representative urban environment in southern China. Based on the new dataset, we find that non - formaldehyde (HCHO) OVOCs can contribute large fractions (22-44%) of total RO[sub X]radical production, comparable or larger than the contributions from nitrous acid and formaldehyde. We demonstrate that constraints using OVOCs observations are essential in modeling radical and ozone production, as modelled OVOCs can be substantially lower than measurements, potentially due to primary emissions and/or missing secondary sources. Our results show that models without OVOC constraints using ambient measurements will underestimate P(RO[sub x] and ozone production rate, and may also affect the determination of sensitivity regime in ozone formation. Therefore, a thorough quantification of photodegradable OVOCs species is in urgent need to understand accurately the ozone chemistry and to develop effective control strategies. [ABSTRACT FROM AUTHOR]

Effective density is one of the most important physical properties of atmospheric particles. It is closely linked to particle chemical composition and morphology, and could provide special information on particle emissions and aging processes. In this study, size-resolved particle effective density was measured with a combined DMA-CPMA-CPC system in Multiphase chemistry experiment in Fogs and Aerosols in the North China Plain (McFAN) in autumn 2019. With a new developed flexible Gaussian fit algorithm, frequent (77%-87%) bimodal distribution of particle effective density is identified, with a low-density mode (named sub-density mode) accounting for 22-27?% of total observed particles. The prevalence of the sub-density mode is closely related to fresh black carbon (BC) emissions. The geometric mean for the main-density mode (ρ¯eff,main) increases from 1.18gcm-3 (50nm) to 1.37?g?cm-3 (300nm) due to larger fraction of high-density components and more significant restructuring effect at large particle sizes, but decreases from 0.89?g?cm-3 (50nm) to 0.62gcm-3 (300nm) for the sub-density mode (ρ¯eff,sub) ascribed to the agglomerate effect. ρ¯eff,main and ρ¯eff,sub show similar diurnal cycles with peaks in the early afternoon, mainly attributed to the increasing mass fraction of high material density components associated with secondary aerosol production, especially of secondary inorganic aerosol (SIA). To investigate the impact of chemical composition, bulk particle effective density was calculated based on measured chemical composition (ρeff,ACSM) and compared with the average effective density at 300nm (ρ¯eff,tot,300nm). The best agreement between the two densities is achieved when assuming a BC effective density of 0.60?g?cm-3. The particle effective density is highly dependent on SIA and BC mass fractions. The influence of BC on the effective density is even stronger than SIA, implying the importance and necessity of including BC in the estimate of effective density for ambient particles. [ABSTRACT FROM AUTHOR]

Little is known about the formation processes of nitrooxy-organosulfates (nitrooxy-OSs) by nighttime chemistry. Here we characterize nitrooxy-OSs at a molecular level in firework- related aerosols in urban Beijing during Chinese New Year. High-molecular-weight nitrooxy-OSs with relatively low H / C and O / C ratios and high unsaturation, which are potentially aromatic-like nitrooxy-OSs, considerably increased during the New Year's Eve. We find that large quantities of carboxylic-rich alicyclic molecules possibly formed by nighttime reactions. The sufficient abundance of aliphatic-like and aromatic-like nitrooxy-OSs demonstrates that both anthropogenic and biogenic volatile organic compounds are essential precursors of urban secondary organic aerosols (SOA). Besides, more than 98 % of nitrooxy-OSs were extremely low-volatile organic compounds that could easily partition into and consist in the particle phase, and affected the volatility, hygroscopicity, and even toxicity of urban aerosols. Our study provides new insights into the formation of nitrooxy-organosulfates from anthropogenic emissions through nighttime chemistry in the urban atmosphere. [ABSTRACT FROM AUTHOR]

Aerosol mixing state regulates the interactions between water molecules and particles and thus controls the aerosol activation and hygroscopic growth, which thereby influences the visibility degradation, cloud formation, and its radiative forcing. Current studies on the mixing structure effects on aerosol hygroscopicity, however, is few reported. Here we investigated the effect of phthalic acid (PA) coatings on the hygroscopic behavior of the core-shell mixtures of ammonium sulfate (AS) with PA using a coating-hygroscopicity tandem differential mobility analyzer (coating-HTDMA). The slow increase in the hygroscopic growth factor of core-shell particles is observed with increasing thickness of coating PA prior to the DRH of AS. At RH above 80 %, a decrease i24 n hygroscopic growth factor of particles occurs as the thickness of PA shell increases, which indicates that the increase of PA mass fractions leads to a reduction of the overall core-shell particle hygroscopicity. In addition, the use of the ZSR relation leads to the underestimation for the measured growth factors of core-shell particles without consideration of the morphological effect of core-shell particles. For the AS/PA well mixed particles, a shift of deliquescence relative humidity (DRH) of AS to lower relative humidity (RH) is observed due to the presence of PA in the well-mixed particles. The predicted hygroscopic growth factor using the ZSR relation is consistent with the measured hygroscopic growth factor of the well-mixed particles. Moreover, we compared and discussed the influence of mixing states on the water uptake of AS/PA aerosol particles. It is found that the hygroscopic growth factor of the core-shell particles is slightly higher than that of the well-mixed particles with the same mass fractions of PA at RH above 80%. For our observation of AS/PA particles may contribute to a growing field of knowledge regarding the influence of coating properties and mixing structure on water uptake. [ABSTRACT FROM AUTHOR]

Biomass burning (BB) aerosols can influence regional and global climate through interactions with radiation, clouds, and precipitation. Here, we investigate the impact of BB aerosols on the energy balance and hydrological cycle over the Amazon Basin during the dry season. We performed WRF-Chem model simulations for a range of different BB emission scenarios to explore and characterize nonlinear effects and individual contributions from aerosol-radiation interactions (ARI) and aerosol-cloud interactions (ACI). The ARI of BB aerosols tend to suppress low-level liquid clouds by local warming and increased evaporation, and to facilitate the formation of high-level ice clouds by enhancing updrafts and condensation at high altitudes. In contrast, the ACI of BB aerosol particles tend to enhance the formation and lifetime of low-level liquid clouds by providing more cloud condensation nuclei (CCN), and to suppress the formation of high-level ice clouds by reducing updrafts and condensable water vapor at high altitudes (> 8 km). [ABSTRACT FROM AUTHOR]

Understanding the mechanism of haze formation is crucial for the development of deliberate pollution control strategies. Multiphase chemical reactions in aerosol water have been suggested as an important source of particulate sulfate during severe haze (Cheng et al., 2016; Wang et al., 2016). While the key role of aerosol water has been commonly accepted, the relative importance of different oxidation pathways in the aqueous phase is still under debate, mainly due to questions about aerosol pH. To investigate the spatio-temporal variability of aerosol pH and sulfate formation during winter in the North China Plain (NCP), we have developed a new aerosol water chemistry module (AWAC) for the WRF-Chem model (Weather Research and Forecasting model coupled with Chemistry). Using the WRF-Chem-AWAC model, we performed a comprehensive survey of the atmospheric conditions characteristic for wintertime in the NCP, focusing on January 2013. We find that aerosol pH exhibited a strong vertical gradient and distinct diurnal cycle, which was closely associated with the spatio-temporal variation in the relative abundance of acidic and alkaline fine particle components and their gaseous counterparts. Over Beijing, the average aerosol pH at the surface layer was ~ 5.4 and remained nearly constant around ~ 5 up to ~ 2 km above ground level; further aloft, the acidity rapidly increased to pH ~ 0 at ~ 3 km. The pattern of aerosol acidity increase with altitude persisted over the NCP, while the specific levels and gradients of pH varied between different regions. In the region north of ~ 41° N, the mean pH values at surface level were typically > 6 and the main pathway of sulfate formation in aerosol water was S(IV) oxidation by ozone. South of ~ 41° N, the mean pH values at surface level were typically in the range of 4.4 to 5.7, and different chemical regimes and reaction pathways of sulfate formation prevailed in four different regions, depending on reactant concentrations and atmospheric conditions. The NO2 reaction pathway prevailed in the megacity region of Beijing and the large area of Hebei Province to the south and west of Beijing, as well as part of Shandong Province. The transition metal ion (TMI) pathway dominated in the inland region to the west and the coastal regions to the east of Beijing, and the H2O2 pathway dominated in the region extending further south (Shandong and Henan Provinces). In all of these regions, the O3 and TMI pathways in aerosol water as well as the gas-particle partitioning of H2SO4 vapor became more important with increasing altitude. Although pH is sensitive to the abundance of NH3 and crustal particles, we show that the rapid production of sulfate in the NCP can be maintained over a wide range of aerosol acidity (e.g., pH = 4.2-5.7) with transitions from TMI pathway dominated to NO2/O3 pathway dominated regimes. [ABSTRACT FROM AUTHOR]

