Submicron aerosol (PM1) species measured by aerosol mass spectrometers have been widely used to validate chemical transport models; however, the uncertainties due to chemical differences between PM1 ...and PM2.5 are poorly constrained. Here we characterized such differences in a highly polluted environment in north China in winter. Our results showed that the changes in PM1/PM2.5 ratios as a function of relative humidity (RH) were largely different for primary and secondary species. Secondary organic and inorganic aerosol (SOA and SIA) presented clear decreases in PM1/PM2.5 ratios at RH > 60% during periods with high SIA contributions (>50%), likely driven by the changes in aerosol hygroscopicity and phase states, while the traffic and coal combustion OA had limited dependence on RH. Thermodynamic modeling showed negligible impacts of PM differences on predictions of particle acidity, yet these impacts can cause a difference in aerosol water content by up to 50–70%.
Plain Language Summary
Current air pollution studies rely largely upon aerosol mass spectrometers that provide real‐time measurements of submicron aerosol (PM1) species, and in many studies, PM1 aerosol species are used to validate those of PM2.5 in chemical transport models and estimate particle acidity and aerosol water content which are key parameters in studying heterogeneous reactions. However, the uncertainties in air pollution studies due to the chemical differences of PM1 and PM2.5 are poorly constrained, particularly in highly polluted environment, for example, China and India. We found that the changes in PM1/PM2.5 ratios as a function of relative humidity were largely different for primary and secondary aerosol species in highly polluted environment, which was likely driven by the changes in aerosol hygroscopicity and phase states. The chemical differences of PM1/PM2.5 ratios were also found to have negligible impacts on predictions of particle acidity, yet these impacts can cause a difference in aerosol water content by up to 50–70%. Considering the rapid increases in the deployments of aerosol mass spectrometers worldwide, the results in this study are of great importance for a better understanding of the uncertainties in both modelling and observations.
Key Points
We observed large chemical differences between PM1 and PM2.5 under high relative humidity periods in highly polluted environment
The RH dependence of PM1/PM2.5 ratios of primary and secondary species was largely different due to different hygroscopicity
The chemical differences between PM1 and PM2.5 have negligible influences on particle acidity prediction, yet have a large impact on AWC
Activation of biomass burning aerosols (BBA) and fossil fuel combustion aerosols (FFA) in fogs and clouds significantly impact regional air quality through aqueous chemistry and climate by affecting ...cloud microphysics. However, we lack direct observations of how these aerosols behave in fogs and clouds. Using a newly developed aerosol‐cloud sampling system, we conducted observations during fog events and found that BBA, despite their high organic content, effectively contributed to super‐micron interstitial aerosols and fog droplets in low supersaturation fogs. In contrast, FFA, predominantly externally mixed organic, did not grow beyond the super‐micron size in fogs due to their near‐hydrophobic nature. Measurements conducted under supersaturations relevant for cloud formation revealed that portions of FFA could serve as cloud condensation nuclei, but only when supersaturation exceeded ∼0.14%. These findings have broad implications for future investigations into the influence of BBA and FFA on fog and cloud chemistry and their interactions with clouds.
Plain Language Summary
Tiny particles, known as aerosols, emitted from combustion of biomass and fossil fuels, can impact air quality and global climate by interacting with fog and clouds. However, we lack direct observational evidence of how these aerosols behave in these conditions. In our study, we developed an advanced aerosol‐cloud sampling system to observe aerosol activation during fog events. Our findings highlight a crucial factor of aerosol activation: the mixing of these aerosols. Biomass burning aerosols with an internal mixture of organic and inorganic components activate more easily, even in conditions with low supersaturation like fog. In contrast, fossil fuel combustion aerosols are often externally mixed and almost water‐repellent, requiring higher supersaturation to become cloud condensation nuclei. Understanding these distinctions has significant implications for regional air quality and the intricate interactions between aerosols and clouds. By gaining insights into how various aerosols interact with fog and clouds, we can enhance our understanding of their impact on our environment and climate.
