Showing papers by "Chao Yan published in 2021"

Is reducing new particle formation a plausible solution to mitigate particulate air pollution in Beijing and other Chinese megacities

Abstract: Atmospheric gas-to-particle conversion is a crucial or even dominant contributor to haze formation in Chinese megacities in terms of aerosol number, surface area and mass. Based on our comprehensive observations in Beijing during 15 January 2018-31 March 2019, we are able to show that 80-90% of the aerosol mass (PM2.5) was formed via atmospheric reactions during the haze days and over 65% of the number concentration of haze particles resulted from new particle formation (NPF). Furthermore, the haze formation was faster when the subsequent growth of newly formed particles was enhanced. Our findings suggest that in practice almost all present-day haze episodes originate from NPF, mainly since the direct emission of primary particles in Beijing has considerably decreased during recent years. We also show that reducing the subsequent growth rate of freshly formed particles by a factor of 3-5 would delay the buildup of haze episodes by 1-3 days. Actually, this delay would decrease the length of each haze episode, so that the number of annual haze days could be approximately halved. Such improvement in air quality can be achieved with targeted reduction of gas-phase precursors for NPF, mainly dimethyl amine and ammonia, and further reductions of SO2 emissions. Furthermore, reduction of anthropogenic organic and inorganic precursor emissions would slow down the growth rate of newly-formed particles and consequently reduce the haze formation.

Role of iodine oxoacids in atmospheric aerosol nucleation.

Abstract: Iodic acid (HIO3) is known to form aerosol particles in coastal marine regions, but predicted nucleation and growth rates are lacking Using the CERN CLOUD (Cosmics Leaving Outdoor Droplets) chamber, we find that the nucleation rates of HIO3 particles are rapid, even exceeding sulfuric acid–ammonia rates under similar conditions We also find that ion-induced nucleation involves IO3− and the sequential addition of HIO3 and that it proceeds at the kinetic limit below +10°C In contrast, neutral nucleation involves the repeated sequential addition of iodous acid (HIO2) followed by HIO3, showing that HIO2 plays a key stabilizing role Freshly formed particles are composed almost entirely of HIO3, which drives rapid particle growth at the kinetic limit Our measurements indicate that iodine oxoacid particle formation can compete with sulfuric acid in pristine regions of the atmosphere

Sulfuric acid–amine nucleation in urban Beijing

Abstract: . New particle formation (NPF) is one of the major sources of atmospheric ultrafine particles. Due to the high aerosol and trace gas concentrations, the mechanism and governing factors for NPF in the polluted atmospheric boundary layer may be quite different from those in clean environments, which is however less understood. Herein, based on long-term atmospheric measurements from January 2018 to March 2019 in Beijing, the nucleation mechanism and the influences of H2SO4 concentration, amine concentrations, and aerosol concentration on NPF are quantified. The collision of H2SO4 –amine clusters is found to be the dominating mechanism to initialize NPF in urban Beijing. The coagulation scavenging due to the high aerosol concentration is a governing factor as it limits the concentration of H2SO4 –amine clusters and new particle formation rates. The formation of H2SO4 –amine clusters in Beijing is sometimes limited by low amine concentrations. Summarizing the synergistic effects of H2SO4 concentration, amine concentrations, and aerosol concentration, we elucidate the governing factors for H2SO4 –amine nucleation for various conditions.

The Synergistic Role of Sulfuric Acid, Bases, and Oxidized Organics Governing New-Particle Formation in Beijing