Firework (FW) emission has strong impacts on air quality and public health. However, little is known about the molecular composition of FW-related airborne particulate matter (PM) especially the organic fraction. Here we describe the detailed molecular composition of Beijing PM collected before, during, and after a FW event in New Year's Eve evening in 2012. Subgroups of CHO, CHNO, and CHOS were characterized using ultrahigh resolution Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry. These subgroups comprise substantial fraction of aromatic-like compounds with low O/C ratio and high degrees of unsaturation, some of which plausibly contributed to the formation of brown carbon in Beijing PM. Moreover, we found that the number concentration of sulfur-containing compounds especially the organosulfates was increased dramatically by the FW event, whereas the number concentration of CHO and CHON doubled after the event. The co-variation of CHO, CHON, and CHOS subgroups was suggested to be associated with multiple atmospheric aging processes of aerosols including the multiphase redox chemistry driven by NOx, O3, and •OH. These findings highlight that FW emissions can lead to a sharp increase of high molecular weight compounds particularly aromatic-like substances in urban PM, which may affect the light absorption properties and adverse health effects of atmospheric aerosols. [ABSTRACT FROM AUTHOR]

Biomass burning and primary biological aerosol particles account for an important part of urban aerosols. Floods of studies have been conducted on the chemical compositions of fine aerosols (PM2.5) in megacities where the haze pollution is one of the severe environmental issues in China. However, little is known about their size distributions in atmospheric aerosols in the urban boundary layer. Here, size-segregated aerosol samples were collected in Beijing during haze and clear days from April 2017 to January 2018. Three anhydrosugars, six primary saccharides and four sugar alcohols in these samples were identified and quantified by gas chromatography/mass spectrometry (GC/MS). Higher concentrations of a biomass burning tracer, levoglucosan, were detected in autumn and winter than other seasons. Sucrose, glucose, fructose, mannitol and arabitol were more abundant in the bloom and glowing seasons. Particularly high level of trehalose was found in spring, which was largely associated with the Asian dust outflows. Anhydrosugars, xylose, maltose, inositol and erythritol are mainly existed in the fine mode (< 2.1 µm), while the others showed the coarse mode privilege. The concentrations of measured tracers of biomass burning particles and primary biological particles in the haze events were higher than those in the non-hazy days, with an enrichment factor of 2-10. Geometric mean diameters (GMD) of molecular markers of biomass burning and primary biological aerosols showed that there was no significant difference in the coarse mode (> 2.1 µm) between the haze and non-haze samples, while a size shift towards large particles and large GMDs in the fine fraction (< 2.1 µm) was detected during the hazy days, which highlights that the stable meteorological conditions with high relative humidity in urban Beijing may favor the condensation of organics onto coarse particles. The contributions of reconstructed primary organic carbon (POC) by tracer-based methods from plant debris, fungal spores, and biomass burning to aerosol OC in the total mode particles were in the ranges of 0.09-0.30% (on average 0.21%), 0.13-1.0% (0.38%) and 1.2-7.5% (4.5%), respectively. This study demonstrates that the contribution of biomass burning was significant in Beijing throughout the whole year with the predominance in the fine mode, while the contributions of plant debris and fungal spores dominated in spring and summer in the coarse mode, especially in the size of > 5.8 µm. Our observations demonstrate that the sources, abundance, and chemical composition of urban aerosol particles are strongly size dependant in Beijing, which are important to better understand their environmental and health effects of urban aerosols and should be considered in air quality and climate models. [ABSTRACT FROM AUTHOR]

Abstract. The hygroscopicity of organic aerosols (OA) is important for investigation of its climatic and environmental impacts. However, the hygroscopicity parameter κOrg remains poorly characterized, especially in the relatively polluted environment on the North China Plain (NCP). Here we conducted simultaneous wintertime measurements of bulk aerosol chemical compositions of PM2.5 and PM1 and bulk aerosol hygroscopicity of PM10 and PM1 on the NCP using a capture vaporizer time-of-flight aerosol chemical speciation monitor (ToF-ACSM) and a humidified nephelometer system which measures aerosol light scattering enhancement factor f(RH). A method for calculating κOrg based on f(RH) and bulk aerosol chemical composition measurements was developed. We found that κOrg varied in a wide range with significant diurnal variations. The derived κOrg ranged from almost 0.0 to 0.25 with an average (± 1σ) of 0.08 (± 0.06) for the entire study. The derived κOrg was highly correlated with f44 (fraction of m/z 44 in OA), an indicator of oxidation degree of OA (R = 0.79), and the relationship can be parameterized as κOrg = 1.04 × f44 − 0.02. On average, κOrg reached the minimum (0.02) in the morning near 07:30 and then increased rapidly reaching the peak value of 0.16 near 14:30. The diurnal variations of κOrg were highly and positively correlated with those of mass fractions of oxygenated OA (R = 0.95), indicating that photochemical processing played a dominant role for the increase of κOrg in winter on NCP. Results in this study demonstrate the potential wide applications of humidified nephelometer system together with aerosol composition measurements for investigating the hygroscopicity of OA in various environments, and highlight that the parameterization of κOrg as a function of OA aging processes needs to be considered in chemical transport models for better evaluating the impacts of OA on cloud formation, atmospheric chemistry and radiative forcing. [ABSTRACT FROM AUTHOR]

Natural sea-salt aerosols, when interacting with anthropogenic emissions, can enhance the formation of particulate nitrate. This enhancement has been suggested to increase the direct radiative forcing of nitrate, called mass-enhancement effect. Through a size-resolved dynamic mass transfer modelling approach, we show that interactions with sea-salt shift the nitrate from sub- to super-micron sizes (re-distribution effect), and hence lower its efficiency for light extinction and reduce its lifetime. The re-distribution effect overwhelms the mass-enhancement effect and significantly moderates nitrate cooling; e.g., the nitrate associated aerosol optical depth can be reduced by 10-20 % over European polluted regions during a typical sea-salt event, in contrast to an increase by ~ 10 % when only accounting for the mass-enhancement effect. Global model simulations indicate significant re-distribution over coastal and offshore regions world-wide. Our study suggests a strong buffering by natural sea-salt aerosols that reduces the climate forcing of anthropogenic nitrate, which had been expected to dominate the aerosol cooling by the end of the century. Comprehensive considerations of this re-distribution effect foster better understandings of climate change and nitrogen deposition. [ABSTRACT FROM AUTHOR]

Hygroscopicity is one of the most important physicochemical properties of aerosol particles, and also plays indispensable roles in many other scientific and technical fields. A myriad of experimental techniques, which differ in principles, configurations and cost, are available for investigating aerosol hygroscopicity under subsaturated conditions (i.e., relative humidity below 100%). A comprehensive review of these techniques is provided in this paper, in which experimental techniques are broadly classified into four categories, according to the way samples under investigation are prepared. For each technique, we describe its operation principle and typical configuration, use representative examples reported in previous work to illustrate how this technique can help better understand aerosol hygroscopicity, and discuss its advantages and disadvantages. In addition, future directions are outlined and discussed for further technical improvement and instrumental development. [ABSTRACT FROM AUTHOR]

