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Severe haze hovered over Harbin during the heating season of 2019–2020, making it one of the ten most polluted Chinese cities in January of 2020. Here we focused on the optical properties and sources of brown carbon (BrC) during the extreme atmospheric pollution periods. Enhanced formation of secondary BrC (BrCsec) was evident as relative humidity (RH) became higher, accompanied with a decrease of ozone but concurrent increases of aerosol water content and secondary inorganic aerosols. These features were generally similar to the characteristics of haze chemistry observed during winter haze events in the North China Plain, and indicated that heterogeneous reactions involving aerosol water might be at play in the formation of BrCsec, despite the low temperatures in Harbin. Although BrCsec accounted for a substantial fraction of brown carbon mass, its contribution to BrC absorption was much smaller (6 vs. 28%), pointing to a lower mass absorption efficiency (MAE) of BrCsec compared to primary BrC. In addition, emissions of biomass burning BrC (BrCBB) were inferred to increase with increasing RH, coinciding with a large drop of temperature. Since both the less absorbing BrCsec and the more absorbing BrCBB increased as RH became higher, the MAE of total BrC were largely unchanged throughout the measurement period. This study unfolded the contrast in the source apportionment results of BrC mass and absorption, and could have implications for the simulation of radiative forcing by brown carbon.

To investigate the impacts of relative humidity (RH) on secondary organic aerosol (SOA) concentrations and chemical reactions, the carbonaceous aerosol components [i.e., organic carbon (OC) and element carbon (EC)] were quantified in daily PM2.5 samples collected at a background site in East China during summer 2015. Based on the method of EC-tracer, the concentration of secondary organic carbon (SOC) demonstrated an obvious negative relationship with RH higher than 60%. Moreover, the ratio of SOC/EC also exhibited obvious decreasing trends with increasing RH, indicating negative effects for chemical production of SOA under high RH conditions. Due to high RH, photochemistry was weakened, gaseous oxidant concentrations was lowered (e.g., significantly decreased O3 levels), and the production rates of SOA were relatively low. On the other hand, because of more water uptake under higher RH conditions, the aerosol droplet acidity was reduced and enhancement of SOA formation by acidity was accordingly absent. In addition, high RH also plays an important role in changing viscosity of pre-existing aerosol coatings, which can affect reactive uptake yield of SOA. Overall, the results from this study imply that SOA production may be more associated with photochemical processes, while aqueous-phase chemistry is not very important for some SOA formation in a moist ambient environment. In the ambient atmosphere, oxidant concentrations, reaction rates, airborne species, etc., are highly variable. How do these factors affect SOA yields under given ambient environment warrants further detailed investigations.

The study of atmospheric nitrous acid (HONO), which is the primary source of OH radicals, is crucial with respect to understanding atmospheric photochemistry and heterogeneous chemical processes. Heterogeneous NO2 chemistry under haze conditions has been identified as one of the missing sources of HONO on the North China Plain, and also produces sulfate and nitrate. However, controversy exists regarding the various proposed HONO production mechanisms, mainly regarding whether SO2 directly takes part in the HONO production process and what roles NH3 and the pH value play. In this paper, never before seen explosive HONO production was reported and evidence was found – for the first time in field measurements during fog (usually with 4

Black carbon (BC) is important due to its complex influences on the environment and on climate in particular. However, reported BC data are largely dependent on measurement techniques due to the multitude of measurement principles. Here we focused on thermal-optical method which has been widely used to determine BC mass (as elemental carbon, EC). Several factors influencing EC measurement were investigated. Results from source samples representing vehicle engine emissions pointed to a continuum of EC components in thermal stability and provided direct observational evidence for the premature evolution of EC in inert atmosphere. It was also found that EC masses may be substantially underestimated for the vehicle exhaust samples if the adopted protocol requires an oxidizing atmosphere to define the split point between organic carbon (OC) and EC. Results from a field campaign conducted during winter in Beijing showed that the optical attenuation (ATN; i.e., the filter transmittance signal, I) was largely saturated for the samples with relatively high loadings, indicating their EC results were unreliable. Improved measurement of EC was achieved by extracting these heavily loaded filters using methanol, given that ATN was considerably reduced by the extraction and, moreover, saturation of ATN (or I) became not evident for the extracted samples. The methanol extraction also significantly reduced the transformation of OC to char-OC, by removing the majority (i.e., ~85%) of the deposited organic aerosols. Higher EC were measured for the extracted samples compared with the untreated ones, indicating that EC tends to be underestimated due to the charring-induced uncertainties. In addition, the methanol extraction largely reduced the inter-protocol discrepancy in the EC measurement results. Similar effects of methanol extraction have been observed during summer in Beijing, despite the seasonal variations of aerosol sources and compositions. This study indicates the potential benefits of methanol extraction for EC measurement.