Key Points
Advanced Aerosol‐Cloud sampling system was developed to characterize aerosol activation in fogs
Biomass burning aerosols efficiently form fog droplets, while fossil fuel combustion aerosols are almost hydrophobic and don't contribute
Fossil fuel combustion organic aerosols can serve as CCN in high supersaturation conditions (>0.14%)
The commercial continuous‐flow streamwise thermal‐gradient cloud condensation nuclei (CCN) counter is widely used in aerosol CCN activity measurements which are critical for investigating ...aerosol‐cloud interactions. In CCN measurements, a critical threshold is needed for distinguishing interstitial aerosols and activated droplets, and a default threshold of 0.75 μm was set in CCN counter. Theoretically, interstitial aerosols could also grow larger than 0.75 μm in CCN counter at low supersaturations, thus theoretical thresholds derived from the Köhler theory were suggested. Here, we report that droplet growth in the CCN counter is kinetically limited and CCN‐active droplets does not reach their theoretical diameters under low supersaturations. Neglecting the kinetic growth limitation in CCN identification would count less CCN and underestimate the aerosol hygroscopicity parameter κ $\kappa $ by up to 50% for supersaturations lower than 0.1%. We recommend that thresholds considering kinetic limitations should be used as new criteria for CCN identification under low supersaturations.
Plain Language Summary
Accurate Cloud Condensation Nuclei (CCN) predictions in climate and regional models are crucial for the assessment of climate effects of aerosol‐cloud interactions. Direct measurements of CCN activity, varied by aerosol size, hygroscopicity, and supersaturation, are important for model verifications and improvements. The commercial continuous‐flow streamwise thermal‐gradient CCN counter (CCNC) is widely used in CCN activity measurements. We found that the droplet growth in the CCNC is kinetically limited under low supersaturations (<0.14%), thus not reaching the theoretical critical activation diameter, which significantly affect CCN identification. Kinetic model results revealed that the wet diameter thresholds of CCN identification that took the kinetic limitations of droplet growth into account were independent of dry aerosol size and hygroscopicity thus could be directly used as new criteria for CCN identification. Neglecting the kinetic growth limitation in CCN identification would result in significant bias in CCN activity measurements and thus supersaturated aerosol hygroscopicity retrievals. Findings of this research have significant impacts on CCN activity measurements and supersaturated aerosol hygroscopicity investigations.
Key Points
Limited hygroscopic growth of cloud condensation nuclei (CCN) in CCN counter is recognized and evaluated based on model analysis
Kinetic limitations in CCN counter impacts significantly on size‐resolved CCN activity measurements
Kinetic limitations in CCN counter impacts significantly on aerosol hygroscopicity measurements under low supersaturation
NH3, SO2, NOx and the inorganic ions of PM2.5 in winter 2009, 2014 and 2016 were examined to investigate the change in NH3 and aerosol chemistry in Beijing, China. NH3 concentrations showed an ...increase by 59% on average, in contrast to the decrease of SO2 by 63% from winter 2009 to 2016. The mean mass ratio of NH3/NHx was 0.83 ± 0.12 in 2016, which is higher than those obtained in 2009 and 2014, implying more NHx remaining as free NH3 in 2016 winter. Our findings suggest that vehicles exhaust emissions are an important NH3 source in urban central atmosphere of Beijing in winter. Despite the observed NOx presenting declining trends from 2014 to 2016, nitrate concentrations even exhibited a significant increasing trend, which may be largely attributable to high NH3 levels. An in-depth analysis of measured NH3 and aerosol species in a heavy pollution episode in December 2016, combined with the acidity predicted by ISORROPIA II model demonstrated abundant NH3 most of the time in air, where NH3 is not only a precursor for NH4+ but also effect the neutralization of SO42− and NO3− in PM2.5. With high RH and low photochemical activity, elevated NO3− concentration was attributed to an enhanced heterogeneous conversion of NOx to HNO3 to form NH4NO3 in pollution transport stage. The decrease in NOx from high level and the increase in NH3, with peaks of SO42− occurring were observed in pollution cumulative stage. The aqueous-phase oxidation of SO2 by NO2 to sulfate might play an important role with high pH values. Our results suggested that the simultaneous control of NH3 emissions in conjunction with SO2 and NOx emissions would be more effective in reducing particulate matter PM2.5 formation.