Abstract: Intense and frequent new particle formation (NPF) events have been observed in polluted urban environments, yet the dominant mechanisms are still under debate. To understand the key species and governing processes of NPF in polluted urban environments, we conducted comprehensive measurements in downtown Beijing during January–March, 2018. We performed detailed analyses on sulfuric acid cluster composition and budget, as well as the chemical and physical properties of oxidized organic molecules (OOMs). Our results demonstrate that the fast clustering of sulfuric acid (H2SO4) and base molecules triggered the NPF events, and OOMs further helped grow the newly formed particles toward climateand health-relevant sizes. This synergistic role of H2SO4, base species, and OOMs in NPF is likely representative of polluted urban environments where abundant H2SO4 and base species usually co-exist, and OOMs are with moderately low volatility when produced under high NOx concentrations. Plain Language Summary Atmospheric new particle formation (NPF) is a dominant source of atmospheric ultrafine particles worldwide. Those particles profoundly influence climate and human health. NPF includes two consecutive processes, that is, the formation of new particles (∼2 nm in diameter) and their subsequent growth to larger sizes. Extensive studies conducted in the laboratory and in forested areas have shown that many gaseous species can participate in NPF, such as sulfuric acid, ammonia, amines, and oxidize organic molecules. However, the actual roles of these vapors may vary significantly from location to location and are largely unclear in urban environments. Here, based on measurements of sulfuric acid, sulfuric acid clusters, and oxidize organic molecules, we demonstrate that sulfuric acid and base molecules were responsible for the initial formation of new particles during a wintertime field campaign in Beijing. The majority of oxidized organic molecules had a minor YAN ET AL. © 2021. The Authors. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. The Synergistic Role of Sulfuric Acid, Bases, and Oxidized Organics Governing New-Particle Formation in Beijing Chao Yan , Rujing Yin, Yiqun Lu , Lubna Dada , Dongsen Yang, Yueyun Fu, Jenni Kontkanen , Chenjuan Deng, Olga Garmash , Jiaxin Ruan, Rima Baalbaki , Meredith Schervish, Runlong Cai, Matthew Bloss, Tommy Chan , Tianzeng Chen , Qi Chen, Xuemeng Chen, Yan Chen, Biwu Chu, Kaspar Dällenbach, Benjamin Foreback, Xucheng He, Liine Heikkinen , Tuija Jokinen, Heikki Junninen, Juha Kangasluoma , Tom Kokkonen , Mona Kurppa, Katrianne Lehtipalo , Haiyan Li, Hui Li , Xiaoxiao Li, Yiliang Liu, Qingxin Ma , Pauli Paasonen , Pekka Rantala , Rosaria E. Pileci , Anton Rusanen, Nina Sarnela , Pauli Simonen , Shixian Wang, Weigang Wang , Yonghong Wang , Mo Xue, Gan Yang, Lei Yao, Ying Zhou, Joni Kujansuu, Tuukka Petäjä, Wei Nie , Yan Ma, Maofa Ge , Hong He, Neil M. Donahue , Douglas R. Worsnop, Veli-Matti Kerminen, Lin Wang , Yongchun Liu, Jun Zheng , Markku Kulmala , Jingkun Jiang , and Federico Bianchi Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Aerosol and Haze Laboratory, Beijing University of Chemical Technology, Beijing, China, Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, University of Helsinki, Helsinki, Finland, School of Environment, State Key Joint Laboratory of Environment Simulation and Pollution Control, Tsinghua University, Beijing, China, Department of Environmental Science & Engineering, Fudan University, Shanghai, China, Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, Nanjing University of Information Science & Technology, Nanjing, China, Center for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, PA, USA, Research Center for EcoEnvironmental Sciences, Chinese Academy of Science, Beijing, China, School of Environmental Sciences, Peking University, Beijing, China, Institute of Chemistry, Chinese Academy of Sciences, Beijing, China, Laboratory of Environmental Physics, Institute of Physics, University of Tartu, Tartu, Estonia, Finnish Meteorological Institute, Helsinki, Finland, Laboratory of Atmospheric Chemistry, Paul Scherrer Institute (PSI), Villigen, Switzerland, Aerosol Physics Laboratory, Physics Unit, Tampere University, Tampere, Finland, School of Atmospheric Sciences, Joint International Research Laboratory of Atmospheric and Earth System Sciences, Nanjing University, Nanjing, China, Aerodyne Research Inc., Billerica, MA, USA Key Points: • Process-level understanding of new particle formation (NPF) in wintertime Beijing was obtained based on measurement state-of-theart instruments • The analysis of sulfuric acid cluster composition and budget showed that sulfuric acid-base clustering initiated NPF • Condensable organic vapors were characterized and demonstrated to have a crucial influence on the growth of newly formed particles Supporting Information: Supporting Information may be found in the online version of this article.

