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Atmospheric aerosol particles exert significant influence on the earth's radiation budget, and accurate measurement of aerosol optical properties can help reduce the uncertainties in estimating aerosol radiative forcing. Therefore, year-round measurements of aerosol optical properties from August 2016 to July 2017 at an urban site in Beijing were analysed. The results show that the annual mean scattering coefficient (σsp at 550 nm, 131 ± 135 Mm−1) was approximately half of the measurement in Beijing from 2008 to 2009 and from 2005 to 2006, mainly due to the reduction of PM2.5 and scattering aerosols in recent years. Meanwhile, annual mean of σabs (637 nm) and SSA was 14 ± 11 Mm−1 and 0.85 ± 0.06, respectively. Seasonally, σsp during winter was approximately two times higher than in other seasons, while σabs was higher during autumn and winter, which may be attributed to biomass and fossil fuel burning under a shallow and stable boundary layer. Diurnal variation of σsp showed an evening peak and a slight morning peak in spring and summer, as well as a night peak in autumn and winter. σabs in four seasons were basically unimodal, with a morning peak in spring and summer, and a night peak in autumn and winter. In addition, the scattering Angstrom exponent and backscattering Angstrom exponent suggested the dominance of micro-size aerosols. Seasonal variations of SAE, BAE and the backscatter fraction indicated a larger proportion of isotropic scattering and fine-mode particles in summer and autumn, as well as a dominance of coarse particles in spring and a complicated aerosol type distribution and emission sources in winter. Furthermore, radiative forcing was estimated by a radiative transfer model. Radiative forcing varied between −6.69 and −235.67 Wm-2 (with the average of −54.93 ± 39.92 Wm-2) at the surface, with a mean of 35.56 ± 22.88 Wm-2 in the atmosphere.

Particle number size distribution measurements were conducted during the summer of 2012 at City-Centre Urban Background (Patras-C), Urban Background (ICE-HT in Patras, DEM in Athens, EPT in Thessaloniki), and Regional Background stations (FIN in Crete). At the City-Centre Urban Background station, the average number distribution had a geometric mean diameter peak approximately at 60 nm and the highest number concentration, whereas at the Regional Background station and the Urban Background stations it displayed a major peak approximately at 100 nm, with the Regional Background station exhibiting the lowest number concentration. The particle number size distribution at each site was divided into size fractions and, based on their diurnal variation and previous studies, we concluded that the main sources for the City-Centre Urban Background station are traffic and the regional background concentration, for the Urban Background stations fresh traffic, aged traffic, cooking and the regional background concentration, and for the Regional Background station local activities (tourism, cooking) and regional background concentration. The median number concentration that is attributed to regional background concentration for the City-Centre Urban Background, the Urban Background and the Regional Background stations are respectively 13, 29 and 45% of the total number concentration. Nucleation events were identified at DEM station, where the newly formed particles accounted for 4% of the total particle concentration for the measurement period in the size range 10–20 nm, EPT, where they accounted for 12%, and FIN, where they accounted for 1%, respectively. New Particle Formation events contribution during summer to Condensation Cloud Nuclei were therefore insignificant in the Eastern Mediterranean. Modal analysis was performed on the number distributions and the results were classified in clusters. At the City-Centre Urban Background station, the cluster-source that dominated number concentration and frequency is related to fresh and aged traffic emissions, at the Urban Background stations aged traffic emissions, while at the Regional Background station number and frequency were dominated by the regional background concentration. Based on cluster analysis, 18% of the median number distribution was due to long range transport at the City-Centre Urban Background site, 37% at the Urban Background sites, and 59% at the Regional Background site. The Flexible Particle Dispersion Model (FLEXPART) was used in order to acquire geographic origin clusters and we concluded that the Etesian flow increases the median regional background number concentration in the Mediterranean basin by a factor of 2.5–4.

Highly time-resolved measurements of non-refractory submicron particulate matter (NR-PM1) were performed using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) over a big urban city (Ahmedabad, 23.0°N, 72.6°E, 49 m amsl) of western India during the post-monsoon season. All the components of NR-PM1 [i.e., organic aerosol (OA) SO 4 2 − , NO 3 − , NH 4 + and Cl − ] showed a strong diurnal variation with the overall dominance of OA (58%) in NR-PM1, and the OA composition was found to be a mixture of fresh and aged species. O/C varied from 0.04 to 0.91 whereas, H/C varied from 1.34 to 2.26. A strong diurnal variation in O/C caused a large variability in OM/OC ratio (ranging from 1.2 to 2.3) during the study period, which suggests that a constant conversion factor for OM estimation (from OC) can be a source of large uncertainty in their load assessment over the study region. Furthermore, the observed slope (−0.78 ± 0.01) of Van Krevelen (VK) diagram suggests that plausible major functional groups of ambient oxidized OA could be the mixture of alcohols and carboxylic groups. Further, a drastic change in NR-PM1 concentrations and composition was observed during Diwali, a festival when huge amount of firecrackers are burnt across India. Three distinct sources of OA [i.e., primary OA (POA), semi-volatile oxygenated OA (SV-OOA), and low volatility OOA (LV-OOA)] were identified via positive matrix factorization (PMF) analysis. The OA mass was dominated by POA (42%), followed by LV-OOA (33%) and SV-OOA (25%). Diurnal variations in PMF factors suggest that OOA (SV-OOA + LV-OOA) were high during early morning and afternoon hours; whereas, POA was most abundant during traffic rush hours. Observations also revealed that the high OA loading events were dominated by POA during the study period. This study provides new insights on the atmospheric aging of OA, for the first time, over western India, which would be helpful in understanding the formation mechanism of secondary OA over this region.

For the first time, atmospheric carbonyls were identified in an island in the South China Sea during biomass burning episodes to investigate their chemical characteristics and possible sources. PM2.5 and its chemical species, including water-soluble ions, water-soluble organic compounds, organic carbon and elemental carbon, were also identified. The levels of atmospheric carbonyls in Weizhou Island were significantly higher than those in other remote areas and most polluted cities in China during the sampling period. The sum of formaldehyde (27.23 ± 12.25 μg/m3), acetaldehyde (10.67 ± 4.46 μg/m3) and isovaleraldehyde (2.35 ± 1.21 μg/m3) accounted for ∼84% of the total 16 carbonyls. Different from other studies, isovaleraldehyde ranked the third highest carbonyl in this study, while it was extremely low in other rural areas, and it was believed to be a photochemical production from industrial pollutants. Backward trajectory analysis showed that transport from Southeast Asia could be possible sources of isovaleraldehyde. Contrary to many previous studies, diurnal variation of carbonyls at this site showed that the concentrations of carbonyls were high in the early morning and evening (82.00 μg/m3 and 43.18 μg/m3, respectively) but low in the morning and afternoon (47.99 μg/m3 and 32.16 μg/m3, respectively), indicating that transport and intense photolysis might be responsible for the diurnal variation of carbonyls in Weizhou Island. The ozone formation potential and propene-equivalent concentration were 345 μg/m3 and 20 μg/m3, respectively, suggesting that carbonyls had a significant contribution to ozone formation in Weizhou Island. Diagnostic ratios and correlation analysis together with backward trajectory analysis showed that anthropogenic activities and biomass burning emissions from Southeast Asia and northern Gulf of China were two main sources of carbonyls at the sampling site.

In a scenario when haze and smog events are wreaking havoc in many parts of the world, accurate information on ambient sulfur dioxide (SO2) sources is a required input to models for prediction and mitigation of air pollution. Globally, India is one of the highest emitters of anthropogenic SO2. However, the Ozone Monitoring Instrument (OMI) data reveal that unlike many other anthropogenic air pollutants, the strongest anthropogenic SO2 sources are not located in the Indo-Gangetic plain (IGP) but over the coal belts of North Eastern peninsular India. In this study, we report SO2 measurements conducted at Bhubaneswar, a rapidly developing smart city in the Eastern Coast of India. The city is ideally located just downwind of the Indian SO2 hotspots during winter, when SO2 concentrations are the highest, reaching 14.3 ppbv for hourly averaged data. The measurements, reported here from the summer of 2010 to the summer of 2012, show that despite being a rapidly developing location, SO2 over Bhubaneswar is dominated by regional emissions rather than local sources like vehicular exhausts. The influence of regional emissions is strongest in winter as the city is directly downwind of 6 of the 10 largest thermal power plants and 7 out of the 13 largest integrated steel plants in India with a mean SO2 concentration of 3.2 ppbv. This is evinced by multiple pointers viz. Concentration-Weighted Trajectory (CWT) analysis for surface SO2 data, correlation analysis for OMI SO2 data, analysis of surface concentrations of SO2, organic carbon, black carbon and carbon monoxide; which reveal SO2 emissions from the industrial and mining regions of Chhattisgarh and north-west Odisha. This is the same region that shows up as hotspots of SO2 in OMI retrievals over India. However, despite being downwind of major SO2 sources during winter, SO2 concentrations in Bhubaneswar are lower than over some other cities (Kolkata, Kanpur) in the IGP, due to influence of strong local sources in the later cases. Unlike most other urban regions in India, the diurnal variation of SO2 over Bhubaneswar shows elevated daytime values due to predominant influence of regional emissions.

A tethered balloon-based measurement was conducted to investigate vertical distribution of O3 over Shanghai, China, during May of 2016. The time-series and diurnal variation of ground-level air pollutants (including O3, NO, NO2 and VOCs) were analyzed systematically. By using TrajStat (a GIS-based software), the potential distant contributions of O3 might come from East China Sea, and the Yangtze River Delta (YRD). Within 1000 m above ground, the average vertical profiles of O3 exhibited obviously diurnal characteristics. Most of the NO and O3 profiles measured on high-ozone days demonstrated anti-correlation. The launches on May 11, 2016 revealed that the Atmospheric Boundary Layer (ABL) and shifting of wind direction contributed to the peaks of O3 at the upper altitude. A typical case of vertical mixing of O3 was caught on May 18, 2016, when the ground-level O3 was greatly influenced by the O3 transported from high altitude (1500 m above sea level). The present study provides an important insight of the vertical distribution of O3 over Eastern China, which implied that coupling effects of horizontal and vertical transportations on ground-level ozone should be investigated in future studies.