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•NH3 and the chemistry components of PM2.5 change significantly from 2009 to 2016.•Vehicles exhaust is an important contributor to NH3 in central Beijing in winter.•Increasing nitrate trend may be largely attributable to high NH3 levels.
The first Tropospheric Ozone Assessment Report (TOAR) provides information on present-day distributions and long-term trends of ozone metrics relevant for climate change, human health, and ...vegetation. However, only few results are available in TOAR for China due to limited long-term ozone observations. Here, we present an integrated analysis of long-term measurements of surface ozone from eight sites distributed in the North China Plain (NCP) and Yangtze River Delta (YRD), the relatively underdeveloped region Northeast China, and the remote regions in Northwest and Southwest China. Trends and present-day values for seven annual and five seasonal ozone metrics were calculated following the TOAR methodologies. We compare the seasonal and diurnal cycles of ozone concentrations as well as the present-day values of ozone among sites and discuss the long-term trends in the ozone metrics. Large and significant increases of ozone are detected at the background site in the NCP, moderate increases at the global baseline site in western China, significant decreases at the northwestern edge of China, and nearly no trend at other sites. Extremely high values of ozone occurred in the NCP and YRD, particularly in warmer seasons. The present-day levels of summer ozone metrics in the NCP are much higher than the thresholds set in TOAR for the highest value groups of ozone metrics. The summer ozone metrics at the Shangdianzi background site in the NCP indicate increases at rates of more than 2%/yr during 2004–2016. In contrast, ozone at the Lin’an background site in the YRD was constant over the period 2006–2016. Our results fill some knowledge gaps in spatiotemporal changes of ozone in China and may be of useful in the assessment of ozone impacts on human health and vegetation.
Biomass burning activities are ubiquitous in China. In order to better understand the biomass burning sources and their impacts on aerosol 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 the 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 LG/MN and LG/nss-K+ ratios, biomass burning aerosols in East China were identified to be predominately associated with burning of rice crop residues. Moreover, based on the relationship between biomass burning tracers and other components in aerosols, biomass burning plays an important role in regulating chemical properties of aerosols in background areas in East China, especially for carbonaceous components.
•Biomass burning pollution at background site in East China was investigated.•Intensive fireworks activity resulted in high K+ levels during the Spring Festival.•Rice crop residue was the major biomass burning type in East China.•Impacts of biomass burning in the background areas of East China can not be ignored.
Fine-particle pollution associated with winter haze threatens the health of more than 400 million people in the North China Plain. The Multiphase chemistry experiment in Fogs and Aerosols in the ...North China Plain (McFAN) investigated the physicochemical mechanisms leading to haze formation with a focus on the contributions of multiphase processes in aerosols and fogs. We integrated observations on multiple platforms with regional and box model simulations to identify and characterize the key oxidation processes producing sulfate, nitrate and secondary organic aerosols. An outdoor twin-chamber system was deployed to conduct kinetic experiments under real atmospheric conditions in comparison to literature kinetic data from laboratory studies. The experiments were spanning multiple years since 2017 and an intensive field campaign was performed in the winter of 2018. The location of the site minimizes fast transition between clean and polluted air masses, and regimes representative for the North China Plain were observed at the measurement location in Gucheng near Beijing. The consecutive multi-year experiments document recent trends of PM
2.5
pollution and corresponding changes of aerosol physical and chemical properties, enabling in-depth investigations of established and newly proposed chemical mechanisms of haze formation. This study is mainly focusing on the data obtained from the winter campaign 2018. To investigate multiphase chemistry, the results are presented and discussed by means of three characteristic cases: low humidity, high humidity and fog. We find a strong relative humidity dependence of aerosol chemical compositions, suggesting an important role of multiphase chemistry. Compared with the low humidity period, both PM
1
and PM
2.5
show higher mass fraction of secondary inorganic aerosols (SIA, mainly as nitrate, sulfate and ammonium) and secondary organic aerosols (SOA) during high humidity and fog episodes. The changes in aerosol composition further influence aerosol physical properties,
e.g.