Biogenic particles formed in the Himalaya as an important source of free tropospheric aerosols

Abstract: Aerosols of biogenic and anthropogenic origin affect the total radiative forcing of global climate. Poor knowledge of the pre-industrial aerosol concentration and composition, in particular of particles formed directly in the atmosphere from gaseous precursors, constitutes a large uncertainty in the anthropogenic radiative forcing. Investigations of new particle formation at pre-industrial-like conditions can contribute to the reduction of this uncertainty. Here we present observations taken at the remote Nepal Climate Observatory Pyramid station at 5,079 m above sea level, a few kilometres from the summit of Everest. We show that up-valley winds funnel gaseous aerosol precursors to higher altitudes. During this transport, these are oxidized into compounds of very low volatility, which rapidly form a large number of aerosol particles. These are then transported into the free troposphere, which suggests that the whole Himalayan region may act as an ‘aerosol factory’ and contribute substantially to the free tropospheric aerosol population. Aerosol production in this region occurs mainly via organic precursors of biogenic origin with little evidence of the involvement of anthropogenic pollutants. This process is therefore likely to be essentially unchanged since the pre-industrial period, and may have been one of the major sources that contributes to the upper tropospheric aerosol population during that time. Newly formed biogenic particles in the Himalaya increase free-tropospheric background aerosol concentration by a factor of up to two.

Formation and growth of sub-3 nm particles in megacities: impact of background aerosols

Abstract: New particle formation (NPF) occurs frequently in various atmospheric environments and contributes majorly to the aerosol number budget. In megacities, the high concentrations of gaseous precursors and background aerosols add complexity to this process. Based on long-term measurements (373 days) in urban Beijing, we examine the formation and growth of sub-3 nm particles under the effects of background aerosols, as indicated by the condensation sink (CS) or the Fuchs surface area. The median CS and the median PM2.5 mass concentration for the days with NPF events were 0.03 s−1 and 34 μg m−3, respectively. The high loss rates of both molecular clusters and sub-3 nm particles to background aerosols reduce their atmospheric residence time and suppress their survival. As the key clusters for H2SO4–base nucleation, sulfuric acid dimer and trimer concentrations in Beijing decrease significantly when CS increases and the scavenging becomes stronger. The occurrence of NPF events and the formation of sub-3 nm particles in Beijing is governed by CS. 95% of the observed NPF days occurred with CS values below 0.03 s−1. During NPF events, high concentrations of sub-3 nm particles were formed and they mostly ranged from 103 to 105 cm−3 with a median value of 6.2 × 103 cm−3. Driven by the fast H2SO4–base nucleation, the daily maximum formation rate of 1.5 nm particles in Beijing has a mean value of 77 cm−3 s−1 and is much higher than that in clean environments. However, the mean growth rate of sub-3 nm particles in Beijing was only 2.6 nm h−1, not significantly different from that in clean environments. The relatively low growth rate and the high level of scavenging by background aerosols result in low survival of newly formed particles. The analyses also reinforce prior results on the need to correct conventional methods to adequately quantify the formation and growth rates when analyzing data from megacities with strong coagulation scavenging due to background aerosols. The conventional balance formula underestimates the formation rate of 1.5 nm particles, while the conventional appearance time method overestimates the growth rate of sub-3 nm particles. These findings highlight the governing role of background aerosols in urban NPF.

The driving factors of new particle formation and growth in the polluted boundary layer

Abstract: . New particle formation (NPF) is a significant source of atmospheric particles, affecting climate and air quality. Understanding the mechanisms involved in urban aerosols is important to develop effective mitigation strategies. However, NPF rates reported in the polluted boundary layer span more than 4 orders of magnitude, and the reasons behind this variability are the subject of intense scientific debate. Multiple atmospheric vapours have been postulated to participate in NPF, including sulfuric acid, ammonia, amines and organics, but their relative roles remain unclear. We investigated NPF in the CLOUD chamber using mixtures of anthropogenic vapours that simulate polluted boundary layer conditions. We demonstrate that NPF in polluted environments is largely driven by the formation of sulfuric acid–base clusters, stabilized by the presence of amines, high ammonia concentrations and lower temperatures. Aromatic oxidation products, despite their extremely low volatility, play a minor role in NPF in the chosen urban environment but can be important for particle growth and hence for the survival of newly formed particles. Our measurements quantitatively account for NPF in highly diverse urban environments and explain its large observed variability. Such quantitative information obtained under controlled laboratory conditions will help the interpretation of future ambient observations of NPF rates in polluted atmospheres.

Acid-Base Clusters during Atmospheric New Particle Formation in Urban Beijing.