In the summer of 2017, heavy ozone pollution swamped most of the North China Plain (NCP), with the maximum regional average of daily maximum 8-h ozone concentration (MDA8) reaching almost 120ppbv. In light of the continuing reduction of anthropogenic emissions in China, the underlying mechanisms for the occurrences of these regional extreme ozone episodes are elucidated from two perspectives: meteorology and biogenic emissions. The significant positive correlation between MDA8 and temperature, which is amplified during heat waves concomitant with stagnant air and no precipitation, supports the crucial role of meteorology in driving high ozone concentrations. We also find that biogenic emissions are enhanced due to factors previously not considered. During the heavy ozone pollution episodes in June 2017, biogenic emissions driven by high vapor pressure deficit (VPD), land cover change and urban landscape yield an extra mean MDA8 ozone of 3.08, 2.79 and 4.74ppbv, respectively over the NCP, which together contribute as much to MDA8 ozone as biogenic emissions simulated using the land cover of 2003 and ignoring VPD and urban landscape. In Beijing, the biogenic emission increase due to urban landscape has a comparable effect on MDA8 ozone to the combined effect of high VPD and land cover change between 2003 and 2016. This study highlights the vital contributions of heat waves, land cover change and urbanization to the occurrence of extreme ozone episode, with significant implications for ozone pollution control in a future when heat wave frequency and intensity are projected to increase under global warming. [ABSTRACT FROM AUTHOR]

Quantifying the relative importance of gas uptake on the ground and aerosol surfaces helps to determine which processes should be included in atmospheric chemistry models. Gas uptake by aerosols is often characterized by an effective uptake coefficient (γeff), whereas gas uptake on the ground is usually described by a deposition velocity (Vd). For efficient comparison, we introduce an equivalent uptake coefficient (γeqv) at which the uptake flux of aerosols would equal that on the ground surface. If γeff is similar to or larger than γeqv, aerosol uptake is important and should be included in atmospheric models. In this study, we compare uptake fluxes in the planetary boundary layer (PBL) for different reactive trace gases (O3, NO2, SO2, N2O5, HNO3, H2O2), aerosol types (mineral dust, soot, organic aerosol, sea salt aerosol), environments (urban, agricultural land, Amazon forest, water body), seasons, and mixing heights. For all investigated gases, γeqv ranges from 10-6~10-4 in polluted urban environments to 10-4~10-1 under pristine forest conditions. In urban areas, aerosol uptake is relevant for all species (γeff≥γeqv) and should be considered in models. On the contrary, contributions of aerosol uptakes in Amazon forest are minor compared to the dry deposition. Phase state of aerosols could be one of the crucial factors influencing the uptake rates. Current models tend to underestimate the O3 uptake on liquid organic aerosols which can be important especially over regions with γeff≥γeqv. H2O2 uptakes on a variety of aerosols is yet to be measured at laboratory and evaluated. Given the fact that most models have considered their uptakes on the ground surface, we suggest also considering the N2O5 uptake by all types of aerosols, HNO3 and H2O2 uptakes by mineral dust, O3 uptake by liquid organic aerosols and NO2, SO2, HNO3 uptakes by sea salt aerosols in atmospheric models. [ABSTRACT FROM AUTHOR]

The effect of black carbon (BC) on air quality and the climate is still unclear, which is partly because of the poor understanding regarding the BC aging process in the atmosphere. In this work, we developed a new approach to simulate the BC mixing state (i.e., other species coated on the BC surface) based on an emissions inventory and back-trajectory analysis. The model tracks the evolution of the BC aging degree (characterized by the ratio of the whole particle size and BC core) during atmospheric transport. Using the models, we quantified the mass-averaged aging degree of total BC particles transported to a receptor (e.g., an observation site) from various emission origins (i.e., 0.25°×0.25° grids). The simulations showed good agreement with the field measurements, which validated our model calculation. Modeling the aging process of BC during atmospheric transport showed that it strongly dependent on emission levels. BC particles from extensive emission origins (i.e., polluted regions) were characterized by a higher aging degree during atmospheric transport due to more co-emitted coating precursors. On the other hand, high-emission regions also controlled the aging process of BC particles that were emitted from cleaner regions and passed through these polluted regions during atmospheric transport. The simulations identified the important roles of extensive emission regions in the BC aging process during atmospheric transport, implying the enhanced contributions of extensive emission regions to BC light absorption. This revealed that emission reductions in polluted regions could achieve more benefits for improving air pollution and climate change. Emission reductions in polluted regions not only decreased the aging degree of BC emitted from these regions but also reduced the aging process of BC emitted from other origins during atmospheric transport. Moreover, emissions reduction in polluted regions may be more efficient in counteract the suppression of planetary boundary layer (PBL) by BC particles (e.g., Zdunkowski et al., 1976; Jacobson, 1998; Wendisch et al., 2008; Barbaro et al., 2013; Ding et al., 2016; Wang et al., 2018), because a greater decrease in the BC aging degree during atmospheric transport would weaken the light absorption capability of BC in the upper PBL. The simulation of the BC aging degree during atmospheric transport provided more clues for improving air pollution and climate change. [ABSTRACT FROM AUTHOR]

Non-methane volatile organic compounds (NMVOC) are important ozone and secondary organic aerosol precursors and play important roles in tropospheric chemistry. In this work, we estimated the total and speciated NMVOC emissions from China’s anthropogenic sources during 1990–2017 by using a bottom-up emission inventory framework, and investigated the main drivers behind the trends. We found that, anthropogenic NMVOC emissions in China have been increased continuously since 1990 due to the dramatic growth in activity rates and absence of effective control measures. We estimated that, anthropogenic NMVOC emissions in China increased from 9.76 Tg in 1990 to 28.5 Tg in 2017, mainly driven by persistent growth from the industry sector and solvent use. In the meanwhile, emissions from the residential and transportation sectors declined after 2005, partly offset the total emission increase. During 1990–2017, mass-based emissions of alkanes, alkenes, alkynes, aromatics, oxygenated VOCs (OVOC) and other species increased by 274 %, 88 %, 4 %, 387 %, 91 %, and 231 % respectively. Following the growth in total NMVOC emissions, the corresponding ozone formation potential (OFP) increased form 38.2 Tg-O3 in 1990 to 99.7 Tg-O3 in 2017. We estimated that aromatics accounted for the largest share (43 %) of total OFP, followed by alkenes (37 %) and OVOC (10 %). Growth in China's NMVOC emissions were mainly driven by the transportation sector before 2000, while industrial sector and solvent use dominated the emission growth during 2000–2010. After 2010, although emissions from the industry sector and solvent use kept growing, strict control measures on transportation and fuel transition in residential stoves have successfully slowed down the increase trend, especially after the implementation of China's clean air action since 2013. However, compared to large emission decreases of other criteria air pollutants in China (e.g., SO2, NOx, and primary PM) during 2013–2017, the relatively flat trend in NMVOC emissions and OFP revealed the absence of effective control measures, which might have contributed to the increase of ozone during the same period. Given their high contributions to emissions and OFP, tailored control measures for solvent use and industrial sources should be developed, and collaborative control strategies should be designed to mitigate both PM2.5 and ozone pollution simultaneously. [ABSTRACT FROM AUTHOR]

Volatile organic compounds (VOCs) play a key role in atmospheric chemistry. Emission and deposition on soil have been suggested as important sources and sinks of atmospheric trace gases. The exchange characteristics and heterogeneous chemistry of VOCs on soil, however, are not well understood. We used a newly designed differential coated-wall flow tube system to investigate the long-term variability of bidirectional air-soil exchange of 13 VOCs at ambient air conditions of an urban background site in Beijing. Sterilized soil was investigated to address physicochemical processes and heterogeneous/multiphase reactions independently from biological activity. Most VOCs revealed net deposition with average uptake coefficients (γ) in the range of 10-7–10-6 (referring to the geometric soil surface area), corresponding to deposition velocities (Vd) of 0.0013–0.01cms-1 and soil surface resistances (Rc) of 98–745scm-1, respectively. Formic acid, however, was emitted at a long-term average rate of ~6×10-3nmolm-2s-1, suggesting that it was formed and released upon heterogeneous oxidation of other VOCs. The soil-atmosphere exchange of one individual VOC species can be affected by both its surface degradation/depletion caused by surface reactions and by competitive uptake or heterogeneous formation/accommodation of other VOC species. Overall, the results show that physicochemical processing and heterogeneous oxidation on soil and soil-derived dust can act as a sink or as a source of atmospheric VOCs, depending on molecular properties and environmental conditions. [ABSTRACT FROM AUTHOR]