, with higher aerosol hygroscopicity parameter
κ
and single scattering albedo SSA under high humidity and fog cases. The campaign-averaged aerosol pH is 5.1 ± 0.9, of which the variation is mainly driven by the aerosol water content (AWC) concentrations. Overall, the McFAN experiment provides new evidence of the key role of multiphase reactions in regulating aerosol chemical composition and physical properties in polluted regions.
High-RH-favored multiphase reactions can significantly change the chemical composition of fine particles and thereby modify their physicochemical properties.
Effective emission reductions of some primary pollutants have brought down aerosol loadings but led to increasing relative importance of secondary pollutants, as was indicated by the rising O3 levels ...during warm seasons within urban and suburban areas of China, which has received much attention in recent years, especially in the North China Plain (NCP). This has raised serious concerns on its agricultural impacts, which were mainly evaluated based upon O3 model simulations or urban/suburban measurements due to a lack in long-term rural observations. In this study, we present highly valuable continuous O3 observations at a rural NCP site during 2013–2019. Compared to nearby urban/suburban sites, which experienced increased O3 levels, rural observations exhibited decreasing O3 mole fractions. While O3 mole fractions and AOT40 widely increased at urban/suburban NCP sites from 2013 to 2019, O3 observations in the rural NCP site (GC) revealed decreases, especially during summer and autumn with greater rates for AOT40. A reassessment of O3 agricultural impacts in the NCP region was performed using rural observations, resulting in wheat, maize and soybean averaged relative yield losses of 37 ± 14, 8 ± 4 and 30 ± 13% yr–1, respectively. O3 impacts on crop yields and resulting economic losses did not increase as was suggested by previous estimations based on urban/suburban O3 data. Our analyses indicated high overestimations (i.e., average relative differences in estimated crop production loss reaching 53%, 112% and 75%, respectively, for wheat, maize, and soybean). Despite alleviated O3 agricultural impacts, the total economic cost loss in Hebei province still took up 0.89% of the gross domestic production (3.47 × 1012 USD) in Hebei province. Since the China National Environmental Monitoring Center mainly aims at monitoring O3 levels in populated areas, observation sites representative of agricultural regions are lacking across China. The current study highlights the urgent necessity for the establishment of rural O3 observation networks and encourages extensive field experiments on exposure–response relationships of different crops varieties to O3 for more accurate agricultural impact evaluations. Additionally, explorations into the underlying mechanisms behind the reversed O3 temporal variation between rural and urban areas should be conducted for future development of pollution control strategies.
Solar radiation triggers atmospheric nitrous acid (HONO) photolysis, producing OH radicals, thereby accelerating photochemical reactions, leading to severe secondary pollution formation. Missing ...daytime sources were detected in the extensive HONO budget studies carried out in the past. In the rural North China Plain, some studies attributed those to soil emissions and more recent studies to dew evaporation. To investigate the contributions of these two processes to HONO temporal variations and unknown production rates in rural areas, HONO and related field observations obtained at the Gucheng Agricultural and Ecological Meteorological Station during spring and autumn were thoroughly analyzed. Morning peaks in HONO frequently occurred simultaneously with those of ammonia (NH
) and water vapor both during spring and autumn, which were mostly caused by dew and guttation water evaporation. In spring, the unknown HONO production rate revealed pronounced afternoon peaks exceeding those in the morning. In autumn, however, the afternoon peak was barely detectable compared to the morning peak. The unknown afternoon HONO production rates were attributed to soil emissions due to their good relationship to soil temperatures, while NH
soil emissions were not as distinctive as dew emissions. Overall, the relative daytime contribution of dew emissions was higher during autumn, while soil emissions dominated during spring. Nevertheless, dew emission remained the most dominant contributor to morning time HONO emissions in both seasons, thus being responsible for the initiation of daytime OH radical formation and activation of photochemical reactions, while soil emissions further maintained HONO and associated OH radial formation rates at a high level, especially during spring. Future studies need to thoroughly investigate the influencing factors of dew and soil emissions and establish their relationship to HONO emission rates, form reasonable parameterizations for regional and global models, and improve current underestimations in modeled atmospheric oxidation capacity.
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