Abstract: Molecular clustering is the initial step of atmospheric new particle formation (NPF) that generates numerous secondary particles. Using two online mass spectrometers with and without a chemical ionization inlet, we characterized the neutral clusters and the naturally charged ion clusters during NPF periods in urban Beijing. In ion clusters, we observed pure sulfuric acid (SA) clusters, SA-amine clusters, SA-ammonia (NH3) clusters, and SA-amine-NH3 clusters. However, only SA clusters and SA-amine clusters were observed in the neutral form. Meanwhile, oxygenated organic molecule (OOM) clusters charged by a nitrate ion and a bisulfate ion were observed in ion clusters. Acid-base clusters correlate well with the occurrence of sub-3 nm particles, whereas OOM clusters do not. Moreover, with the increasing cluster size, amine fractions in ion acid-base clusters decrease, while NH3 fractions increase. This variation results from the reduced stability differences between SA-amine clusters and SA-NH3 clusters, which is supported by both quantum chemistry calculations and chamber experiments. The lower average number of dimethylamine (DMA) molecules in atmospheric ion clusters than the saturated value from controlled SA-DMA nucleation experiments suggests that there is insufficient DMA in urban Beijing to fully stabilize large SA clusters, and therefore, other basic molecules such as NH3 play an important role.

Contribution of Atmospheric Oxygenated Organic Compounds to Particle Growth in an Urban Environment.

Abstract: Gas-phase oxygenated organic molecules (OOMs) can contribute substantially to the growth of newly formed particles. However, the characteristics of OOMs and their contributions to particle growth rate are not well understood in urban areas, which have complex anthropogenic emissions and atmospheric conditions. We performed long-term measurement of gas-phase OOMs in urban Beijing during 2018-2019 using nitrate-based chemical ionization mass spectrometry. OOM concentrations showed clear seasonal variations, with the highest in the summer and the lowest in the winter. Correspondingly, calculated particle growth rates due to OOM condensation were highest in summer, followed by spring, autumn, and winter. One prominent feature of OOMs in this urban environment was a high fraction (∼75%) of nitrogen-containing OOMs. These nitrogen-containing OOMs contributed only 50-60% of the total growth rate led by OOM condensation, owing to their slightly higher volatility than non-nitrate OOMs. By comparing the calculated condensation growth rates and the observed particle growth rates, we showed that sulfuric acid and its clusters are the main contributors to the growth of sub-3 nm particles, with OOMs significantly promoting the growth of 3-25 nm particles. In wintertime Beijing, however, there are missing contributors to the growth of particles above 3 nm, which remain to be further investigated.

An indicator for sulfuric acid–amine nucleation in atmospheric environments

Abstract: New particle formation (NPF) occurs frequently in various atmospheric environments and it is a major source of ultrafine particles. This study proposes an indicator, I, for the occurrence of NPF in...

Secondary Production of Gaseous Nitrated Phenols in Polluted Urban Environments.

Abstract: Nitrated phenols (NPs) are important atmospheric pollutants that affect air quality, radiation, and health. The recent development of the time-of-flight chemical ionization mass spectrometer (ToF-CIMS) allows quantitative online measurements of NPs for a better understanding of their sources and environmental impacts. Herein, we deployed nitrate ions as reagent ions in the ToF-CIMS and quantified six classes of gaseous NPs in Beijing. The concentrations of NPs are in the range of 1 to 520 ng m-3. Nitrophenol (NPh) has the greatest mean concentration. Dinitrophenol (DNP) shows the greatest haze-to-clean concentration ratio, which may be associated with aqueous production. The high concentrations and distinct diurnal profiles of NPs indicate a strong secondary formation to overweigh losses, driven by high emissions of precursors, strong oxidative capacity, and high NOx levels. The budget analysis on the basis of our measurements and box-model calculations suggest a minor role of the photolysis of NPs (<1 ppb h-1) in producing OH radicals. NPs therefore cannot explain the underestimated OH production in urban environments. Discrepancies between these results and the laboratory measurements of the NP photolysis rates indicate the need for further studies aimed at understanding the production and losses of NPs in polluted urban environments.