Reducing black carbon (BC) emissions has been recognized as an efficient way to simultaneously improve air quality and mitigate climate change. However, the benefits of BC emission controls are not well quantified partly due to a lack of understanding of the changes in BC light absorption as a result of emission reductions. In this work, we discussed the effects of multi-pollutant emission reductions on the BC light absorption based on a field campaign study conducted before, during and after the 2014 APEC (Asia-Pacific Economic Cooperation) meeting in Beijing, China. When emission restrictions were in place during APEC, we found that the reduction in the light absorption of BC-containing particles was driven by both the decrease in BC mass concentration and the weakened light-absorption capability of BC. Compared with that before and after APEC, the daytime light absorption of BC-containing particles during APEC reduced by ~ 56 %, of which ~ 48 % was contributed by the decrease in BC mass concentration and the remain ~ 8 % was contributed by a weakening of light-absorption capability for BC. Based on single particle soot photometer (SP2) measurement and Mie calculation, we estimated that the light-absorption capability of BC-containing particles with ~ 80–200 nm refractory BC (rBC) cores at daytime during APEC was reduced by ~ 6–15 % and ~ 10–20 % compared with that before and after APEC, respectively. The decrease in BC light-absorption capability could be attributed to less coating materials on BC surfaces as a result of a decreased chemical production of secondary aerosols. Compared with that before and after APEC, the mass ratio between the coating materials and rBC core (~ 80–200 nm) during APEC decreased by ~ 10–30 % and ~ 31–53 %, respectively, due to reductions in coating precursor emissions, e.g., SO2 and NO2. The results revealed the benefits of emission control on BC light absorption by simultaneously reducing the mass concentration and light-absorption capability of BC, implying that synergetic reduction in multiple-pollutant emission could benefit both air quality and climate. [ABSTRACT FROM AUTHOR]

Simultaneous measurements of aerosol hygroscopicity and particle phase chemical composition were performed at a suburban site over the Pearl River Delta Region in the late summer of 2016 using a self-assembled Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA) and an Aerodyne Quadruple Aerosol Chemical Speciation Monitor (ACSM), respectively. Hygroscopic growth factor (HGF) of Aitken (30 nm, 60 nm) and accumulation mode (100 nm, 145 nm) particles was obtained under 90 % relative humidity (RH). An external mixture was observed for all-sized particles during this study, with a dominant mode of more hygroscopic (MH) particles as aged aerosols dominated due to the anthropogenic influence. The HGF of less hygroscopic (LH) mode particles increased, while their number fractions decreased, during the daytime due to a reduced degree of external mixing probably from the condensation of gaseous species. These LH mode particles in the early morning or late afternoon could be possibly dominated by carbonaceous material emitted from local automobile exhaust during the rush hours. During polluted days with air masses mainly from the coastal areas, the chemical composition of aerosols had a clear diurnal variation and a strong correlation with the mean HGF. Closure analysis was carried out between the HTDMA-measured HGF and the ACSM-derived hygroscopicity using various approximations for hygroscopic growth factor of organic compounds (HGForg). The result with HGForg of below 1.1 suggests that the organic material in the aerosol from this suburban site, which represents a more anthropogenically influenced aerosol, is close to being hydrophobic even under 90 % RH. According to the closure analysis, a simple linear relationship between HGForg and the oxidation level inferred from the O : C ratio of the organic material was suggested for the current type of aerosols during this study. Compared with the results from other environments, HGForg obtained from our measurements appeared to be less sensitive to the variation of its oxidation level. This finding suggests it is important to consider the differences in the relationship between HGForg and the O : C atomic ratio observed in different laboratory and field environments. Moreover, this result provides an improved hygroscopicity parameterization of organic material in anthropogenically influenced aerosols that can be incorporated into large-scale modeling frameworks, especially for the suburban context in China. [ABSTRACT FROM AUTHOR]

Brown carbon (BrC), a certain group of organic carbon (OC) with strong absorption from the visible to ultraviolet (UV) wavelengths, makes considerable contribution to light absorption on both global and regional scales. High concentration and proportion of OC has been reported in China, but studies of BrC absorption based on long-term observations are rather limited in this region. In this study, we reported 3-year results of light absorption of BrC based on continuous measurement at the Station for Observation Regional Processes of the Earth System (SORPES) in the Yangtze River Delta, China combined with Mie-theory calculation. Light absorption of BrC was obtained using an improved Absorption Ångstrom exponent (AAE) segregation method to calculate AAE of pure and non-absorbing coated black carbon (BC) at each time step based on Mie-theory simulation and measurement of multi-wavelength aerosol light absorption. By using this improved method, the variation of AAE over time is taken into consideration, making it applicable for long-term analysis. The yearly average light absorption of BrC (babs_BrC) at 370 nm was 4.3 Mm−1 at the SORPES station. The contribution of BrC to total aerosol absorption (PBrC) at 370 nm ranged from 6 % to 18 % (10th and 90th percentiles, respectively), and reached up to ~ 28 % in biomass burning-dominant season and winter. Both babs_BrC and PBrC exhibited clear seasonal cycles with two peaks in later spring/early summer (May–June, babs_BrC ~ 4 Mm−1, PBrC ~ 11 %) and winter (December, babs_BrC ~ 12 Mm−1, PBrC ~ 17 %), respectively. Lagrangian modeling and chemical signature observed at the site suggested that open biomass burning and residential emissions were the dominate sources influencing BrC in the two seasons. [ABSTRACT FROM AUTHOR]

The impacts of black carbon (BC) aerosols on air quality, boundary layer dynamic and climate depend not only on the BC mass concentration but also on the light absorption capability of BC. It well known that the light absorption capability of BC depends on the amount of coating materials (namely other species on BC by condensation and coagulation). However, the difference of light absorption capability of ambient BC-containing particles under different air pollution conditions (e.g., the air clean and polluted conditions) remains unclear due to the complex aging process of BC in the atmosphere. In this work, we investigated the evolution of light absorption capability for BC-containing particles with changing pollution levels in urban Beijing, China. During the campaign period (17 to 30 November 2014), with the growth of PM1 concentration from ~ 10 μg m−3 to ~ 230 μg m−3, we found that the aging degree and light absorption capability of BC-containing particles with refractory BC cores of ~ 75–200 nm increased by 26–73 % and 13–44 % respectively, indicating stronger light absorption capability of BC-containing particles under more polluted conditions due to more coating materials on the BC surface. By using effective emission intensity (EEI) model, we further found that aging during the regional transport plays an important role in the difference among the light absorption capability of BC-containing particles under different air pollution levels. During the pollution episode, ~ 63 % of the BC over Beijing originated from regional sources outside of Beijing. These regionally sourced BC-containing particles were characterized by more coating materials on BC surface due to accelerated aging process within more polluted air, which contributed ~ 78 % of the increase in light absorption capability of BC observed in Beijing during the polluted period (PM1 of ~ 230 μg m−3) comparing to that in the clean period (PM1 of ~ 10 μg m−3). Due to the increase of light absorption capability of BC associated with air pollution, the direct radiative forcing of BC was estimated to be increased by ~ 20 % based on a simple radiation transfer model. Our work identified an amplification of light absorption and direct radiative forcing under more air polluted environment due to more coating pollutants on BC. The air pollution control measures may, on the other hand, break the amplification effect by reducing both emissions of BC and the coating materials and achieve co-benefits of both air quality and climate. [ABSTRACT FROM AUTHOR]