To investigate aerosol optical properties relevant to pollutant level controls in place in Beijing, particulate matters with diameter σ a p ) and scattering ( σ s p ) coefficients were continuously measured from January 2014 to March 2017 at an urban site Peking University Atmosphere Environment MonitoRing Station (PKUERS). Our results showed that PM2.5, σ a p , and σ s p all decreased, especially from 2014 to 2015. Note that, a significant decrease of σ a p in autumn and σ s p in winter was responsible for the decrease of the year by year average. PM2.5, σ a p and σ s p exhibited a pronounced bi-peak diurnal variation, which was attributed to the traffic emissions. The wind dependence analysis revealed that σ a p was mainly influenced by local emissions but σ s p was influenced by both local emissions and regional transport. The potential spatial source origins of σ s p in Beijing were investigated using the potential source contribution function (PSCF). The results indicated that the potential source regions that influence σ s p of Beijing were mainly from south and west of Beijing but slightly varied with different years and seasons.

The planetary boundary-layer (PBL) structure was investigated using observations from single aerosol lidar, eddy covariance (EC) system, and automatic meteorological station (AWS) in the north of Nanjing city during an air pollution episode in 2016–2017 winter. Based on seven days’ observations under clear to polluted day, we present the temporal variations of the aerosol extinction profiles observed by lidar, and then inter-compare PBL depth retrieved from individual gradient methods. The results show that the gradient method (GM) generated the lowest PBL depth. In contrast to the cubic root gradient method (CRGM), which determined PBL depth ranging from 172 m to 1575 m during the observation period, the logarithm gradient method (LGM) and normalized gradient method (NGM) generated similar results and both tended to overestimate PBL depth on polluted days. The CRGM performed better than LGM and NGM in case of multiple backscatter layers and could detect low level layers, while the GM was biased at low heights probably due to the effect of lidar overlap function. Based on these measurements, the evolution of boundary layer structures and PBL depth over clean days and polluted days were compared. The results show that (1) on clean days, the strong surface turbulence exchange make the PBL depth fully developed and the PBL depth had obvious characteristics of diurnal variation; the maximum depth of PBL was 1560 m for CRGM; and (2) on polluted days, the high pressure system and lower wind was favorable to the accumulation of air pollutants, and thus generating less turbulence by reducing surface radiation. These conditions on polluted days led to smaller PBL depth than those on clean days, and the maximum depth of PBL was 660 m for CRGM. Besides, the diurnal variation of PBL depth on polluted days was weaker than those on clean days.

A reference height that often needs to be assumed in aerosol retrieval from Raman lidar tends to cause high uncertainty in retrieving the vertical distribution of aerosol optical properties. Here, a novel method is proposed to determine the height-revolved reference height, which is then used to retrieve aerosols from Raman lidar. This method can automatically avoid the atmospheric layers with the presence of aerosols, clouds and low signal to noise ratio (SNR). Based on elastic (at 355 nm) and inelastic (at 387 nm) signals collected during the period from 5 December 2016 to 5 March 2017 by a ground-based Raman lidar in Beijing, China, the aerosol optical properties, such as extinction coefficient, backscattering coefficient and lidar ratio have been successfully retrieved. Results show that the averaged nighttime aerosol optic depth (AOD) from Raman lidar is in good agreement with early morning AOD retrieved from a collocated sunphotometer. The AOD exhibits a strong diurnal variation with a peak at 1500 Beijing time. On average, the nighttime AOD at 355 nm is 0.32, whereas the daytime AOD is 0.72 over Beijing during the study period. The column averaged lidar ratio is 44 sr at 355 nm, roughly consistent with previous studies. Our findings shed light on the pathways towards improving the retrieval of vertical distribution of aerosols optical properties during nighttime.

Oxalic acid is the most abundant dicarboxylic acid in the atmosphere, but the processes involved in its formation in ambient aerosols are still uncertainty. We used single particle aerosol mass spectrometer to investigate the sources and formation mechanisms of oxalic acid-containing (C2) particles in ambient aerosols in Chengdu during summer and winter seasons. C2 particles accounted for 2.9 and 0.7% of the total amount of fine particles detected in summer and winter, respectively. The C2 particles in each season were classified as nine different types of particle with significant seasonal differences in their contribution to total C2 particles. The oxalic acid-sulfate (C2-SO4) particles and particles related to biomass burning (i.e., oxalic acid-organic nitrogen (C2-CN) and oxalic acid-elemental carbon (C2-EC)) represented 78% of the total C2 particles in summer, whereas the contributions of oxalic acid-dust (C2-D), oxalic acid-sulfate and nitrate (C2-SN), and oxalic acid-nitrate (C2-NO3) particles clearly increased in winter. Although the peak diameter of the average size distribution of total C2 particles was larger in winter than in summer, there was no significant seasonal difference in the number fraction of the size-resolved particles for each type of particle. The peak concentration of total C2 particle in summer occurred in the afternoon, but there was no obvious diurnal variation in winter. We concluded that local photochemical oxidation in the daytime favored the production of C2 particles in summer, whereas in-cloud processing with subsequent long-distance transport had an important effect on C2 particles in winter. The potential source contribution function (PSCF) analysis indicated that local sources from the eastern, southeastern, and southern regions of Sichuan Province dominated the C2 particles in summer, whereas the contribution from the long-distance transport of aerosols originating outside Sichuan Province was significantly enhanced in winter. These findings will help us to understand the formation of secondary organic aerosols in the atmosphere over the Sichuan Basin.

A dual dynamic chamber system combined with incoherent broadband cavity enhanced absorption spectroscopy (IBBCEAS) was first developed to measure nitrous acid (HONO) emission fluxes from agricultural fields in the North China Plain. The concentrations of HONO were measured with two IBBCEAS instruments in the sample and blank chambers, respectively. To ensure the accuracy and reliability of flux measurement, before the dynamic chamber system was deployed in an agricultural field, the correlation and accuracy of the two IBBCEAS instruments were tested, and the mixing uniformity of the gas in the chamber was verified. Within a week after fertilization, the observed HONO flux exhibited a pronounced diurnal variation profile with a maximum of 20.25 ng N m−2 s−1 at noontime and a minimum of −0.86 ng N m−2 s−1 in the early morning, one order of magnitude higher than the flux observed in previously reported literature from fields. During this period, the concentrations of HONO in both of the chambers was higher in the daytime than that nighttime, and the concentration of HONO in blank chamber reached the maximum of 18.88 ppbv at noon on the 5th day after fertilization. HONO and NO2 in the sample chambers showed opposite diurnal profiles. The field observation results suggested that the high HONO flux is mainly from the direct emission of soil rather than from the heterogeneous reaction of NO2. Agricultural fertilization will affect the balance of the HONO budget, and then affect the atmospheric oxidation in the North China Plain.

Temporal and seasonal variabilities in black carbon (BC) mass concentrations, equivalent BC from fossil fuel ( BC f f ) and wood burning ( BC w b ) are investigated using multiwavelength aethalometer measurements made over urban (Ahmedabad) and high altitude remote (Gurushikhar) sites in western India during 2015–2016. BC, BC f f and BC w b mass concentrations exhibit strong diurnal variation over Ahmedabad compared to Gurushikhar. Annual mean contribution of BC f f to total BC mass concentration is estimated to be 80 and 72% respectively over Ahmedabad and Gurushikhar, which indicates the dominance of fossil fuel emissions. To delineate the impact of BC aerosols on the Earth-atmosphere radiation budget aerosol radiative forcing due to composite aerosols, and BC aerosols alone is estimated. Maximum atmospheric forcing due to BC is observed during December (15 W m − 2 ) and November (8 W m − 2 ) over Ahmedabad and Gurushikhar respectively, because BC mass concentration is highest in the respective months. Surface composite forcing is higher during postmonsoon (−27 W m − 2 ) and premonsoon (−16 W m − 2 ) over Ahmedabad and Gurushikhar respectively due to high aerosol optical depths. BC aerosols contribute ≃ 60% to the shortwave atmospheric forcing. The present study shows that the large spatial and temporal variation in BC mass concentrations over an urban and high altitude remote site can produce significant regional variabilities in aerosol radiative effects.

As a characteristic biogenic volatile organic compound, isoprene is directly emitted by plants and participates in the atmospheric chemical process promptly. In virtue of differential optical absorption spectroscopy (DOAS) technique, isoprene was measured from spring to autumn of 2018 in urban area of Shanghai, China. The concentration of isoprene reached its peak in July with monthly average exceeded 0.3 ppbv. Besides, a distinct diurnal variation was observed and peaked around 13:00, up to 0.6 ppbv. The observed isoprene was found closely related to meteorological factors, and showed a good correlation with emission active factor γ calculated by temperature and radiation (R2 = 0.87), indicating that the urban isoprene emission is mainly emitted from biogenic sources and regulated by temperature and radiation. According to the proportional relationship between isoprene and its major intermediate oxidation products, it suggested that vast majority of daytime methyl vinyl ketone (MVK) and methacrolein (MACR) were derived from the oxidation of isoprene. To further understand the oxidation processes of isoprene, six cases covering different atmospheric conditions were selected to simulate atmospheric radicals relied on the box model. The daily peak concentration of simulated ·OH and NO3 ranged from 5.98 × 106 to 1.19 × 107 molecule cm−3 and 2.35 × 108 to 1.49 × 109 molecule cm−3, presenting a medium-to-high level of atmospheric oxidation capacity. And oxidants of ·OH, O3 and NO3 shared 66–95%, 1–4% and 3–30% of isoprene oxidation, respectively. The comparison of estimated initial and measured isoprene concentration implied that at least half of the isoprene was oxidized between emission and measurement. The short daytime atmospheric lifetime (0.4–1.1 h) also suggested the high reactivity of isoprene. This study indicates that the urban ambient isoprene is fast oxidized and fully involved in the atmospheric oxidation processes, presenting some subtle differences under different air pollution conditions.