Despite the growing interest in understanding haze formation in Chinese megacities, air pollution has been largely overlooked for the Harbin–Changchun (HC) metropolitan area, located in the severe cold climate region in northeast China. In this study, we unfolded significant variations of fine particulate matter (PM2.5) in HC's central city (Harbin) during two sequential heating seasons of 2018–2019 and 2019–2020, and we explored major drivers for the observed variations. The two campaigns showed comparable organic carbon (OC) levels but quite different OC sources. The biomass burning (BB) to OC contribution decreased substantially for 2019–2020, which was attributed primarily to the transition of local policies on agricultural fires, i.e., from the “legitimate burning” policy released in 2018 to the “strict prohibition” policy in 2019. Meanwhile, the contribution of secondary OC (OCsec) increased significantly, associated with the much more frequent occurrences of high relative humidity (RH) conditions during the 2019–2020 measurement period. Similar to OCsec, the major secondary inorganic ions, i.e., sulfate, nitrate and ammonium (SNA), also exhibited RH-dependent increases. Given the considerable aerosol water contents predicted for the high-RH conditions, heterogeneous reactions were likely at play in secondary aerosol formation even in the frigid atmosphere in Harbin (e.g., with daily average temperatures down to below −20 ∘C). In brief, compared to 2018–2019, the 2019–2020 measurement period was characterized by a policy-driven decrease of biomass burning OC, a RH-related increase of OCsec and a RH-related increase of SNA, with the first two factors generally offsetting each other. In addition, we found that open burning activities were actually not eliminated by the strict prohibition policy released in 2019, based on a synthesis of air quality data and fire count results. Although not occurring during the 2019–2020 measurement period, agricultural fires broke out within a short period before crop planting in spring of 2020, and this resulted in off-the-charts air pollution for Harbin, with 1 h and 24 h PM2.5 concentrations peaking at ∼ 2350 and 900 µg/m3, respectively. This study indicates that sustainable use of crop residues remains a difficult challenge for the massive agricultural sector in northeast China.

Formation of secondary inorganic aerosol (SIA) was investigated during a six-month long heating season in Harbin, China. Enhanced sulfate formation was observed at high relative humidity (RH), with the same threshold RH (80%) for both colder and warmer measurement periods. Compared to wintertime results from Beijing, the threshold RH was considerably higher in Harbin, whereas the RH-dependent enhancement of sulfur oxidation ratio (SOR) was less significant. In addition, the high RH events were rarely encountered, and for other periods, the SOR were typically as low as ∼0.1. Therefore, the sulfate formation was considered inefficient in this study. After excluding the several cases with high RH, both SOR and the nitrogen oxidation ratio (NOR) exhibited increasing trends as the temperature increased, with the increase of NOR being sharper. The nitrate to sulfate ratio tended to increase with increasing temperature as well. Based on a semi-quantitative approach, this trend was attributed primarily to the temperature-dependent variations of precursors including SO2 and NO2. The influence of biomass burning emissions on SIA formation was also evident. With stronger impact of biomass burning, an enhancement in NOR was observed whereas SOR was largely unchanged. The different patterns were identified as the dominant driver of the larger nitrate to sulfate ratios measured at higher concentrations of fine particulate matter.