Direct field evidence of autocatalytic iodine release from atmospheric aerosol

Abstract: Reactive iodine plays a key role in determining the oxidation capacity, or cleansing capacity, of the atmosphere in addition to being implicated in the formation of new particles in the marine boundary layer. The postulation that heterogeneous cycling of reactive iodine on aerosols may significantly influence the lifetime of ozone in the troposphere not only remains poorly understood but also heretofore has never been observed or quantified in the field. Here, we report direct ambient observations of hypoiodous acid (HOI) and heterogeneous recycling of interhalogen product species (i.e., iodine monochloride [ICl] and iodine monobromide [IBr]) in a midlatitude coastal environment. Significant levels of ICl and IBr with mean daily maxima of 4.3 and 3.0 parts per trillion by volume (1-min average), respectively, have been observed throughout the campaign. We show that the heterogeneous reaction of HOI on marine aerosol and subsequent production of iodine interhalogens are much faster than previously thought. These results indicate that the fast formation of iodine interhalogens, together with their rapid photolysis, results in more efficient recycling of atomic iodine than currently considered in models. Photolysis of the observed ICl and IBr leads to a 32% increase in the daytime average of atomic iodine production rate, thereby enhancing the average daytime iodine-catalyzed ozone loss rate by 10 to 20%. Our findings provide direct field evidence that the autocatalytic mechanism of iodine release from marine aerosol is important in the atmosphere and can have significant impacts on atmospheric oxidation capacity.

Ammonium nitrate promotes sulfate formation through uptake kinetic regime

Abstract: . Although the anthropogenic emissions of SO 2 have decreased significantly in China, the decrease in SO 4 2 - in PM 2.5 is much smaller than that of SO 2 . This implies an enhanced formation rate of SO 4 2 - in the ambient air, and the mechanism is still under debate. This work investigated the formation mechanism of particulate sulfate based on statistical analysis of long-term observations in Shijiazhuang and Beijing supported with flow tube experiments. Our main finding was that the sulfur oxidation ratio (SOR) was exponentially correlated with ambient RH in Shijiazhuang (SOR = 0.15 + 0.0032 × exp⁡ ( RH / 16.2 ) ) and Beijing (SOR = - 0.045 + 0.12 × exp⁡ ( RH / 37.8 ) ). In Shijiazhuang, the SOR is linearly correlated with the ratio of aerosol water content (AWC) in PM 2.5 (SOR = 0.15 + 0.40 × AWC / PM 2.5 ). Our results suggest that uptake of SO 2 instead of oxidation of S(IV) in the particle phase is the rate-determining step for sulfate formation. NH 4 NO 3 plays an important role in the AWC and the change of particle state, which is a crucial factor determining the uptake kinetics of SO 2 and the enhanced SOR during haze days. Our results show that NH 3 significantly promoted the uptake of SO 2 and subsequently the SOR, while NO 2 had little influence on SO 2 uptake and SOR in the presence of NH 3 .

Formation of Nighttime Sulfuric Acid from the Ozonolysis of Alkenes in Beijing

Abstract: . Gaseous sulfuric acid (SA) has received a lot of attention for its crucial role in atmospheric new particle formation (NPF), and for this reason, studies until now have mainly focused on daytime SA when most NPF events occur. While daytime SA production is driven by SO2 oxidation of OH radicals from photochemical origin, the formation of SA during night and its potential influence on particle formation remains poorly understood. Here we present evidence for significant nighttime SA production in urban Beijing during winter, yielding concentrations between 1.0 and 3.0 × 106 cm−3. We found a high frequency (~ 30 %) of nighttime SA events, which are defined by the appearance of a distinct SA peak observed between 20:00 and 04:00 local time, and with the maximum concentration exceeding 1.0 × 106 cm−3. These events mostly occurred during unpolluted nights with low vapor condensation sink. Furthermore, we found that under very clean conditions (visibility > 16.0 km) with abundant ozone (concentration > 2.0 × 1011 cm−3, ~ 7 ppb), the overall sink of SA was strongly correlated with the products of O3, alkenes and SO2 concentrations, suggesting that the ozonolysis of alkenes played a major role in nighttime SA formation under such conditions. This is in light with previous studies showing that the ozonolysis of alkenes can form OH radical and stabilized Criegee intermediate (sCI), both of which are able to oxidize SO2 leading to SA formation. However, we also need to point out that there exist additional sources of SA under more polluted condition, which are not investigated in this study. Moreover, there was a strong correlation between SA concentration and the number concentration of sub-3 nm particles in both clean and polluted nights. Different from forest environments, where oxidized biogenic vapors are the main driver of nighttime clustering, our study demonstrates that the formation of nighttime cluster mode particles in urban environments is mainly driven by nighttime SA production.