This study explored the spatiotemporal variation, chemical characteristics, and long-range transport of marine fine particles (PM2.5) at two remote islands in the South China Sea (SCS). Daytime and nighttime PM2.5 was simultaneously sampled at the Dongsha and Nansha Islands for determining its mass concentrations and chemical fingerprints. Field measurement results showed that high PM2.5 level occurred in winter and spring, particularly during the northeastern monsoon periods (NMPs), which blew anthropogenic PM2.5 to the SCS via continental outflow. In terms of diurnal variation, daytime PM2.5 concentrations were higher than those at nighttime in all seasons. Chemical analysis indicated that PM2.5 was dominated by water-soluble ions (WSIs) which were dominated by secondary inorganic aerosols (SIAs) (NO3−, SO42−, and NH4+). Crustal elements (Ca, K, Mg, Fe, and Al) were abundant of elements in PM2.5, but enrichment factor (EF) showed that trace elements (Ti, Cr, Mn, Ni, Cu, and Zn) were mainly originated from anthropogenic sources. The concentrations of organic carbon (OC) in PM2.5 were always higher than those of elemental carbon (EC). High concentrations of levoglucosan and organic acids were observed in winter and spring. The major source of PM2.5 was sea salts (16.8–24.8%), and followed by fugitive dust, secondary aerosols, oil-fired boilers, automobile sources, and biomass burning. Clustered trajectories indicated that polluted air masses were transported from mainland China, Korean Peninsula, and Japan Islands, particularly passing through industrialized and urbanized regions in East Asia during the NMPs. A superimposition phenomenon of local emissions and long-range transport (LRT) was observed in the SCS. The LRT of anthropogenic aerosols accounted for 54–79% of marine PM2.5. Overall, PM2.5 at the Dongsha Islands was highly influenced by LRT from Asian continental outflow, while PM2.5 concentrations observed at the Nansha Islands did not vary seasonally.

Volatile organic compounds (VOCs) and oxygenated VOCs (OVOCs) were measured in Ho Chi Minh City (HCMC) and Hanoi, the two largest and most populous cities in Vietnam. The purpose of this study is to better understand the VOC atmospheric composition and their role in ground-level ozone formation. Online measurements of a wide range of VOCs and other pollutants were conducted using numerous instruments during different seasons (dry and rainy) in HCMC and Hanoi (spring). Our results show that the mean mixing ratio of total measured VOCs in Hanoi was 80.8 ± 40.7 ppb (mean ± standard deviation), and was similar to that observed during the rainy season (75.2 ± 44.8 ppb) in HCMC. During the dry season campaign in HCMC, which was coincident with the Hanoi campaign, total VOC was around 50% lower (40.7 ± 19.5 ppb), largely a result of increased planetary boundary layer (PBL) height and the direction of the prevailing wind. VOC profiles in both cities were dominated by alkanes (31–35%) and OVOCs (27–33%) and the proportion of alkenes (13–17%) and aromatics (12–19%) were comparable. Similarities in diurnal variation for most VOC species (except for isoprene) are seen in both cities with two clear peaks during the morning (7:00–8:00 a.m.) and evening (18:00 p.m.) rush hours, as observed for vehicular-combustion tracers (acetylene and CO). Comparisons of the ambient ratios of paired VOCs, namely i/n-pentane, and toluene/benzene, with those reported in motorcycle exhaust, roadside and gasoline samples indicate that motorcycle-related emission is likely a major contributor to VOC pollution. According to the propylene-equivalent concentration (PE conc.) and maximum incremental reactivity (MIR) methods, alkenes and aromatics were determined to be the main contributors to reactivity and ozone potential formation. Furthermore, the initial mixing ratio of VOC species was estimated based on the photochemical age method. The consumed VOCs (initial VOCs minus measured VOCs) has a similar variation trend to ground-level ozone, and a good correlation is observed in HCMC. In contrast, this result was not seen in Hanoi despite relatively high levels of PE conc. and MIR.

Mesoscale variation of the chemical composition of submicron aerosol concentrations and their influence on cloud condensation nuclei (CCN) activation have been examined over a tropical coastal location in peninsular India during the winter season. The land-sea breeze circulation, boundary layer evolution, and photochemical processes significantly influence the diurnal variation of the aerosol chemical composition over the region. Broadly, the organic aerosols (mean ∼37.50 ± 27.00 μg m−3) dominated the submicron mass loading with a mass fraction (MF) of 0.66, followed by sulfate (∼10.92 ± 6.13 μg m−3; MF ∼0.19), ammonium (3.46 ± 1.71 μg m−3; MF∼0.06) and a small contribution from nitrate (1.7 ± 1.5 μg m−3; MF ∼0.03). The average mass concentration of organic carbon (OC) is 8.84 ± 6.42 μg m−3 with a mean organics/OC ratio of ∼3, indicating the presence of aged, highly oxidized organics. The diurnal variation of all the aerosol species, except sulfate, depicted daytime-low and nighttime-high. The photochemical formation of sulfate aerosols overwhelms the reduction in concentration due to boundary layer mixing and sea-land breeze circulation during the daytime. The decrease in the organics/sulfate ratio during the daytime has significant implications on the CCN concentration and its activation properties. The average CCN number concentration and activation ratio (ratio of CCN to total aerosol number concentration) at 0.38% supersaturation during the study period are ∼3756 ± 1609 cm−3 and 0.52 ± 0.17, respectively. The diurnal variation of CCN concentrations and activation ratio is distinctly different. The change in the mode diameter of the number size distribution between sea-land breeze was found to be less (∼8 nm) compared to the large variation in chemical composition. This suggested the importance of mesoscale variations of aerosols, especially the contrasting diurnal variation of organics and sulfate aerosols in CCN activation.

Humic-like substances (HULIS) in particulate matter (PM) play critical roles in the atmospheric changes in our environment. In this study, high time resolution PM2.5 samples were collected to insight the abundances, spectroscopic characters, chemical groups, and oxidative potential of HULIS in Xi'an, China. The average mass concentrations of HULIS in term of carbon (HULIS-C) was 11.55 ± 5.85 and 8.28 ± 2.23 μg C m−3 in winter and summer, respectively. The diurnal variations of HULIS displayed three peaks (03:00–07:00, 10:00–14:00, and 18:00–21:00 LT) in winter, but a single peak (08:00–12:00 LT) in summer. The optical parameters show obvious difference between winter and summer. Fourier Transform infrared (FT-IR) spectra revealed that the HULIS mainly consisted of aliphatic chains, aromatic rings, and carboxylic groups. The FT-IR spectra proved that the aromatic compounds and carboxylic acids dominated the diurnal variation in winter, while carboxylic acids were responsible for that in summer. The normalized oxidative potential of HULIS exhibited an inverse trend with the HULIS-C concentration measured by the DCFH assay. The diurnal reactive oxygen species (ROS) productions from HULIS presents three valleys (03:00–07:00, 10:00–14:00, and 18:00–21:00 LT) in winter but only a single valley (08:00–12:00 LT) in summer. In addition, a positive correlation (R2 ≈ 0.6, p＜0.01) between the oxidative potential and specific ultraviolet absorbance at a wavelength of 254 nm (SUVA254) in winter suggested that the oxidative power of HULIS could be more related to the compounds with high light absorbance, high aromaticity, and molecular weight. The results of this study offer more solid knowledge on the spectral and chemical characteristics and oxidative potential of the HULIS in PM2.5 of the typical city in northwest China.

Net ecosystem methane exchanges (NEME) from lowland rice often show significant variations in diurnal, seasonal and yearly scales and with crop phenology. Air and soil temperatures, standing water, and radiation are believed to be primary drivers of such variations. We studied the dynamics of net ecosystem methane exchange in three consecutive years (2014–16) using eddy covariance technology. Wet season (July–October) showed higher diurnal methane flux variations than that of dry season (December–March) due to prolonged anoxic condition and higher methanogens in the former. Significant diurnal variations in NEME were also noticed during different crop growth stages. Variations were higher in flowering stage followed by ripening in 2015 and 2016. But in 2014, it was highest at panicle initiation (PI) stage. Principal component bivariate plot analysis and performance analytic correlation were performed for variables such as net ecosystem carbon dioxide exchange (NEE), NEME, sensible heat (Hs), latent heat (LE), photosynthetically active radiation (PAR), net radiation (NR), air temperature and soil temperature (5 cm depth) in order to assess their relationships. The NEME was positively correlated to air and soil temperatures in all the years. However, in year 2016, NEME also showed positive correlation to PAR and NR. These relationships could provide valuable information to partitioning and process-based simulation modelling for projecting and up scaling of NEME to regional and global scales.

Ten years (2008–2017) of Black Carbon (BC) observations obtained using Aethalometer (AE-31) are analyzed to investigate the seasonal trends and temporal variabilities over a tropical site Gadanki (13.5° N, 79.2° E) located in south-east India. Diurnal variations of BC have two peak structures one in the morning (∼08 IST) in all seasons and second in the evening (∼20 IST) only during the pre-monsoon (March–May). Intra-annual variation in BC indicated February and March months as the bio-mass burning with highest BC mass concentration (3000–5000 ng/m3). About 46% of air parcel back trajectories found passing across the in-land regions of southern peninsular India brining transported aerosol to the source location during pre-monsoon. The lowest BC (∼1500 ng/m3) is noticed during the monsoon months (June–September). The average BC (2200 ng/m3) represents observational site as a typical rural site. The inter-annual variability of BC did not show any significant trend. However, trends in the maximum (March) and minimum (July) BC values show statistically significant decreasing trend suggesting reduction in bio-mass burning sources during March supported by the decrease in the fire counts. Diurnal variation in the absorption angstrom exponent indicates that the morning and evening peaks are contributed by the bio-mass combustion with values above threshold of 1. However, angstrom exponent values are found below 1 during noon time of monsoon season suggesting fossil fuel contribution. Strong coupling is found between aerosol concentration and tropospheric dynamics, meteorology in addition to the sources. The present study is expected to provide valuable input to the modelers and observational physicists as BC is climate sensitive variable.