In the Amazonian atmosphere, the aerosol coarse mode comprises a complex, diverse, and variable mixture of bioaerosols emitted from the rain forest ecosystem, long-range transported Saharan dust, marine aerosols from the Atlantic Ocean, and coarse smoke particles from deforestation fires. For the rain forest, the coarse mode particles are of significance with respect to biogeochemical and hydrological cycling as well as ecology and biogeography. However, detailed knowledge on the physicochemical and biological properties as well as the ecological role of the Amazonian coarse mode is still sparse. This study presents results from multi-year coarse mode measurements at the remote Amazon Tall Tower Observatory (ATTO) site. It combines online aerosol observations, selected remote sensing and modelling results, as well as dedicated coarse mode sampling and analysis. The focal points of this study are a systematic characterization of aerosol coarse mode abundance and properties in the Amazonian atmosphere as well as a detailed analysis of the frequent, pulse-wise intrusion of African long-range transport (LRT) aerosols (comprising Saharan dust and African biomass burning smoke) into the Amazon Basin. We find that, on a multi-year time scale, the Amazonian coarse mode maintains remarkably constant concentration levels (with 0.4 cm-3 and 4.0 µg m-3 in the wet vs. 1.2 cm-3 and 6.5 µg m-3 in the dry season) with rather weak seasonal trends (in terms of abundance and size spectrum), which is in stark contrast to the pronounced biomass burning-driven seasonality of the submicron aerosol population and related parameters. For most of the time, bioaerosol particles from the forest biome account for a major fraction of the coarse mode background population. However, from Dec to Apr there are episodic intrusions of African LRT aerosols, comprising Saharan dust, sea salt particles from the transatlantic passages, and African biomass burning smoke. Remarkably, during the core period of this LRT season (i.e., Feb-Mar), the presence of LRT influence, occurring as a sequence of pulse-like plumes, appears to be rather the norm than an exception. The LRT pulses increase the coarse mode concentrations drastically (up to 100 µg m-3) and alter coarse mode composition as well as its size spectrum. Efficient transport of the LRT plumes into the Amazon Basin takes place in response to specific mesoscale circulation patterns in combination with the episodic absence of rain-related aerosol scavenging en route. Based on a modelling study, we estimated a dust deposition flux of 5-10 kg ha-1 a-1 in the region of the ATTO site. Furthermore, a chemical analysis quantified the substantial increase of crustal and sea salt elements under LRT conditions in comparison to the background coarse mode composition. With these results, we estimated the deposition fluxes of various elements that are considered as nutrients for the rain forest ecosystem. These estimates range from few g ha-1 a-1 up to several hundreds of g ha-1 a-1 in the ATTO region. The long-term data presented here provide a statistically solid basis for future studies of the manifold aspects of the dynamic coarse mode aerosol cycling in the Amazon. Thus, it may help to understand its biogeochemical relevance in this ecosystem as well as to evaluate to what extent anthropogenic influences have altered the coarse mode cycling already. [ABSTRACT FROM AUTHOR]

Discerning mechanisms of sulfate formation during fine-particle pollution (referred to as haze hereafter) in Beijing is important for understanding the rapid evolution of haze and for developing cost-effective air pollution mitigation strategies. Here we present the first observations of the oxygen-17 excess of PM2.5 sulfate (Δ17O(SO42-)) collected in Beijing haze from October 2014 to January 2015, to constrain possible sulfate formation pathways. Throughout the sampling campaign, the 12h-averaged PM2.5 concentrations ranged from 16 to 323 μg m-3 with a mean of (141 ± 88 (1σ)) μg m-3, with SO42- representing 8-25 % of PM2.5 mass. The observed Δ17O(SO42-) varied from 0.1 ‰ to 1.6 ‰ with a mean of (0.9 ± 0.3) ‰.Δ17O(SO42-)increased with PM2.5 levels in October 2014 while the opposite trends were observed in November 2014 to January 2015. Heterogeneous sulfate production rate (Phet) on aerosols was estimated to enhance with PM2.5 levels, generally dominating sulfate formation during haze days when cloud liquid water content (LWC) was low. When LWC was high, however, in-cloud reactions would dominate haze sulfate formation with a fractional contribution up to 68 %. For the specific mechanisms of heterogeneous oxidation of SO2, chemical reaction kinetics calculations suggest S(IV) (= SO2•H2O + HSO3- + SO32-) oxidation by H2O2 in aerosol water accounted for 5-13 % of Phet. The relative importance of heterogeneous sulfate production by other mechanisms was constrained by our observed Δ17O(SO42-). Heterogeneous sulfate production via S(IV) oxidation by O3 was estimated to contribute 21-22 % of Phet on average. Heterogeneous sulfate production pathways that result in zero-Δ17O(SO42-), such as S(IV) oxidation by NO2 in aerosol water and/or by O2 on acidic microdroplets via a radical chain mechanism, contributed the remain 66-73 % of Phet. The assumption about the thermodynamic state of aerosols (stable or metastable) was found to significantly influence the calculated aerosol pH (7.6 ± 0.1 or 4.7 ± 1.1, respectively), and thus influence the relative importance of heterogeneous sulfate production via S(IV) oxidation by NO2 and by O2 on acidic microdroplets. Our calculation suggests sulfate formation via NO2 oxidation can be the dominant pathway in aerosols at high pH-conditions calculated assuming stable state while S(IV) oxidation by O2 on acidic microdroplets can be the dominant pathway providing that highly acidic aerosols (pH ≤ 3) exist. Our results also illustrate the utility of Δ17O(SO42-) for quantifying sulfate formation pathways and its inclusion in models may improve our understanding of rapid sulfate formation during haze events. [ABSTRACT FROM AUTHOR]

Sodium chloride (NaCl) is one of the key components of atmospheric aerosols. The surface tension of aqueous NaCl solution (σNaCl,sol) and its concentration dependence are essential to determine the equilibrium water vapor pressure of aqueous NaCl droplets. Supersaturated NaCl solution droplets are observed in laboratory experiments and under atmospheric conditions, but the experimental data for σNaCl,sol are mostly limited up to sub-saturated solutions. In this study, the surface tension of aqueous NaCl is investigated by molecular dynamics (MD) simulations and pressure tensor method from dilute to highly supersaturated solutions. We show that the linear approximation of concentration dependence of σNaCl,sol at molality scale can be extended to the supersaturated NaCl solution until a molality of ~9.6 mol kg-1 (i.e., solute mass fraction (xNaCl) of ~0.36). Energetic analyses show that this monotonic increase of surface tension is driven by the increase of excessive surface enthalpy (ΔH) as the solution becomes concentrated. After that, the simulated σNaCl,sol remains almost unchanged until xNaCl of ~0.47 (near the concentration upon efflorescence). The existence of the inflection point at xNaCl of ~0.36 and the stable surface tension of xNaCl between ~0.36 and ~0.47 can be attributed to a competitive growth of excessive surface entropy term (T · ΔS) and the excessive surface enthalpy term (ΔH). After a second inflection point at xNaCl of ~0.47, the simulated σNaCl,sol gradually regains the growing momentum with a tendency to approach the surface tension of molten NaCl (~148.4 mN m-1 at 298.15 K, MD simulation based extrapolation). This fast increase of σNaCl,sol at xNaCl > 0.47 is primarily still an excessive surface enthalpy-driving process, although contribution from concurrent fluctuation of excessive surface entropy is expected but in a relatively smaller scale. Our results reveal different regimes of concentration dependence of the surface tension of aqueous NaCl at 298.15 K: a water-dominated regime (xNaCl from 0 to ~0.36), a transition regime (xNaCl from ~0.36 to ~0.47) and a molten NaCl-dominated regime (xNaCl from ~0.47 to 1). [ABSTRACT FROM AUTHOR]