International audience; Sustainable use of crop residues remains a challenge in main agricultural regions of China such as the Northeast Plain. Here we investigated the impacts of biomass burning on fine particle (PM2.5) during a six-month long heating season in the Harbin-Changchun (HC) metropolitan area, China's only national-level city cluster located in the severe cold climate region. Temporal variation of PM2.5 was found to coincide with that of levoglucosan. This was attributed to the strong contribution of biomass burning to organic aerosol (the dominant component in PM2.5), as supported by the source apportionment results and high levoglucosan-to-organic carbon (OC) ratios. Furthermore, the variation of biomass burning contribution was inferred to be driven mainly by agricultural fires with relatively low combustion efficiencies, based on a synthesis of the relationship between OC and elemental carbon (EC), the dependence of EC on carbon monoxide, and the relative abundances of different tracers for biomass burning. Nitrate formation was enhanced during biomass burning episodes whereas no evidence was observed to indicate enhanced sulfate formation or net increase of OC mass due to secondary formation. This study demonstrates the importance of open burning as a source of haze pollution in the HC region.

The characteristics of secondary inorganic aerosol including sulfate, nitrate and ammonium (SNA) were investigated during a six-month long heating season in the Harbin-Changchun metropolitan area, i.e., China’s only national-level city cluster located in the severe cold climate region. The contribution of SNA to fine particulate matter (PM2.5) tended to decrease with increasing PM2.5 concentration, opposite to the trend repeatedly observed during winter in Beijing. Heterogeneous sulfate formation was still evident when the daily average temperature was as low as below −10 °C, with the preconditions of high relative humidity (RH; above ∼80%) and high nitrogen dioxide (above ∼60 μg/m3). Both the sulfur oxidation ratio (SOR) and nitrogen oxidation ratio (NOR) were enhanced at high RH, reaching ∼0.3. However, the high RH conditions were not commonly seen during the heating season, which should be responsible for the overall lack of linkage between the SNA contribution and PM2.5 temporal variation.

Biomass burning (BB) activities are ubiquitous in China, especially in North China, where there is an enormous rural population and winter heating custom. In order to better understand their impacts on aerosol chemical characteristics in rural and agricultural areas of the North China Plain, BB tracers (i.e., levoglucosan (LG), mannosan (MN) and potassium (K+)), as well as other chemical components were quantified at a rural site (Gucheng, GC) from 15 October to 30 November, during a transition heating season, when the field burning of agricultural residues was becoming intense. The measured daily average PM2.5 concentrations of LG, MN and K+ during this study were 0.79 ± 0.75 μg m−3, 0.03 ± 0.03 μg m−3 and 1.52 ± 0.62 μg m−3. Due to the planetary boundary layer development, carbonaceous components and BB tracers showed higher levels at nighttime than daytime, while OM and secondary inorganic ions were enhanced during daytime, likely due to enhanced photochemical activity. An episode with high levels of BB tracers was encountered at the end of October, 2016, with high LG at 4.37 μg m−3. Based on the comparison of chemical components during different BB periods, it appeared that biomass combustion can obviously elevate carbonaceous components levels, whereas there seems to be essentially no effect on secondary inorganic ions in the ambient air. Moreover, the LG / MN ratios in different BB periods were consistent, while the LG / K+ ratio during intensive BB periods was significantly elevated at times, with K+ not increasing as much as LG during intensive BB episodes. This indicated that there were other sources of K+ in the study region, such as fireworks, fertilizer use, or soil resuspension, which don't have variable contributions of K+ during the intensive BB periods; however, local soft wood and vegetation combustion can't be excluded, which have efficient formation of levoglucosan during flaming fires.