Nitrogen dioxide (NO_2) and sulfur dioxide (SO_2) are important pollution gases which can affect air quality, human health and even climate change. Based on the combination of tropospheric NO_2 and SO_2 column density products derived from OMI satellite with the ground station observations, this study analyzed the spatial-temporal distribution of NO_2 and SO_2 amount in Inner Mongolia urban agglomerations. It shows that NO_2 increased continually from 2005 to 2011 at a rate of 14.3% per year and then decreased from 2011 to 2016 at a rate of −8.1% per year. SO_2 increased from 2005 to 2007 with a rate 9.7% per year. While with a peak value in 2011, SO_2 generally showed a decreasing trend of −1.6% per year from 2007 to 2016. With regard to the spatial pattern, the highest levels of NO_2 occur in Hohhot and Baotou, followed by Wuhai and Ordos, the least in Bayannur. Compared with NO_2, the spatial distribution of SO_2 is slightly different. The pollution of SO_2 is the most serious in Wuhai, followed by Hohhot and Baotou, and the lightest in Ordos and Bayannur. The diurnal variations of NO_2 and SO_2 are basically the same, which decrease from 0:00 to 6:00, then increase to a peak value at 8:00, and decrease from 8:00 to 15:00. The diurnal variation of NO_2 and SO_2 is highly related to the diurnal variation of both anthropogenic emission and boundary layer height. Differently, the long-term spatial-temporal distribution of NO_2 and SO_2 are more closely related to human activities.

Traffic emits a considerable amount of aerosol particles in urban areas and has also recently been shown to be a significant source of sub-3 nm particles. In this study, the concentration of sub-3 nm particles, here referred to as nanocluster aerosol (NCA), was continuously measured with 1 s resolution in a busy street canyon in the city of Helsinki, Finland, for a month period in 2017. NCA concentrations were carefully analyzed with respect to the time of day, the wind direction, the condensation sink, the concentration of sub-7 nm particles, the total particle size distribution, and the CO2 concentration, from which the emission factors for the NCA were calculated. The concentration of the NCA seemed to follow a similar trend to that of sub-7 nm particles. Diurnal variation of the NCA concentration divided into weekdays and weekends and sorted according to the wind direction followed the amount traffic. The NCA concentration was at highest when wind was blowing directly from the road, during the rush hours or when the condensation sink calculated from the particle size distributions was low. The NCA concentration was in line with the traffic-related nucleation mode of the size distribution and its diurnal variation, and the NCA fraction comprised a relatively large part of the total particle number concentration. Average emission factors for the NCA and sub-7 nm particles were 9.36 ⋅ 1014 kg fuel − 1 and 2.73 ⋅ 1015 kg fuel − 1 , respectively. Diurnal variation of the emission factors showed an increase in the night, which may result from a dependency of the emission factors on traffic composition, temperature, condensation sink or the driving mode of vehicles.

Particulate matter (PM), especially those with an aerodynamic particle size of less than 10 μm (PM10), is typically emitted from transboundary forest fires. A large-scale forest fire may contribute to a haze condition known as a high particulate event (HPE), which has affected Southeast Asia, particularly Peninsular Malaysia, for a long time. Such event can alter the photochemical reactions of secondary pollutants. This work investigates the influence of PM on ground-level ozone (O3) formation during HPE. Five continuous air quality monitoring stations from different site categories (i.e., industrial, urban and background) located across Peninsular Malaysia were selected in this study during the HPEs in 2013 and 2014. Result clearly indicated that O3 concentrations were significantly higher during HPE than during non-HPE in all the sites. The O3 diurnal variation in each site exhibited a similar pattern, whereas the magnitudes of variation during HPE and non-HPE differed. Light scattering and atmospheric attenuation were proven to be associated with HPE, which possibly affected O3 photochemical reactions during HPE. Critical conversion time was used as the main determining factor when comparing HPE and non-HPE conditions. A possible screening effect that resulted in the shifting of the critical transformation point caused a delay of approximately of 15–30 min. The shifting was possibly influenced by the attenuation of sunlight in the morning during HPE. A negative correlation between O3 and PM10 was observed during the HPE in Klang in 2013 and 2014, with −0.87. Essentially, HPE with a high PM concentration altered ground-level O3 formation.

Size-resolved effective densities of ambient aerosols in Riverside, CA were determined over 4 periods during 2015–2016. A DMA-CPMA-CPC technique was used to measure effective density for particles with selected diameters of 50, 70, 101 and 152 nm. A catalytic stripper (CS) was used alternately to remove the volatile fraction of aerosol before density measurements. Aerosol non-refractory composition measurement was conducted in June 2016 campaign to understand the effect of chemical composition on particle density. The average densities for particles over all the measurement campaigns over BP mode (i.e. bypassing the CS) were 1.17 g/cm3 at 50 nm and 1.25–1.28 g/cm3 at 70, 101 and 152 nm. The average density after CS conditioning (CS mode) showed a decreasing trend from 1.22 g/cm3 to 1.04 g/cm3, with increase in the selected size, and a mass fractal dimension (Df) of 2.85. Both the BP and CS mode particles showed the lowest effective density at 6–9 am and highest density at 11 a.m.-3 pm. The diurnal variation of density for both modes became more intensive as particle size increased. The variation was also more intense for the CS mode compared to the BP mode. Organic aerosol and ammonium nitrate mass in the size range of density measurements correlated well positively (R2 = 0.78) and negatively (R2 = 0.62), respectively with BP mode effective density. The study provides an update to the aerosol density profiles of a well-known receptor site (Riverside, CA) and investigates the transformation of particles in different seasons. The effective density profiles will be used in a follow-up study to better estimate the respiratory-deposited ambient aerosol mass.

Upon the effect of atmospheric stability on air pollution, this study highlights the weakened diurnal evolution of atmospheric boundary layer (ABL), which is crucial to the formation of regional fog-haze and air pollution episodes over eastern China (ECN). The decrease of atmospheric visibility (Vis) during 1973–2012 is found to be related to the more stable ABL with weakened diurnal evolution that is characterized by a concurrent weakening of surface wind, a decrease in ABL height, and a reduction of diurnal temperature range (DTR, – 0.26 and – 0.092 °C/10-years in the winter and summer, respectively). With a general synchronous variation, the increase in both aerosol loading and cloud cover might work in concert to reduce solar radiation reaching the surface, stabilize ABL and weaken the diurnal ABL evolution, thereby weakening the turbulent mixing of pollutants and enhancing air pollution. This conjecture of “aerosol, cloud – ABL – air pollution” feedback, with particular emphasis on the effect of cloud (in addition to the effect of aerosol) on the ABL evolution, is supported by our analysis of 40 years trends in the cloud, Vis and DTR from surface observation, the aerosol optical depth from MODIS, and the validated MERRA-2 reanalysis aerosol, cloud and boundary layer height (by surface observation and routine radiosonde sounding). Regarding the linkage between fog-haze and diurnal evolution of the ABL, the result emphasizes the effect of a persistent high relative humidity (RH), which is found to be important to the formation of regional persistent fog-haze episodes and must be carefully considered in future study. Therefore, a chain of processes is suggested to interpret the occurrence of regional persistent fog-haze episodes over ECN. First, during a polluted day, because of aerosol radiative effects, the high ABL stability and high RH can persist throughout the day, and consequently, favoring the accumulation of pollutants as well as the secondary formation and hygroscopic growth of aerosol. Second, the maintenance of such more stable, humid and polluted ABL is in contrast to a clear day when the ABL is more convective in the afternoon and facilitates the diffusion and dilution of pollutants. Third, under a persistent stable synoptic system, the increase of air pollution associated with the weakened diurnal evolution of ABL due to the “aerosol, cloud – ABL – air pollution” feedback, as described above, can be further strengthened, thereby renders more persistent and severe air pollution events. The DTR variation in relationship with the aerosol, cloud, ABL and air pollution is also discussed.

Precipitation susceptibility of monsoon clouds to changes in aerosols has been studied initially by utilizing ground based observations from a High Altitude Cloud Physics Laboratory (HACPL), Mahabaleshwar, India collected during monsoon seasons and later compared with the results obtained from satellite observations. Cloud condensation nuclei (CCN) concentration, rain intensity and integrated liquid have been taken from ground based observations for calculating precipitation susceptibility. As a first step, we have analyzed the temporal variation of CCN concentration, rain intensity and integrated liquid water during monsoon season. It is observed that all three parameters exhibit strong diurnal variation with two peaks i.e. in morning and afternoon. CCN showed strong morning peak whereas other two showed during afternoon hours. Later precipitation susceptibility has been estimated using CCN as aerosol proxy and it is higher ∼0.65 for the integrated liquid water ranging from 0.6 to 0.8 mm. Secondly, we have compared ground based observation with satellite observations carried out using aerosol optical depth (AOD), rain rate and liquid water path. Satellite observations showed that precipitation susceptibility, S0, using AOD as aerosol proxy is higher (S0 = 0.98) for liquid water path ranging from 800 to 1000 gm-2. It is also seen that the results are in agreement with the earlier reported results especially with the report from tropical region. Present analysis also showed that the precipitation is getting suppressed at medium range of integrated liquid/liquid water path. This is the first such study from Indian region explicitly using ground-based observations.