Ground based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) and lidar measurements were performed in Shanghai, China during May 2016 to investigate the summertime atmospheric pollutants vertical distribution. In this study, vertical profiles of aerosol extinction coefficient, nitrogen dioxide (NO2) and formaldehyde (HCHO) concentrations were retrieved from MAX-DOAS measurement using the Heidelberg Profile (HeiPro) algorithm, while vertical distribution of ozone (O3) was obtained from an ozone lidar. Sensitivity study of the MAX-DOAS aerosol profile retrieval shows that the a priori aerosol profile shape has significant influences on the aerosol profile retrieval. Aerosol profiles retrieved from MAX-DOAS measurements with Gaussian a priori demonstrate the best agreements with simultaneous lidar measurements and vehicle-based tethered-balloon observations among all a priori aerosol profiles. MAX-DOAS measured tropospheric NO2Vertical Column Densities (VCDs) show a good agreement with OMI satellite observations with Pearson correlation coefficient (R) of 0.95. In addition, measurements of the O3 vertical distribution indicate that the ozone productions do not only occur at surface level but also at higher altitudes (about 1.1 km). Planetary boundary layer (PBL) height, horizontal and vertical wind fields information were integrated to discuss the ozone formation at upper altitudes. The results reveal that enhanced ozone concentrations at ground and upper altitudes are not directly related to horizontal and vertical transportations. Similar patterns of O3 and HCHO vertical distributions were observed during this campaign, which implies that the ozone productions near to the surface and at higher altitudes are mainly influenced by the abundance of volatile organic compounds (VOCs) in the lower troposphere. [ABSTRACT FROM AUTHOR]

Polycyclic aromatic hydrocarbons (PAHs) are hazardous pollutants, with increasing emissions in pace with economic development in East Asia, but their distribution and fate in the atmosphere have not yet been well understood. We extended the regional atmospheric chemistry model WRF/Chem (Weather Research Forecast model with Chemistry module) to comprehensively study the atmospheric distribution and the fate of low concentrated, slowly degrading semivolatile compounds. The WRF/Chem-PAH model reflects the state-of-the-art understanding of current PAHs studies with several new or updated features. It was applied for PAHs covering a wide range of volatility and hydrophobicity i.e., phenanthrene, chrysene and benzo(a)pyrene, in East Asia. Temporally highly resolved PAH concentrations and particulate mass fractions were evaluated against observations. The WRF/Chem-PAH model is able to reasonably well simulate the concentration levels and particulate mass fractions of PAHs near the sources and at a remote outflow region of East Asia, in high spatial and temporal resolutions. Sensitivity study shows that the heterogeneous reaction with ozone and the homogeneous reaction with the nitrate radical significantly influence the fate and distributions of PAHs. The methods to implement new tracers and to correct the transport problems can be applied to other newly implemented tracers in WRF/Chem. [ABSTRACT FROM AUTHOR]

Hygroscopic growth factors of organic surrogate compounds representing biomass burning and mixed organic-inorganic aerosol particles exhibit variability during dehydration experiments depending on their chemical composition, which we observed using a hygroscopicity tandem differential mobility analyzer (HTDMA). We observed that levoglucosan and humic acid aerosol particles release water slightly in the range of 90-5 % relative humidity (RH). 4-Hydroxybenzoic acid aerosol particles, however, remain in the solid state without diameter growth and even slightly shrinking at higher RH compared to the dry size. The measurements were accompanied by RH-dependent thermodynamic equilibrium calculations using the AIOMFAC and the E-AIM models, the ZSR relation, and a fitted hygroscopicity expression. We observed several effects of organic components on the hygroscopicity behavior of mixtures containing ammonium sulfate (AS) in relation to the different mass fractions of organic compounds: (1) a shift of efflorescence relative humidity (ERH) of ammonium sulfate to higher RH due to the presence of 25 wt % levoglucosan in the mixture. (2) There is a distinct efflorescence transition at 25 % RH for mixtures consisting of 25 wt % of 4-hydroxybenzoic acid compared to the ERH at 35 % for organic-free AS particles. (3) There is indication for a liquid-to-solid phase transition of 4-hydroxybenzoic acid in the mixed particles during dehydration. (4) A humic acid component shows no significant effect on the efflorescence of AS in mixed aerosol particles. In addition, relatively good measurement-model agreement in the case of the AIOMFAC and E-AIM models is mainly due to composition-dependent consideration of crystallization of AS in the model prediction. The use of the ZSR relation leads to good agreement with measured diameter growth factors of aerosol particles containing humic acid and ammonium sulfate. A similarity of the hygroscopicity parameter κ for organic surrogate compounds mixed with ammonium sulfate for different mass fractions during the different seasonal periods in the Amazon is observed. The RH-dependent hygroscopicity parameter κ measured at sub-saturated is consistent with kappa values for observations in the central Amazon Basin at the same environment, which suggests the similar O : C ratios and ammonium sulfate mass fraction in the laboratory and field observation conditions. [ABSTRACT FROM AUTHOR]

Soil and biological soil crusts can emit nitrous acid (HONO) and nitric oxide (NO). The terrestrial ground surface in arid and semi-arid regions is anticipated to play an important role in the local atmospheric HONO budget, deemed to represent one of the unaccounted HONO sources frequently observed in field studies. In this study HONO and NO emissions from a representative variety of soil and biological soil crust samples from the Mediterranean island Cyprus were investigated under controlled laboratory conditions. A wide range of fluxes was observed, ranging from 0.6 to 264ngm-2s-1 HONO-N at optimal soil water content (20-30% of water holding capacity, WHC). Maximum NO-N at this WHC fluxes were lower (0.8-121ngm-2s-1). Highest emissions of both reactive nitrogen species were found from bare soil, followed by light and dark cyanobacteria-dominated biological soil crusts (biocrusts), correlating well with the sample nutrient levels (nitrite and nitrate). Extrapolations of lab-based HONO emission studies agree well with the unaccounted HONO source derived previously for the extensive CYPHEX field campaign, i.e., emissions from soil and biocrusts may essentially close the Cyprus HONO budget. [ABSTRACT FROM AUTHOR]

Black carbon (BC) aerosol particles play an important role in regulating earth's climate and their climate effects depend on their mixing state. During the CAREBeijing 2013 campaign, we measured the size-resolved mixing state of refractory BC particles in North China Plain and performed intercomparison between a single particle soot photometer (SP2) and a volatility tandem differential mobility analyzer (VTDMA). The intercomparison shows a good agreement between the optical particle diameter determined by SP2 and the mobility particle diameter determined by VTDMA for non-BC as well as for internally mixed refractory BC particles. The VTDMA shows a higher concentration of refractory particles than that of the SP2, which suggests the existence of a large fraction of low volatile non-BC aerosols. Following parameters were constrained by closure studies to improve the inversion of the mixing state of ambient BC (i.e., coating thickness (CT) and shell/core ratio (Dp / Dc)) by SP2: a) refractive indices (RI) of 1.42 and 1.67-0.56i for non-BC and rBC core components, respectively, b) refractory BC (rBC) core density of 1.2 g cm-3 for internally-mixed BC particles, and c) an effective density range of 0.25-0.45 g cm-3 for externally-mixed BC particles. Moreover, the upper limit of the measurable particle size of SP2 was extended by the leading-edge-only (LEO) fit from ~ 400 nm to ~ 550 nm as confirmed by the VTDMA measurement. Based on the improved inversion from SP2 measurement, we found that non-BC containing particles, internally-mixed BC and externally-mixed BC contribute 85-90 %, 5-7 % and 5-10 % of the total aerosol number in the size range of 200 nm to 350 nm. The number fraction of internally-mixed BC in total BC-containing aerosols (Fin) shows pronounced diurnal cycles with a peak around noon time and an apparent turnover rate up to 6-9 % h-1. Such diurnal cycles are similar to the finding of Cheng et al. (2012) suggesting the competing effect of emissions and aging processes. In this study, the observed internally-mixed BC particles in the polluted regional NCP (North China Plain) background site (Xianghe) suggest a rapid aging process of BC on the regional scale. During the intensive field study period, ~ 80 % of internally-mixed BC particles at 200-300 nm showed a Dp / Dc ratio of more than 2, accompanying with an average value of 2.3-2.8. Meanwhile, the CT of internally-mixed BC particles (200-350 nm) with rBC core size of 80-200 nm was in the range of 50-150 nm. Compared with previous measurements in developed countries, the observed BC particles on regional scale (i.e., internally-mixed BC particles) were more-aged, indicating stronger optical and climate effect of BC on the regional scale in northern China. [ABSTRACT FROM AUTHOR]