Carbonaceous aerosol was measured during fall of 2010 in Beijing. Daily variation of organic carbon (OC) was found to coincide with that of relative humidity (RH), and the OC to elemental carbon (EC) ratios were more than doubled during the more humid periods (RH above 0.75) compared to other conditions. This large increase in OC/EC could not be explained by the variations of primary biomass burning emissions but was accompanied by a five-fold increase in the sulfate to EC ratio. It was then inferred that secondary organic aerosol (SOA) formation was enhanced under the more humid conditions, presumably through aqueous-phase processes. This enhanced SOA formation might be partially associated with particles externally mixed with black carbon, as indicated by the RH-dependent relationships between aerosol optical attenuation and EC loading. In addition, organic aerosols exhibited different properties between the more humid and the other periods, such that they were less volatile and charred more significantly during thermal-optical analysis in the former case. These differences coincided with the evidence of enhanced SOA formation under the more humid conditions. This study highlights the necessity of incorporating aqueous-phase chemistry into air quality models for SOA.

With rapid economic development and urbanization, particular attention has been paid to atmospheric chemical studies in the Yangtze River Delta in China. PM2.5 samples were collected by a MiniVol™ air sampler in summer time at a background site of the Yangtze River Delta in China. Carbonaceous components, i.e., OC and EC, levoglucosan and water-soluble inorganic ions, including sulfate, nitrate, ammonium, etc., were quantified. The average concentration of PM2.5 in summer at Lin'an was 30.19 ± 8.86 μg m−3, lower than previous studies reported, confirming that air pollution in China is improving, e.g., by emission control measures and subsequent reduction in PM emissions in China. Investigating the relationship among sulfate, nitrate and ammonium showed that SO42− existed as (NH4)2SO4, while NO3− may have been present as NaNO3 and KNO3. Based on molecular tracers, synoptic data as well as air mass back trajectory analysis, it was revealed that regional transport and stable synoptic conditions both play an important role in controlling the variations of aerosol chemical components. The comparison of chemical species between clean and hazy days showed that secondary organic and inorganic aerosols have different production processes. Secondary organic carbon (SOC) was much more important during clean days, while secondary inorganic aerosol species were readily produced and consequently became more important during polluted periods in Lin'an during summer time.

Saccharides are important constituents of atmospheric particulate matter (PM). In order to better understand the sources and seasonal variations of saccharides in aerosols in Beijing, China, saccharide composition was measured in ambient PM samples collected at an urban site in Beijing. The highest concentrations of total saccharides in Beijing were observed in autumn, while an episode with abnormal high total saccharide levels was observed from 15 to 23 June, 2011, due to extensive agricultural residue burning in northern China during the wheat harvest season. Compared to the other two categories of saccharides, sugars and sugar alcohols, anhydrosugars were the predominant saccharide group, indicating that biomass burning contributions to Beijing urban aerosol were significant. Ambient sugar and sugar alcohol levels in summer and autumn were higher than those in spring and winter, while they were more abundant in PM2.5 during winter time. Levoglucosan was the most abundant saccharide compound in both PM2.5 and PM10, the annual contributions of which to total measured saccharides in PM2.5 and PM10 were 61.5% and 54.1%, respectively. To further investigate the sources of the saccharides in ambient aerosols in Beijing, the PM10 datasets were subjected to positive matrix factorization (PMF) analysis. Based on the objective function to be minimized and the interpretable factors identified by PMF, six factors appeared to be optimal as to the probable origin of saccharides in the atmosphere in Beijing, including biomass burning, soil or dust, isoprene SOA and the direct release of airborne fungal spores and pollen.

Atmospheric ammonia (NH3) as the most important alkaline gas in the atmosphere has attracted much attention in recent years due to its critical role in haze formation, especially on the North China Plain (NCP). Comprehensive studies are needed for investigating diurnal variations of NH3 and underlying mechanisms in different seasons and their potential impacts on atmospheric chemistry. In this study, continuous long-term observation (Mar. 2016 to May 2017) of NH3 at a rural site in the NCP was used to characterize the diurnal variation of NH3 in different seasons and to unveil its causes and potential impacts on atmospheric chemistry. NH3 concentrations displayed rapid increases during the morning, reaching very prominent peaks mostly between 8:00 to 11:00 LT. Such frequent (55%) morning peaks were mainly caused by the evaporation of dew and guttation water droplets. Average dew and guttation water volume concentrations of 750 mL m−2 was estimated for spring, which resulted in approximate NH3 emissions of 800 ng m−2 s− 1. Such high emission fluxes from dew and guttation water evaporation have never been reported before, suggesting dew and guttation droplets to be significant night-time reservoirs and strong morning sources for NH3. In light of recent studies putting forward that NH3 can promote the heterogeneous formation of HONO and nitrate under high humidity conditions, we investigated the differences in HONO and aerosol chemical composition diurnal variations between days with and without NH3 morning spikes during November. HONO, nitrate and sulfate concentrations were significantly higher for days with NH3 morning spikes, with HONO displaying a morning peak near that of NH3. These results demonstrate that the prevailing NH3 morning spikes on the NCP have significant influences on aerosol formation and atmospheric chemistry. NH3 emission mitigation strategies and regulations are urgently needed.