Dust aerosols are the main aerosol components of the atmosphere that affect climate change, but the contribution of dust devils to the atmospheric dust aerosol budget is uncertain. In this study, a new parameterization scheme for dust devils was established and coupled with WRF-Chem, and the diurnal and monthly variations and the contribution of dust devils to the atmospheric dust aerosol budget in East Asia was simulated. The results show that 1) both the diurnal and monthly variations in dust devil emissions in East Asia had unimodal distributions, with peaks in the afternoon and the summer that were similar to the observations; 2) the simulated dust devils occurred frequently in deserts, including the Gobi. The distributed area and the intensity center of the dust devil moved from east to west during the day; 3) the ratio between the availability of convective buoyancy relative to the frictional dissipation was the main factor that limited the presence of dust devils. The position of the dust devil formation, the surface temperature, and the boundary layer height determined the dust devil intensity; 4) the contribution of dust devils to atmospheric dust aerosols determined in East Asia was 30.4 ± 13%, thereby suggesting that dust devils contribute significantly to the total amount of atmospheric dust aerosols. Although the new parameterization scheme for dust devils was rough, it was helpful for understanding the distribution of dust devils and their contribution to the dust aerosol budget.

This study presents the characterization of XCH4 (averaged-column dry air mole fraction) variability and trends using the near-infrared (NIR) solar spectra recorded by the ground-based high-resolution Fourier transform spectrometer (FTS) over Hefei, China. An annual increase of 10.55 ± 2.87 ppb yr−1 (0.56 ± 0.15% yr−1) in CH4 concentrations was deduced from the three years’ worth of FTS measurements. Seasonal cycles and three typical diurnal variation modes were captured. The maximum CH4 concentration occurred in late summer or early autumn, while the minimum CH4 concentration occurred in winter. CH4 concentrations in August were 30–55 ppb higher than those in January. In most cases, the diurnal variability exhibited an increasing trend in summer or early autumn, while it exhibited a decreasing trend or remained relatively constant in other seasons. We compared the FTS data with the GOSAT data and the GEOS-Chem model data. The average difference between the GOSAT and FTS data (GOSAT minus FTS) was 1.60 ppb (0.09%) ±13.0 ppb (0.70%), and between the smoothed GEOS-Chem model data and FTS data (GEOS-Chem minus FTS) was −8.09 ppb (0.45%) ±17.83 ppb (0.95%). The differences between GOSAT and FTS were season-independent; however, obvious season-dependent differences were observed between GEOS-Chem and FTS. The GOSAT and GEOS-Chem models could reproduce the seasonal cycle observed by FTS with correlation coefficients (r) of 0.77 and 0.86, respectively.

To understand greenhouse gas (GHG) flux in reclaimed water intake area impact on urban climate, ‘static chamber’ method was used to investigate the spatio-diurnal variations and the influence factors of GHG fluxes at water-air interface from Jian River to Chaobai River. Results showed that the average fluxes of CO2 from the Jian River and the Chaobai River were 73.46 mg(m2·h)−1 and -64.75 mg(m2·h)−1, respectively. CO2 was emitted the most in the Jian River, but it was absorbed from the atmosphere in the Chaobai River. Unary linear regression analyses demonstrated that Chlorophyll a (Chl a) and pH variation controlled the carbon source and sink from the Jian River to the Chaobai River. The diurnal variation of CO2 fluxes was higher at night than in the daytime in the Jian River, and it was the inverse in the Chaobai River, which highly correlated with dissociative CO2 and HCO3− transformation to CO32-. The average fluxes of CH4 from the Jian River and Chaobai River were 0.973 mg(m2·h)−1 and 5.556 mg(m2·h)−1, respectively, which increased along the water flow direction. Unary and multiple linear regression analyses demonstrated that Chl a and total organic carbon (TOC) controlled the increase of CH4 along the flow direction. The diurnal variation of CH4 fluxes was slightly higher in the daytime than at night due to the effect of water temperature.

Volatile organic compounds (VOCs) are critical precursors of secondary pollutants such as O3 and PM2.5; some VOCs are also harmful to human health directly. In recent years, O3 pollution in summer and PM2.5 pollution in winter have occurred frequently in Chengdu, a megacity in southwest China. Effective pollution control on O3 and PM2.5 require clarification of VOCs pollution characteristics. In this study, 90 VOC species were observed using the online VOC monitoring system in June 2018 and in January 2019 at an urban monitoring station in Chengdu. The positive matrix factorization (PMF) model and the potential source contribution function (PSCF) model were used to analyze the main sources and potential source regions of VOCs in summer and winter. The O3 formation potentials (OFPs) of VOCs from different species, sources, and its contribution to secondary organic aerosols (SOA) were analyzed. Additionally, the health risks of human exposure to VOCs were also assessed. The results showed that the mixing ratio of total VOCs (TVOCs) in winter (53.3 ppbv) was around twice as high as in summer (26.8 ppbv), and TVOCs were mainly composed of alkanes. From the perspective of diurnal variation, the high values of VOCs in the morning and evening were mainly affected by traffic, and the low mixing ratios of VOCs in the afternoon was mainly due to photochemical reactions. The main sources of VOCs in downtown Chengdu was LPG/NG usage, which accounted for more than 30% in both summer and winter. The main potential source regions of TVOCs were from the western and northern regions with industrial contribution. The regional transmission characteristics between the two seasons were different, pollutants diffused in short distance and moved slowly in summer, while they migrated far and spread widely in winter. The most reactive species were alkenes and aromatics which contributed most to O3 formation potentials. VOCs from solvent utilization, vehicle exhaust, and LPG/NG usage were the main precursors of O3. VOC concentration in the atmosphere is unlikely to cause evident non-carcinogenic risk to human health, but still poses potential carcinogenic risk.

Hebei is one of the most air-polluted provinces located in China. In this study, NOx, volatile organic compounds (VOCs), and ozone (O3) observational data obtained at 7 monitoring sites in Hebei Province in the warm season (May to September) in 2019 were analyzed to investigate the spatiotemporal distribution, composition characteristics, and chemical reactivity of VOCs. Results indicated that VOCs concentrations were the highest in Shijiazhuang and Langfang (47.8 and 39.2 ppb, respectively) and the lowest in Zhangjiakou and Chengde (16.6 and 12.0 ppb, respectively). In the warm season in 2019, the monthly average concentrations of VOCs increased initially and decreased subsequently, and the highest concentration found in July and August. The diurnal variation patterns of VOCs exhibited a double-peak curve, with the first peak occurring between 07:00–09:00, influenced by the production and life activities of residents. The second peak occurred between 23:00–0:00 due to the poor atmospheric diffusion conditions at nighttime and the possible transportation from residual layer. Alkanes, haloalkanes, and alkenes were responsible for the largest components to VOCs in Hebei Province, with the average proportions at the 7 monitoring sites of 43.25%, 30.39% and 15.00%, respectively. The ratios of toluene to benzene (T/B) of the monitored sites indicated that the VOCs concentrations at the Shijiazhuang and Xingtai sites were mainly influenced by emissions from coal combustion. In comparison, the VOCs concentrations at the Baoding and Langfang sites were mainly influenced by motor vehicle emissions, while the sites at Zhangjiakou and Chengde were mainly influenced by solvent-based coating and industrial process emissions. Alkenes had the highest chemical reactivity of the VOCs in Hebei Province, ethylene, propene, toluene, and 1-butene were also the high chemical reactivity species. Lastly, tailored schemes for the reduction of O3 precursor were proposed based on the NOx, VOCs, and O3 data obtained from the various monitoring sites.

We investigated the temporal and spatial variations of aerosol optical properties over the Korean peninsula during the KORUS-AQ (KORea–United States Air Quality) experiment with ground-based aerosol optical properties measured by remote and in-situ techniques. On the ground, AErosol RObotic NETwork (AERONET) and ground-level particulate matter (PM) concentration from air quality monitoring stations were used. From the NASA DC-8 research aircraft, the airborne Spectrometer for Sky-Scanning, Sun-Tracking Atmospheric Research instrument (4STAR), an aerosol mass spectrometer (AMS), and a single particle soot photometer (SP2) provided aerosol information. Average hourly fine- and coarse-mode aerosol optical depth (AOD) and fine mode fraction (FMF), were divided into four clusters (background, Seoul Metropolitan Area (SMA), southwest, and east) representing different temporal/spatial variations; the results of those clusters were similar to the clustering results using PM from air quality monitoring stations. The downwind region of SMA was dominant by light-scattering fine-mode aerosols likely due to secondary aerosol formation with high fine-mode AOD and single scattering albedo compared to other regions, even Seoul. The fine-mode aerosols were more spatially homogeneous than coarse-mode aerosols, especially in the west-to-east direction, because fine-mode aerosols are usually transported on a regional scale by westerlies rather than emitted from local emission sources. However, the aerosol size distribution was spatially more homogeneous because of a consistent contribution of fine- and coarse-mode AOD to total AOD regardless of direction between the AERONET sites. During high-aerosol loading episodes, the temporal and spatial variations of aerosol optical properties were similar to those derived from 4STAR and ground-level PM concentrations, providing detailed information on aerosol behavior and characteristics. Using missed-approach flight segments (touchdown and take-off without a full-stop), investigation of diurnal variations over the SMA revealed a significant increase in AOD and Angstrom exponent (AE) in the afternoon compared to morning and noon, especially in the downwind region, because of more active secondary formation resulting from advected and/or emitted local pollutants and precursors. The diurnal variation of PM1 in the downwind region was similar to that of AOD; it was mainly increased during the day by secondary organic aerosols and ammonium nitrate due to large amounts of isoprene and meteorological conditions that supported secondary aerosol formation. Ammonium sulfate also partially led to increasing the PM1, but its behavior was unclear, because RH decreased during the day, which would imply the reduced aqueous-phase oxidation (the major pathway for sulfate production). Compared to the downwind region, ammonium nitrate and refractory black carbon decreased in Seoul because of a slightly decreased traffic volume, dissociation of ammonium nitrate, and increased boundary layer height according to meteorological conditions in the afternoon. The results of this study provide detailed information on aerosol behavior given the synergy of the various measurement platforms used. Along with the evidence of active photochemical reactions taking place in the downwind region, such data will be useful in formulating policies that improve air quality in Korea.