Coated-wall flow tube reactors are frequently used to investigate gas uptake and heterogeneous or multiphase reaction kinetics under laminar flow conditions. Coating surface roughness may potentially distort the laminar flow pattern, induce turbulence and introduce uncertainties in the calculated uptake coefficient based on molecular diffusion assumptions (e.g., Brown/CKD/KPS methods), which hasn't been sufficiently addressed in previous applications. Here we investigate the influence of surface roughness and local turbulence on coated-wall flow tube experiments for gas uptake and kinetic studies. According to laminar boundary theory and considering the specific flow conditions in a coated-wall flow tube, we suggest using a critical height Δc to evaluate turbulence effects in the design and analysis of coated-wall flow tube experiments. When a coating thickness εmax is larger than Δc, the roughness elements of the coating may cause local turbulence and result in overestimation of the real uptake coefficient (γ). We collect εmax values in previous coated-wall flow tube studies and evaluate their roughness effects using the criterion of Δc. In most cases, the roughness doesn't generate turbulence and has negligible effects. When turbulence is generated, the calculated effective uptake coefficient (γeff) can bear large difference compared to the real uptake coefficient (γ). Their difference becomes less for gas reactants with lower uptake (i.e., smaller γ), or/and for a smaller ratio of the coating thickness εmax/R0 to the flow tube radius R0, (εmax/R0). On the other hand, the critical height Δc can also be adjusted by optimizing flow tube configurations and operating conditions (i.e., tube diameter, length and flow velocity), to ensure not only an unaffected laminar flow pattern but also a flexible residence time of gas reactants in flow tube reactors. [ABSTRACT FROM AUTHOR]

Proteins can be nitrated by air pollutants (NO2), enhancing their allergenic potential. This work provides insight into protein nitration and subsequent decomposition in the present of solar radiation. We also investigated light-induced formation of nitrous acid (HONO) from protein surfaces that were nitrated either online with instantaneous gas phase exposure to NO2 or offline by an efficient nitration agent (tetranitromethane, TNM). Bovine serum albumin (BSA) and ovalbumin (OVA) were used as model substances for proteins. Nitration degrees of about 1 % were derived applying NO2 concentrations of 100 ppb under VIS/UV illuminated condition, while simultaneous decomposition of (nitrated) proteins was also found during long-term (20 h) irradiation exposure. Gas exchange measurements of TNM-nitrated proteins revealed that HONO can be formed and released even without contribution of instantaneous heterogeneous NO2 conversion. However, fumigation with NO2 was found to increase HONO emissions substantially. In particular, a strong dependence of HONO emissions on light intensity, relative humidity (RH), NO2 concentrations and the applied coating thickness were found. The 20 hours long-term studies revealed sustained HONO formation, even if concentrations of the intact (nitrated) proteins were too low to be detected after the gas exchange measurements. A reaction mechanism for the NO2 conversion based on the Langmuir-Hinshelwood kinetics is proposed. [ABSTRACT FROM AUTHOR]

The hygroscopicity parameter κ has been intensively used in the investigation of the water uptake, cloud condensation nuclei (CCN) activity and chemical composition of atmospheric aerosol particles. A representative value of κ is often assigned to individual species or sources. Such treatment may lead to confusion in closure studies of κ derived from hygroscopic growth factor measurements (κgf) and CCN activity measurements (κCCN), and in studies of aerosols at the sub-10 nm size range. Here we show that for particles of the same dry composition, κ may differ as a function of water content, solute concentration and particle size. The concentration- and size-dependence of κ are demonstrated for representative inorganic and organic compounds, i.e., ammonium sulfate (AS), sodium chloride (NaCl) and sucrose. Our results illustrate that an absolute closure between κgf and κCCN should not be expected, and how the deviations observed in field and laboratory experiments can be quantitatively explained and reconciled. The difference between κgf and κCCN increases as particle size decreases reaching up to 40 % and 30 % for 10 nm AS and NaCl particles, respectively. Moreover, we show that the deviations of κCCN vary from ~ 10 % for 30 nm and ~ 40 % for 200 nm, indicating a strong dependence on the Köhler models and thermodynamic parameterizations used for instrument calibration (e.g., effective water vapor supersaturation in CCN counter). By taking these factors into account, we can largely explain apparent discrepancies between κgf and κCCN values reported in the scientific literature. Our results help to understand and interpret κ values determined at different water vapor ratios and at different size ranges (especially sub-10 nm). We highlight the importance of self-consistent thermodynamic parameterizations when using AS for calibration aerosol and taking it as a reference substance representing inorganics in closure study between chemical composition and hygroscopicity of aerosol particles. [ABSTRACT FROM AUTHOR]

Heterogeneous hydrolysis of N2O5 on the surface of deliquescent aerosol particles leads to HNO3 formation and acts as a major sink of NOx in the atmosphere during nighttime. The reaction constant of this heterogeneous hydrolysis is determined by temperature (T), relative humidity (RH), aerosol particle composition as well as the surface area concentration (S). However, its parameterization in previous 3-D modelling studies did not comprehensively consider these parameters. In this investigation, we propose a sophisticated parameterization of the heterogeneous hydrolysis of N2O5 with respect to T, RH, aerosol particle compositions and S, based on laboratory experiments. This new parameterization was incorporated into a 3-D fully online coupled model: COSMO-MUSCAT. As case study, we used the data from the HOPE-Melpitz campaign (10-25 September 2013). Here, we investigated the improvement of nitrate prediction over the western and central Europe. The modelled particulate nitrate mass concentrations ([NO3-]) were validated by filter measurements over Germany (Neuglobsow, Schmücke, Zingst, and Melpitz). The modelled [NO3-] were significantly overestimated for this period by a factor of 5-19, with the corrected NH3 emissions (reduced by 50 %) and the original parameterization of N2O5 heterogeneous hydrolysis. The proposed new parameterization significantly reduces the overestimation of [NO3-] by ~ 35 %. Particularly, the overestimation factor was reduced to approximately 1.4 within our case study period (September 12, 17-18 and 25, 2013), when [NO3-] was dominated by local chemical formations. Furthermore, the organic coating effect on a suppression of the N2O5 reaction probability may have been also significantly overestimated in previous modelling studies, due to a strong overestimation of the N2O5 reaction probability on coatings. Based on the original parameterization, previous studies reported a decrease of modelled [NO3-] up to 90 %, where both secondary organic aerosol (SOA) and N2O5 were built-up over western and central Europe. For this case study, the suppression of organic coating was negligible over western and central Europe, with influence on [NO3-] less than 2 % on average and 20 % at the most significant moment. As for a significant impact of the organic coating effect, N2O5, SOA and NH3 need to be present when RH is high and T is low. However, those conditions were rarely fulfilled simultaneously over western and central Europe. Hence, the organic coating effect on reaction probability of N2O5 over Europe may not be as important as expected in previous studies. [ABSTRACT FROM AUTHOR]