Biomass burning activities are ubiquitous in China. In order to better understand the biomass burning sources and their impacts on aerosols components in the background air of East China, biomass burning tracers i.e., levoglucosan (LG), mannosan (MN) and non-sea-salt potassium (nss-K+) were quantified at the Lin'an site. The annual average concentrations of LG, MN and nss-K+ in PM2.5 were 0.13 ± 0.14 μg m−3, 0.009 ± 0.010 μg m−3 and 0.65 ± 0.38 μg m−3, respectively. The yearly variations of anhydrosugar tracers, i.e., LG and MN were fairly consistent, as well as nss-K+. However, due to extensive fireworks activity, nss-K+ was observed during a three-day episode from 7 to 9 February 2016, with abnormally high levels (average at 5.38 ± 3.55 μg m−3). The highest levels of biomass burning tracers were associated with the strong continental outflow from inland China in winter, while lower levels of biomass burning tracers were observed in summer season, influenced by maritime air masses. Combined with the analysis of fire activity data, clear seasonal trends of biomass burning pollution in Lin'an are apparent, ranging from high to low in winter, spring, autumn and summer. By analyzing the ratios of LG/MN and LG/nss-K+, biomass burning aerosols in the East China were identified to be predominately associated with burning of rice crop residues. Moreover, based on the relationship between biomass burning tracers and components in aerosols, biomass burning plays an important role in regulating chemical properties of aerosols in the background site in East China, especially for carbonaceous components.

PM 10 and PM 2.5 samples were collected from November, 2010 to October, 2011 at Tsinghua University in Beijing. Various carbohydrates were quantified by high-performance anion exchange chromatography with pulsed amperometric detection (HPAEC–PAD), including the 2-methyltetrols (2-methylthreitol and 2-methylerythritol). A clear seasonal variation in the ambient 2-methyltetrol concentrations was observed, with the highest levels occurring in the summer, followed by autumn, spring and winter. The average concentrations of the 2-methyltetrols in PM 10 and PM 2.5 were 17.5 ± 15.4 ng m − 3 and 13.8 ± 12.2 ng m − 3 , respectively. The 2-methyltetrols exhibited significant positive correlations with ambient relative humidity and temperature, likely due to the higher isoprene emission strength and enhanced formation yield under higher temperature and humidity conditions. In contrast, there was no relationship between the concentration of 2-methyltetrols and sunshine duration. The significant positive correlation ( R 2 = 0.76) between 2-methyltetrols and SO 4 2 − indicated that high concentrations of SO 4 2− can increase the formation rate of 2-methyltetrols from isoprene. Moreover, 2-methyltetrols were also observed in the winter time in Beijing, illustrating the enhancement of the 2-methyltetrol formation rate by high concentrations of pollutants in ambient aerosol.