Measurements of BVOC emissions, ozone concentration and environmental parameters were carried out from May 2013 to January 2016 in a subtropical Pinus plantation in China. Isoprene and monoterpene emissions were measured using a relaxed eddy accumulation (REA) system and a gradient technique on an above-canopy tower. In 2013, isoprene comprised 21.2% of total terpenoid emissions, while α-pinene, camphene, β-pinene and limonene constituting 51.5%, 2.4%, 9.1%, and 13.0% of total emissions, respectively. Monoterpenes together were the dominant VOCs measured contributing 71.6%. α-pinene, camphene, β-pinene and limonene constituted 67.7%, 3.2%, 11.9%, 17.2% of total monoterpene emissions. Isoprene and monoterpene emissions displayed strong diurnal variations, with lower emissions in the morning and late evening, and the highest emissions around noon. BVOC peak emissions typically occurred a few hours after the noon PAR peak. Isoprene and monoterpene emissions varied with season and were the highest in summer, contributing more than half of the total annual emission, and the lowest emissions were in winter. Evident interannual variations of isoprene, monoterpenes and total BVOCs were observed. Compared to 2013, annual BVOC emissions decreased in 2015, associated with decreases of PAR, Temperature, water vapor, and an increase of all substances in gas, liquid and solid phases in the atmospheric column (e.g., S/Q, the ratio of solar scattered radiation to global radiation). Ozone concentration showed clear diurnal variation with PAR, higher around noon and lower in the early morning and late evening. Generally, there were no evident correlations between ozone concentrations and BVOC emissions, or the vertical gradients of ozone concentrations and BVOC concentrations. Under all sky conditions (including cloudy skies), no strong correlations at a high confidence level or very similar variation patterns were observed between any two following parameters, BVOC emissions, PAR, temperature, water vapor, and S/Q. The major factors controlling BVOC emissions were PAR and temperature but biomass burning smoke and phenology (pine florescence) may also play a role. The mean emission factors at standard conditions determined using the MEGAN model emission algorithms and empirical model of BVOC emissions were 0.71 and 1.19 mg m −2 h −1 for isoprene and 1.39 and 1.65 mg m −2 h −1 for total monoterpenes, respectively.

Continuous ground-based measurements of CO, SO 2 and NO 2 were carried out in a semi-arid rural area, Anantapur [14.62 0 N, 77.65 0 E], Southern India, for the period January 2012–December 2012. The maximum CO concentration was observed in winter (310 ± 17 ppbv) followed by summer (180 ± 21 ppbv) and post monsoon (174 ± 20 ppbv), while the minimum in monsoon (72 ± 9 ppbv). Seasonal mean NO 2 /NOx ratios for monsoon, post monsoon, winter and summer were about 0.88, 0.91, 0.76 and 0.80 respectively, indicating a higher conversion of NO to NO 2 over the measurement site. Monthly mean low SO 2 mixing ratio was found (0.46 ± 0.02 ppbv) in monsoon and high (2.42 ± 0.21 ppbv) in winter. Keeping the emissions aside, the levels of CO, SO 2 and NO 2 were influenced by meteorology, urban effects and trans-boundary transport in the lower troposphere. Atmospheric boundary layer (ABL) had the good correlation coefficient (R = 0.76) with solar radiation during daytime, while it was mainly correlated with wind speed during night time (R = 0.42). Diurnal trend of atmospheric visibility was found to be maximum during noon times at around (14:00–16:00 h) about 76 k.m. and minimum during morning periods (06:00–08:00 h) about 45 k.m. A strong positive correlation was observed between BC and CO (R = 0.71) with an average slope, suggesting common or proximate sources likely to be traffic emissions contribution for the production of BC and CO. The SO 2 /NOx and CO/NOx study were strongly suggested that mobile sources were larger contributors over the site and the evidence of transport of emissions from other surrounding regions. Tropospheric Ultraviolet Visible (TUV) radiative transfer model was used to calculate the Photolysis rate coefficients (J(O 3 ), J(NO 2 )). Chemical box model (NCAR-MM) was used to simulate diurnal variation of CO and the results were reported.

Particle number concentration and size distribution are important for better understanding the characteristics of aerosols. However, their measurements are scarce in western China. Based on the first measurement of particle number size distribution (10–487 nm) in the suburb of Xi'an, northwest China from November 2013 to December 2014, the seasonal, monthly and diurnal average particle number concentrations were investigated, and the characteristics of new particle formation (NPF) events and their dependencies on meteorological parameters also discussed. The results showed that the annual average particle number concentrations in the nucleation (NNUC), Aitken (NAIT), and Accumulation (NACC) size ranges were 960 cm−3, 4457 cm−3, 3548 cm−3, respectively. The mean total particle number concentration (NTOT) was 8965 cm−3 and largely dominated by particles in Aitken mode. The number concentration was dominated by particles around 67.3 nm in spring, summer and fall, while about 89.8 nm in winter. The percentage of the ultrafine size range (UFP, particles of diameter below 100 nm) to total particle number concentration was 63.2%, 69.6%, 62.2% and 58.1% in four seasons. The diurnal variation of the nucleation mode particles was mainly influenced by NPF events in summer, while by both traffic densities and NPF events in spring, fall and winter. The diurnal variation of the number concentration of Aitken mode particles correlated with the traffic emission in spring, fall and winter, while in summer it more correlated with contribution of the growth of the nucleation mode particles. The burst of nucleation mode particles typically started in the daytime (08:15–16:05, LST). The growth rates of nucleated particles ranged from 2.8 to 10.7 nm h-1 with an average of 5.0 ± 1.9 nm h-1. Among observed 66 NPF events from 347 effective measurement days, 85 percent of their air masses came from north or northwest China, resulting in a low concentration of pre-existing particles, and only 15 percent came southerly from Qingling Mountains. Based on their growth rate, 64 and 36 percent of their subsequent particles, corresponding to types 1 and 2 NPF events, grew and seldom grew after the burst of nucleation mode particles. For type 1 NPF event, the nucleated particles could grow up to 40 nm or larger when surface winds shifted from westerly to easterly or southeasterly (from village areas). For type 2 NPF events, the particles kept almost unchanged when the winds stayed westerly. This implied that the surface wind direction with different emissions might play an important role in new particle growth in suburb of Xi'an.

The recently released Moderate Resolution Imaging Spectrometer (MODIS) Collection 6 introduced a fine scale aerosol optical depth (AOD) distribution, the 3 km product, which is expected to perform well in analyzing aerosols and identifying local air pollution, especially in the severely polluted atmosphere of China. However, few detailed evaluations of regional variations have been conducted. In this paper, we evaluate MODIS 3 km and 10 km AOD products for China against ground-based measurements and compare their performance with respect to spatial and temporal variations. The ground validations indicate that the two products are generally correlated well to ground-based observations. Spatially, the 3 km product slightly outperform the 10 km product in well-developed areas of southern China. Temporally, both products perform worse during spring and summer. Atmospheric clouds and underlying surface are two key factors that influence the accuracy and number of retrievals for both products. The comparison analysis reveals the newly introduced AOD product clearly shows good relationships with the coarse resolution retrievals in spatial and temporal variation but significant differences regarding details. The 3 km AOD product provides better aerosol gradients, more retrievals in bare areas of western China and some spikes of diurnal variation in cloudy days. Seasonal comparisons show the 3 km AOD product is higher than the 10 km product in all seasons, especially during spring and summer. Although the 3 km product for China generally performs slightly worse than the 10 km product, the added information of the MODIS 3 km AOD product shows potential for studying local aerosol characterization, and may facilitate studies of air pollution.

The mid-high latitude permafrost peatlands in the Northern Hemisphere is a major natural source of methane (CH4) to the atmosphere. Ecosystem scale CH4 emissions from a typical permafrost peatland in the Great Hing'an Mountains were observed during the growing season of 2014 and 2015 using the open-path eddy covariance method. Relevant environmental factors such as temperature and precipitation were also collected. There was a clear diurnal variation in methane emissions in the second half of each growing season, with significantly higher emission rates in the wet sector of study area. The daily CH4 exchange ranged from 1.8 mg CH4 m−2 d−1 to 40.2 mg CH4 m−2 d−1 in 2014 and ranged from −3.9 to 15.0 mg CH4 m−2 d−1 in 2015. There were no peaks of CH4 fluxes during the spring thawing period. However, large peaks of CH4 emission were found in the second half of both growing seasons. The CH4 emission after Jul 25th accounted for 77.9% of total growing season emission in 2014 and 85.9% in 2015. The total CH4 emission during the growing season of 2014 and 2015 was approximately 1.52 g CH4 m−2 and 0.71 g CH4 m−2, respectively. CH4 fluxes during the growing seasons were significantly correlated with thawing depth (R2 = 0.71, P

The Regional Atmospheric Environmental Modeling System for eastern China (RAEMS) is an operational numerical system to forecast near surface atmospheric pollutants such as PM2.5 and O3 over the eastern China region. This system was based on the fully online coupled weather research and forecasting/chemistry (WRF-Chem) model. Anthropogenic emissions were based on the multi-resolution emission inventory for China (MEIC), and biogenic emissions were online calculated using model of emissions of gases and aerosols from nature (MEGAN2). Authorized by the China Meteorological Administration (CMA), this system started to provide operational forecast in 2013. With a large domain covering eastern China, the system produces daily 72-hr forecast. In this work, a comprehensive evaluation was carried out against measurements for two full years (2014–2015). Evaluation results show that the RAEMS is skillful in forecasting temporal variation and spatial distribution of major air pollutants over the eastern China region. The performance is consistent in different forecast length of 24 h, 48 h, and 72 h. About half of cities have correlation coefficients greater than 0.6 for PM2.5 and 0.7 for daily maximum 8-h averaged (DM8H) ozone. The forecasted PM2.5 is generally in good agreement with observed concentrations, with most cities having normalized mean biases (NMB) within ±25%. Forecasted ozone diurnal variation is very similar to that of observed, and makes small peak time error for DM8H ozone. It also shows good capability in capturing ozone pollution as indicated by high critical success indexes (CSI). The modeling system also exhibits acceptable performance for PM10, NO2, SO2, and CO. Meanwhile, degraded performance for PM2.5 is found under heavy polluted conditions, and there is a general over estimation in ozone concentrations.