Size-resolved long-term measurements of atmospheric aerosol and cloud condensation nuclei (CCN) concentrations as well as hygroscopicity were conducted at the remote Amazon Tall Tower Observatory (ATTO) in the central Amazon Basin over a one-year period and full seasonal cycle (March 2014-February 2015). The presented measurements provide a climatology of CCN properties for a characteristic central Amazonian rain forest site. The CCN measurements were continuously cycled through 10 levels of supersaturation (S = 0.11 to 1.10%) and span the aerosol particle size range from 20 to 245 nm. The observed mean critical diameters of CCN activation range from 43 nm at S = 1.10% to 172 nm at S = 0.11%. The particle hygroscopicity exhibits a pronounced size dependence with lower values for the Aitken mode (κAit = 0.14 ± 0.03), elevated values for the accumulation mode (κAcc = 0.22 ± 0.05), and an overall mean value of κmean = 0.17 ± 0.06, consistent with high fractions of organic aerosol. The hygroscopicity parameter κ exhibits remarkably little temporal variability: no pronounced diurnal cycles, weak seasonal trends, and few short-term variations during long-range transport events. In contrast, the CCN number concentrations exhibit a pronounced seasonal cycle, tracking the pollution-related seasonality in total aerosol concentration. We find that the variability in the CCN concentrations in the central Amazon is mostly driven by aerosol particle number concentration and size distribution, while variations in aerosol hygroscopicity and chemical composition matter only during a few episodes. For modelling purposes, we compare different approaches of predicting CCN number concentration and present a novel parameterization, which allows accurate CCN predictions based on a small set of input data. [ABSTRACT FROM AUTHOR]

Sea salt aerosol (SSA) is one of the major components of primary aerosols and has significant impact on the formation of secondary inorganic aerosol particles on a global scale. In this study, the fully online coupled WRF-Chem model was utilized to evaluate the SSA emission scheme and its influence on the nitrate simulation in a case study in Europe during September 10–20, 2013. Meteorological conditions near the surface, wind pattern, and thermal stratification structure were well reproduced by the model. Nonetheless, coarse mode (PM1–10) particle mass concentration was substantially overestimated due to the overestimation of SSA and nitrate. Compared to filter measurements at 4 EMEP stations (coastal stations: Bilthoven, Kollumerwaard and Vredepeel; inland station: Melpitz), the modeled SSA concentrations were overestimated by a factor of 8–20. We found that the overestimation was mainly caused by the overestimated SSA emission over North Sea during September 16–20. Over the coastal regions, the SSA was injected into the continental free troposphere through an “aloft bridge” (about 500 to 1000 meter above the ground), a result of the different thermodynamic properties and planetary boundary layer (PBL) structure between continental and marine regions. The injected SSA was further transported inland and mixed downward to the surface through downdraft and PBL turbulence. This process broadened the influence of SSA to a larger downwind region, for example, leading to an overestimation of SSA at Melpitz, Germany by a factor of ~20. As a result, nitrate partitioning fraction (ratio between particulate nitrate and the summation of particulate nitrate and gas-phase nitric acid) increased by about 0.2 for the coarse mode nitrate due to the overestimation of SSA at Melpitz, but no significant difference in the partitioning fraction for the fine mode nitrate. About 140% overestimation of the coarse mode nitrate was resulted from the influence of SSA at Melpitz. On the other hand, the overestimation of SSA inhibited the nitrate formation in the fine mode by about 20%, because of the increased consumption of precursors by coarse mode nitrate formation. [ABSTRACT FROM AUTHOR]

Sea salt aerosol (SSA) is one of the major components of primary aerosols and has significant impact on the formation of secondary inorganic aerosol particles on a global scale. In this study, the fully online coupled WRF-Chem model was utilized to evaluate the SSA emission scheme and its influence on the nitrate simulation in a case study in Europe during September 10-20, 2013. Meteorological conditions near the surface, wind pattern, and thermal stratification structure were well reproduced by the model. Nonetheless, coarse mode (PM1-10) particle mass concentration was substantially overestimated due to the overestimation of SSA and nitrate. Compared to filter measurements at 4 EMEP stations (coastal stations: Bilthoven, Kollumerwaard and Vredepeel; inland station: Melpitz), the modeled SSA concentrations were overestimated by a factor of 8-20. We found that the overestimation was mainly caused by the overestimated SSA emission over North Sea during September 16-20. Over the coastal regions, the SSA was injected into the continental free troposphere through an "aloft bridge" (about 500 to 1000 meter above the ground), a result of the different thermodynamic properties and planetary boundary layer (PBL) structure between continental and marine regions. The injected SSA was further transported inland and mixed downward to the surface through downdraft and PBL turbulence. This process broadened the influence of SSA to a larger downwind region, for example, leading to an overestimation of SSA at Melpitz, Germany by a factor of ~20. As a result, nitrate partitioning fraction (ratio between particulate nitrate and the summation of particulate nitrate and gas-phase nitric acid) increased by about 0.2 for the coarse mode nitrate due to the overestimation of SSA at Melpitz, but no significant difference in the partitioning fraction for the fine mode nitrate. About 140% overestimation of the coarse mode nitrate was resulted from the influence of SSA at Melpitz. On the other hand, the overestimation of SSA inhibited the nitrate formation in the fine mode by about 20%, because of the increased consumption of precursors by coarse mode nitrate formation. [ABSTRACT FROM AUTHOR]

Gaseous formaldehyde (HCHO) is an important precursor of OH radicals and a key intermediate molecule in the oxidation of atmospheric volatile organic compounds (VOCs). Budget analyses reveal large discrepancies between modeled and observed HCHO concentrations in the atmosphere. Here, we investigate the interactions of gaseous HCHO with soil surfaces through coated-wall flow tube experiments applying atmospherically relevant HCHO concentrations of ~ 10 to 40 ppbv. For the determination of uptake coefficients (γ), we provide a Matlab code to account for the diffusion correction under laminar flow conditions. Under dry conditions (relative humidity = 0 %), an initial γ of (1.1 ± 0.05) × 10−4 is determined, which gradually drops to (5.5 ± 0.4) × 10−5 after 8-hour experiments. Experiments under wet conditions show a smaller γ that drops faster over time until reaching a plateau. The drop of γ with increasing relative humidity and over time can both be explained by the adsorption theory in which high surface coverage leads to a reduced uptake rate. The fact that γ stabilizes at a non-zero plateau suggests the involvement of irreversible chemical reactions. Further back-flushing experiments show that two thirds of the adsorbed HCHO can be re-emitted into the gas phase while the residual is retained by the soil. This partial reversibility confirms that HCHO uptake by soil is a complex process involving both adsorption/desorption and chemical reactions which must be considered in trace gas exchange (emission or deposition) at the atmosphere-soil interface. Our results suggest that soil and soil-derived airborne particles can either act as a source or a sink for HCHO, depending on ambient conditions and HCHO concentrations. [ABSTRACT FROM AUTHOR]

Gaseous formaldehyde (HCHO) is an important precursor of OH radicals and a key intermediate molecule in the oxidation of atmospheric volatile organic compounds (VOCs). Budget analyses reveal large discrepancies between modeled and observed HCHO concentrations in the atmosphere. Here, we investigate the interactions of gaseous HCHO with soil surfaces through coated-wall flow tube experiments applying atmospherically relevant HCHO concentrations of ~ 10 to 40 ppbv. For the determination of uptake coefficients (γ), we provide a Matlab code to account for the diffusion correction under laminar flow conditions. Under dry conditions (relative humidity = 0%), an initial γ of (1.1 ± 0.05) × 10-4 is determined, which gradually drops to (5.5 ± 0.4) × 10-5 after 8-hour experiments. Experiments under wet conditions show a smaller γ that drops faster over time until reaching a plateau. The drop of γ with increasing relative humidity and over time can both be explained by the adsorption theory in which high surface coverage leads to a reduced uptake rate. The fact that γ stabilizes at a non-zero plateau suggests the involvement of irreversible chemical reactions. Further back-flushing experiments show that two thirds of the adsorbed HCHO can be re-emitted into the gas phase while the residual is retained by the soil. This partial reversibility confirms that HCHO uptake by soil is a complex process involving both adsorption/desorption and chemical reactions which must be considered in trace gas exchange (emission or deposition) at the atmosphere-soil interface. Our results suggest that soil and soil-derived airborne particles can either act as a source or a sink for HCHO, depending on ambient conditions and HCHO concentrations. [ABSTRACT FROM AUTHOR]