We investigated the difference in the concentrations of air pollutants between the period of the Beijing 2008 Olympic Games and the same periods in the prior three years (2005–2007, the reference period), in particular, the chemical composition of the particulate matter. PM10, Black carbon (BC), SO2 and NO2 were reduced by 33%, 31%, 32% and 51%, respectively, whereas PM2.5 was not reduced during the Olympic period when compared to the reference period. Many pollution control measures that had been applied to prevent air pollution from contaminating the air quality in Beijing during the Olympic Games significantly reduced the larger particles (PM10), SO2 and NO2 but it did not as effectively reduce the much smaller particles (PM2.5). The sum of the measured water-soluble ions in total suspended particles (TSP) during the Olympic period was reduced by 35% when compared to the reference period. Each ion was reduced significantly by 43%–79% except for nitrogen-containing species. We estimated that CaSO4 reduction by 52% played a significant role in reducing larger particles during the Olympic period. Although many ions in PM2.5 showed a similar decreasing trend as those in TSP, NO3− in PM2.5 increased significantly. During the Olympic period, the atmospheric nitrogen cycle seemed to have been very complicated. Meteorological conditions such as temperature, relative humidity, or regional-scale air mass trajectory did not seem to be a major reason for the nitrate behavior. Instead, it is possible that the increase of ozone and/or oxidants around the center of Beijing may have accelerated the oxidation of nitrogen oxides to form nitrate. We found a trend in the concentration of high-molecular weight PAHs (HMW-PAHs), which had 5–7 rings decreased during the Olympic period. We considered that the reduction of HMW-PAHs that could be associated with the reduction of BC was due to the reduction of traffic flow resulting from traffic control measures implemented during that time period.

The chemical composition of Beijing aerosol was measured during summer and winter. Two distinct episodes were identified. Water-soluble potassium (K(+)) increased significantly during the firework episode in winter with an episode to non-episode ratio of 4.97, whereas the biomass burning (BB) episode in summer was characterized by high episode to non-episode ratios of levoglucosan (6.38) and K(+) (6.90). The BB and firework episodes had only a minor influence on the water-soluble OC (organic carbon) to OC ratio. Based on separate investigations of episode and non-episode periods, it was found that: (i) sulfate correlated strongly with both relative humidity and nitrate during the typical winter period presumably indicating the importance of the aqueous-phase oxidation of sulfur dioxide by nitrogen dioxide, (ii) oxalate and WSOC during both winter and summer in Beijing were mainly due to secondary formation, and (iii) high humidity can significantly enhance the formation potential of WSOC in winter.

Biological aerosol is one of the most important pollutants in the air, not only influencing public health, and air quality, but also playing an important role in climate change and both chemical and physical processing regulations in the atmosphere. Therefore, the quantification of biological aerosols in the air needs to be more accurate. This article reviews the progress on quantitative assessment methods of biological aerosols in the atmosphere as well as those related merits that each method used.

PM2:5 (particulate matter with aerodynamic diameters less than 2:5 m) and PM10 (particulate matter with aerodynamic diameters less than 10 m) samples were collected by high-volume air samplers simultaneously at a rural site and an urban site in Beijing, China. Various carbohydrates were quantified by high-performance anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD), including the sugar alcohols mannitol and arabitol, recently proposed as molecular tracers for fungal aerosol. The annual average concentrations of arabitol in PM2:5 and PM10 at the urban site were 7:4 9:4 and 21:0 20:4 ng m 3 , and the respective mannitol concentrations were 10:3 9:5 and 31:9 26:9 ng m 3 . During summer and autumn, higher arabitol and mannitol levels than during spring and winter were observed in coarse particles, probably due to different dominant sources of fungal spores in different seasons. In the dry season (i.e., winter and spring) in Beijing, probably only the suspension from exposed surfaces (e.g., soil resuspension, transported dust, etc) can be regarded as the main sources for fungal aerosols. On the other hand, in summer and autumn, fungal spores in the atmosphere can be derived from more complex sources, including plants, vegetation decomposition and agricultural activity, such as ploughing; these fungal spore sources may contribute more to coarse PM. Moreover, statistical analysis according to typical seasonal patterns, including a dry season (December 2010 to March 2011) and a wet season (July to September 2011), revealed different variations of fungal spores in different seasons. Although fungal spore levels at rural sites were reported to be consistently higher than those at urban sites in other studies, our findings showed the opposite pattern, indicating a high abundance of fungal spores in the urban area of this Chinese megacity.