Aerosol physical properties, chemical compositions, hygroscopicity and cloud condensation nuclei (CCN) activities were measured in Seoul, the highly populated capital city of Korea, during the Megacity Air Pollution Studies (MAPS-Seoul) campaign, in May–June 2015. The average aerosol concentration for particle diameters >10 nm was 11787 ± 7421 cm−3 with dominant peaks at morning rush hours and in the afternoon due to frequent new particle formation (NPF) events. The average CCN concentration was 4075 ± 1812 cm−3 at 0.6% supersaturation, with little diurnal variation. The average hygroscopicity parameter ( κ ) value determined using a humidified tandem differential mobility analyzer (HTDMA) ranged 0.17–0.27 for a range of particle diameters (30–150 nm). The κ values derived using the aerosol mass spectrometer (AMS) data with three different methods were 0.32–0.34, significantly higher than those from HTDMA due to the uncertainties in the hygroscopicity values of different chemical compositions, especially organics and black carbon. Factors affecting the aerosol hygroscopicity seemed to be traffic and chemical processes during the NPF events. The CCN concentration predicted based on HTDMA κ data showed very good agreement with the measured one. Because of the overestimation of κ , CCN closure with the predicted CCN concentration based on AMS κ data over-predicted CCN concentration although the linear correlation between measured and predicted CCN concentration was still very good.

Five-year continuous O3 measurements were conducted for the first time at the summit of Nanling Mountains (112.88° E, 24.68° N, 1690 m a.s.l), southern China in 2013–2017. Unlike most mountains over the world with O3 maximum in spring or summer, Nanling showed O3 maximum in mid-autumn (October; 62.8 ± 0.7 ppbv, ± 95% confidence intervals) and minimum in summer (July; 46.6 ± 0.7 ppbv). This distinct seasonal cycle of O3 at Nanling was mainly caused by the special impact of Asian monsoon in southern China, which brought clean maritime background air and fresh emissions of the Pearl River Delta (PRD) region in spring/summer, and polluted northern continental air masses in autumn/winter, respectively. Further comparisons found that O3 of Nanling in four seasons were all higher than other similar mountainous background sites, making the annual mean of O3 (55.0 ± 0.2 ppbv) at Nanling the second highest that ever reported in the mountainous sites worldwide. Moreover, the diurnal variation showed the higher impact of regional transport during nighttime from the free troposphere (FT) and presented the potential air exchange between the FT and local atmosphere in the plenary boundary layer on the summit of Nanling. Both the seasonal variation and high O3 level indicated a strong impact of anthropogenic emissions from alternatively PRD/northern China on O3 pollution in this mountainous background region. This study diversifies the seasonal variations among mountains over the world, and also illustrates the complicated impact of anthropogenic emissions on the mountainous atmosphere in southern China.

As a secondary pollutant with strong oxidation, surface O3 is harmful to human health and ecosystem. Accurate identification in the response of ozone formation to its precursors (O3-NOx-NMHCsensitivity) is significant for policy-making of O3 pollution control. Great progress has been made in O3-NOx-NMHC sensitivity analysis based on model-based methods in previous studies. However, large uncertainties from meteorological factors, emission inventory and photochemical mechanism may lead to the deviation between the simulation results and the observations. Properties of nonlinear coupling detrended fluctuation analysis (CDFA) is the ability to reveal the coupling correlations between O3 and its precursors at different time scales, helping us to quantity the contribution degree of NOx and NMHC to O3 formation in our previous work. Here, we further set up a new index to determine directly the O3-NOx-NMHC sensitivity based on CDFA method, and its availability is test by the comparative analysis of classic Empirical Kinetic Modeling Approach (EKMA). Based on this new evaluation index, seasonal change, time dependence and diurnal variation of O3-NOx-NMHC sensitivity for two traffic sites and two general sites located in the north and south of Taiwan respectively are analyzed. The results confirmed that the new index is able to identify the regional difference in temporal variation of O3-NOx-NMHC sensitivity. Thus, the features of quantification, time-saving and easy-operation make it possible for multifractal methods to become an important potential supplement to the methodology of O3-NOx-NMHC sensitivity analysis in the future.

To investigate the effects of biomass burning and photochemical oxidation on the black carbon (BC) mixing state and light absorption, a tandem centrifugal particle mass analyzer (CPMA) and single-particle soot photometer (SP2) system was used to measure ambient fine particulate BC during EXPLORE-YRD 2018 campaign ( EXP eriment on the e L ucidation of the atmospheric O xidation capacity and aerosol fo R mation, and their E ffects in Yangtze River Delta). The mass ratio (MR) of the non-BC coating to BC core, a morphology-independent parameter to quantify the mixing state of BC-containing aerosols, is derived. Two types of periods were identified, namely non-significant biomass-burning period (NB), and enhanced biomass burning period (EB). The MR was higher during EB (2.3 ± 0.5), compared with during the NB (2.0 ± 0.5). MR showed bimodal diurnal variation with peaks in the early morning and early afternoon, mainly due to the biomass burning and photochemical processes, respectively. The light-absorbing enhancement ranged from 1.0 to 1.19 when MR was from 1.4 to 3.4. The relationship of rBC mixing state and its optical properties was parameterized, which can further be used in the regional model.

A Droplet Measurement Technology (DMT) Cloud Condensation Nuclei Counter (CCNC) was deployed to measure cloud condensation nuclei (CCN) for the first time in the pristine Himalayan region at Himalayan Clouds Observatory (HCO), Swami Ram Tirtha (SRT) Campus (30°34′ N, 78°41′ E, 1706 m AMSL), Hemvati Nandan Bahuguna (HNB) Garhwal University, Badshahithaul, Tehri Garhwal, Uttarakhand, India. The CCN concentration (NCCN) was observed at four supersaturation levels (SS: 0.2, 0.5, 0.8, and 1.0%). In this study, we reported CCN concentration at 0.5% SS in different weather conditions from Aug 01, 2018 to Jun 30, 2019. During this observation period, the monthly averaged value of CCN concentration ranged between 1098.3 ± 448.9 cm−3 (mean ± SD) and 3842.9 ± 2512.9 cm−3. It covers a significantly wide range of daily averaged concentrations from the minimum concentration of 43.84 cm−3 (during heavy wet scavenging due to snowfall) to maximum concentration of 17000 cm−3 (during the events of a forest fire) at the observation site. The highest CCN concentration is observed at the time of sunrise (~07:00 a.m.) and after the sunset (~07:00 p.m.) for the diurnal variation of monsoon, post-monsoon, and winter season. Pre-monsoon season shows peak values at 10:00 a.m. and at 07:00 p.m. with higher concentrations at night hours. The possible reasons for maximum concentration in morning and evening time could be upliftment and settlement of CCN because of the convection process, anthropogenic emission, vehicular emission, and biomass burning in the residential area and valley region adjacent to HCO, Badshahithaul. The highest CCN concentration (3842.9 ± 2513 cm−3) values of the whole observation period were observed in May 2019. It was significantly affected by the heavy fire activities over the Uttarakhand and nearby IGP regions. Diurnal variation of CCN concentration during the HFAD shows higher values in the night time differing from the diurnal pattern of CCN for other months of the observation period. The long-range transport of air mass could also contribute to the high CCN concentration values, as found through the five-day air mass backward trajectory analysis. The lowest value of CCN concentration corresponds to the heavy rains and snowfall days, possibly caused by extensive wet scavenging. Cluster analysis of the air mass trajectories used for the allocation and classification of the possible sources of pollutants reaching the observation site. The highest fraction of CCN concentration (more than 2000 cm−3) corresponds to the air mass from the arid and semi-arid regions of Asian countries. Large air mass fraction (~40–60%) with moderate CCN concentration was received from northwestern IGP region and foothills of central Himalaya.

Despite its important roles in the formation of hydroxyl radical (OH) and ozone (O3), the characteristics and sources of nitrous acid (HONO) in the marine atmosphere are still poorly understood. In this study, the HONO production in maritime air masses was explored based on a field observation conducted at a coastal site in Qingdao, China in the summer of 2019. The “sea case” and the “land case” were carefully distinguished according to wind direction and backward air mass trajectory. About four times larger nocturnal NO2-to-HONO conversion rate and ~60% larger daytime Pother (production rate of HONO other than gas-phase OH + NO reaction) at noon were observed in the “sea case” compared to the “land case” (0.045 ± 0.014 h−1 versus 0.012 ± 0.007 h−1, and 1.83 ± 0.02 ppbv h−1 versus 1.14 ± 0.07 ppbv h−1, respectively). Correlation analysis implied that heterogeneous conversion of NO2 and photolysis of nitrogen-containing compounds were potentially important sources of marine atmospheric HONO in the nocturnal and daytime, respectively, though alkaline oceans are previously considered as sinks of HONO. The impacts of these marine-derived HONO on OH and O3 were comparable to or larger than that of the “land case”. These results suggest that strong marine-derived HONO production may have been overlooked previously, and more studies are required to explore its detailed formation mechanisms in the marine atmosphere.

The diurnal variation of pollutants such as particles and carbon monoxide (CO) in urban environments typically follow the traffic density, with two peaks coinciding with the weekday morning and evening rush-hour periods. However, observations made in central Brisbane, the third largest city in Australia, demonstrated an anomalous diurnal variation with the CO peak in the evening being significantly higher than that in the morning. This imbalance was not observed for particle concentrations. Here, we show that the imbalance is a direct result of the difference in CO emission factors from motor vehicles during warm and cold starts. Over 30,000 cars with warmed-up engines enter Brisbane city centre car parks every weekday morning. They all start their engines from cold and leave the city in the evening, producing the anomalously higher emissions of CO in the city centre. This pattern of air quality, while clearly apparent within the city car parks and within the central business district of the city, was not observed outside the city limits and in the suburbs. To the best of our knowledge, no previous study has drawn attention to this phenomenon and no explanation has been provided in the literature to-date.

Nitrous acid (HONO) is the primary source of hydroxyl radical (OH) during daytime and significantly contribute to the formation of secondary organic compounds (SOA) and atmospheric oxidizing capacity. One month simultaneous observations at ship channel site (SCS) and background site (BGS) in Shanghai, China were conducted to aim at estimating the HONO concentration from ship direct emission and analyzing the nocturnal budget of HONO during non-clean period in SCS. The HONO average concentrations at both BGS and SCS during non-clean period were 0.37 ± 0.42 ppbv and 0.91 ± 0.78 ppbv, respectively. The diurnal variation of HONO from ship direct emission presented a double-peak trend with a maximum value 0.38 ± 0.18 ppbv at 07:00 in the morning. The proportion of HONO emission from ships was in a range of 43%~74% in the daytime, while, the proportions were 42%~63% and 14%~28% at night in clean and non-clean periods, respectively. HONO nocturnal conversion frequencies from NO2 were 0.29%/h at both sites in non-clean period, during which the deposition velocities of HONO were 0.21–1.4 cm/s and 0.34–1.4 cm/s at BGS and SCS, respectively.

Due to a reliance on solar radiation, the aerosol optical depth (AOD) is observed only during the day by passive satellite-based instruments such as the MODerate resolution Imaging Spectroradiometer (MODIS). Research on urban air quality, atmospheric turbidity, and evolution of aerosols in the atmospheric boundary layer, however, requires 24-h measurement of aerosols. A lidar system is capable of detecting the vertical distribution of the aerosol extinction coefficient and calculating the AOD throughout the day, but routinely lidar observation is still quite limited and the results from MODIS and lidar sometimes are contradictory in China. In this study, long-term lidar observations from 2005 to 2009 over Hong Kong were analyzed with a focus on identification of the reasons for different seasonal variation in the AOD data obtained from MODIS and lidar. The lidar-retrieved AOD shows the lowest average level, but has the most significant diurnal variation during the summer. When considering only a 5-h period between 10:00 a.m. and 3:00 p.m. local time to match satellite passages, the average of the lidar-retrieved AOD doubles during the summer and exceeds that during the winter. This finding is consistent with the MODIS observation of a higher AOD during the summer and a lower AOD during the winter. The increase in the aerosol extinction coefficient in the upper level of the mixing layer makes the greatest contribution to the increase in the AOD at midday during the summer. These assessments suggest that large over-estimation may occur when long-term averages of AOD are estimated from passive satellite observations.

International audience; Long-term measurements of aerosol optical depth (AOD) from the Aerosol Robotic Network (AERONET) located in Beijing reveal a strong diurnal cycle of aerosol load staged by seasonal patterns. Such pronounced variability is matter of importance in respect to the estimation of daily averaged direct aerosol radiative forcing (DARF). Polar-orbiting satellites could only offer a daily revisit, which turns in fact to be even much less in case of frequent cloudiness. Indeed, this places a severe limit to properly capture the diurnal variations of AOD and thus estimate daily DARF. Bearing this in mind, the objective of the present study is however to evaluate the impact of AOD diurnal variations for conducting quantitative assessment of DARF using Moderate Resolution Imaging Spectroradiometer (MODIS) AOD data over Beijing. We provide assessments of DARF with two different assumptions about diurnal AOD variability: taking the observed hourly-averaged AOD cycle into account and assuming constant MODIS (including Terra and Aqua) AOD value throughout the daytime. Due to the AOD diurnal variability, the absolute differences in annual daily mean DARFs, if the constant MODIS/Terra (MODIS/Aqua) AOD value is used instead of accounting for the observed hourly-averaged daily variability, is 1.2 (1.3) Wm−2 at the top of the atmosphere, 27.5 (30.6) Wm−2 at the surface, and 26.4 (29.3) Wm−2 in the atmosphere, respectively. During the summertime, the impact of the diurnal AOD variability on seasonal daily mean DARF estimates using MODIS Terra (Aqua) data can reach up to 2.2 (3.9) Wm−2 at the top of the atmosphere, 43.7 (72.7) Wm−2 at the surface, and 41.4 (68.8) Wm−2 in the atmosphere, respectively. Overall, the diurnal variation in AOD tends to cause large bias in the estimated DARF on both seasonal and annual scales. In summertime, the higher the surface albedo, the stronger impact on DARF at the top of the atmosphere caused by dust and biomass burning (continental) aerosol. This indicates that the impact on DARFs estimates is sensitive to assumptions of aerosol type and surface albedo.

Short –term particulate concentrations of 13 polycyclic aromatic hydrocarbons (PAHs) in PM 10 were determined in the urban area of Bilbao (Spain). The analysis was performed by thermal desorption coupled with gas chromatography-mass spectrometry (TD-GC/MS), which enabled to use three diurnal periods of 8 h sampling basis time resolution. A total of 105 PM 10 samples were collected during 5 months in 2013. Diurnal average concentration of total PAHs (∑13 PAHs) ranged from 1.18 to 9.78 ng m −3 ; and from 0.06 to 0.70 ng m −3 for benzo[ a ]pyrene. The presence of high concentrations of benzo[ b ]fluoranthene, pyrene, fluoranthene and chrysene, and the significant PAHs diurnal variations due to the sampling period, pointed out the influence of mixing anthropogenic sources and meteorological conditions. The diurnal pattern of source contributions was assessed by binary diagnostic ratios and principal component analysis (PCA). These results showed the prevalence of pyrogenic sources coming from traffic and coal/coke combustion sources. Moreover, the PCA differentiated a diurnal pattern of source contributions. The influence of meteorological factors was studied by Pearson correlation analysis and multiple linear regression. Three factors, temperature, wind speed and atmospheric pressure, were identified as the most significant ones affecting diurnal PAHs concentrations. Finally, PCA of the PAHs levels, regulated atmospheric pollutants and meteorological parameters showed that diurnal PAHs concentrations were mainly influenced by variations in the emission sources, atmospheric oxidants such as ozone, and temperature conditions. These results provide further insight into the PAHs diurnal patterns in urban areas by using higher temporal resolutions.

Nitrous oxide (N2O) emissions from soil are often measured using the manual static chamber method. Manual gas sampling is labour intensive, so a minimal sampling frequency that maintains the accuracy of measurements would be desirable. However, the high temporal (diurnal, daily and seasonal) variabilities of N2O emissions can compromise the accuracy of measurements if not addressed adequately when formulating a sampling schedule. Assessments of sampling strategies to date have focussed on relatively low emission systems with high episodicity, where a small number of the highest emission peaks can be critically important in the measurement of whole season cumulative emissions. Using year-long, automated sub-daily N2O measurements from three fertilised sugarcane fields, we undertook an evaluation of the optimum gas sampling strategies in high emission systems with relatively long emission episodes. The results indicated that sampling in the morning between 09:00–12:00, when soil temperature was generally close to the daily average, best approximated the daily mean N2O emission within 4–7% of the ‘actual’ daily emissions measured by automated sampling. Weekly sampling with biweekly sampling for one week after >20 mm of rainfall was the recommended sampling regime. It resulted in no extreme (>20%) deviations from the ‘actuals’, had a high probability of estimating the annual cumulative emissions within 10% precision, with practicable sampling numbers in comparison to other sampling regimes. This provides robust and useful guidance for manual gas sampling in sugarcane cropping systems, although further adjustments by the operators in terms of expected measurement accuracy and resource availability are encouraged. By implementing these sampling strategies together, labour inputs and errors in measured cumulative N2O emissions can be minimised. Further research is needed to quantify the spatial variability of N2O emissions within sugarcane cropping and to develop techniques for effectively addressing both spatial and temporal variabilities simultaneously.

Recent investigations suggest that aqueous phase oxidation of hydrophilic organic compounds can be a significant source of secondary organic aerosols (SOA) in the atmosphere. Here we investigate the possibility of nighttime aqueous phase formation of SOA in Los Angeles during winter, through examination of trends in fine particulate matter (PM2.5) carbonaceous content during two contrasting seasons. Distinctive winter and summer trends were observed for the diurnal variation of organic carbon (OC) and secondary organic carbon (SOC), with elevated levels during the nighttime in winter, suggesting an enhanced formation of SOA during that period. The nighttime ratio of SOC to OC was positively associated with the relative humidity (RH) at high RH levels (above 70%), which is when the liquid water content of the ambient aerosol would be high and could facilitate dissolution of hydrophilic primary organic compounds into the aqueous phase. Time-integrated collection and analysis of wintertime particles at three time periods of the day (morning, 6:00 a.m.–9:00 a.m.; afternoon, 11:00 a.m.–3:00 p.m.; night, 8:00 p.m.–4:00 a.m.) revealed higher levels of water soluble organic carbon (WSOC) and organic acids during the night and afternoon periods compared to the morning period, indicating that the SOA formation in winter continues throughout the nighttime. Furthermore, diurnal trends in concentrations of semi-volatile organic compounds (SVOCs) from primary emissions showed that partitioning of SVOCs from the gas to the particle phase due to the decreased nighttime temperatures cannot explain the substantial OC and SOC increase at night. The oxidative potential of the collected particles (quantified using a biological macrophage-based reactive oxygen species assay, in addition to the dithiothreitol assay) was comparable during afternoon and nighttime periods, but higher (by at least ∼30%) compared to the morning period, suggesting that SOA formation processes possibly enhance the toxicity of the ambient particles compared to mobile-source dominated primary emissions in the Los Angeles area.