A 3D study on the amplification of regional haze and particle growth by local emissions

Abstract: The role of new particle formation (NPF) events and their contribution to haze formation through subsequent growth in polluted megacities is still controversial. To improve the understanding of the sources, meteorological conditions, and chemistry behind air pollution, we performed simultaneous measurements of aerosol composition and particle number size distributions at ground level and at 260 m in central Beijing, China, during a total of 4 months in 2015–2017. Our measurements show a pronounced decoupling of gas-to-particle conversion between the two heights, leading to different haze processes in terms of particle size distributions and chemical compositions. The development of haze was initiated by the growth of freshly formed particles at both heights, whereas the more severe haze at ground level was connected directly to local primary particles and gaseous precursors leading to higher particle growth rates. The particle growth creates a feedback loop, in which a further development of haze increases the atmospheric stability, which in turn strengthens the persisting apparent decoupling between the two heights and increases the severity of haze at ground level. Moreover, we complemented our field observations with model analyses, which suggest that the growth of NPF-originated particles accounted up to ∼60% of the accumulation mode particles in the Beijing–Tianjin–Hebei area during haze conditions. The results suggest that a reduction in anthropogenic gaseous precursors, suppressing particle growth, is a critical step for alleviating haze although the number concentration of freshly formed particles (3–40 nm) via NPF does not reduce after emission controls.

Formation of condensable organic vapors from anthropogenic and biogenic volatile organic compounds (VOCs) is strongly perturbed by NO x in eastern China

Abstract: . Oxygenated organic molecules (OOMs) are the crucial intermediates linking volatile organic compounds (VOCs) to secondary organic aerosols (SOAs) in the atmosphere, but comprehensive understanding of the characteristics of OOMs and their formation from VOCs is still missing. Ambient observations of OOMs using recently developed mass spectrometry techniques are still limited, especially in polluted urban atmospheres where VOCs and oxidants are extremely variable and complex. Here, we investigate OOMs, measured by a nitrate-ion-based chemical ionization mass spectrometer at Nanjing in eastern China, through performing positive matrix factorization on binned mass spectra (binPMF). The binPMF analysis reveals three factors about anthropogenic VOC (AVOC) daytime chemistry, three isoprene-related factors, three factors about biogenic VOC (BVOC) nighttime chemistry, and three factors about nitrated phenols. All factors are influenced by NOx in different ways and to different extents. Over 1000 non-nitro molecules have been identified and then reconstructed from the selected solution of binPMF, and about 72 % of the total signals are contributed by nitrogen-containing OOMs, mostly regarded as organic nitrates formed through peroxy radicals terminated by nitric oxide or nitrate-radical-initiated oxidations. Moreover, multi-nitrates account for about 24 % of the total signals, indicating the significant presence of multiple generations, especially for isoprene (e.g., C5H10O8N2 and C5H9O10N3 ). Additionally, the distribution of OOM concentration on the carbon number confirms their precursors are driven by AVOCs mixed with enhanced BVOCs during summer. Our results highlight the decisive role of NOx in OOM formation in densely populated areas, and we encourage more studies on the dramatic interactions between anthropogenic and biogenic emissions.

Determination of the collision rate coefficient between charged iodic acid clusters and iodic acid using the appearance time method

Abstract: Ions enhance the formation rate of atmospheric aerosol particles, which play an important role in Earth’s radiative balance. Ion-induced nucleation involves the stepwise accretion of neutral monome...

Toward Building a Physical Proxy for Gas-Phase Sulfuric Acid Concentration Based on Its Budget Analysis in Polluted Yangtze River Delta, East China

Abstract: Gaseous sulfuric acid (H2SO4) is a crucial precursor for secondary aerosol formation, particularly for new particle formation (NPF) that plays an essential role in the global number budget of aerosol particles and cloud condensation nuclei. Due to technology challenges, global-wide and long-term measurements of gaseous H2SO4 are currently very challenging. Empirical proxies for H2SO4 have been derived mainly based on short-term intensive campaigns. In this work, we performed comprehensive measurements of H2SO4 and related parameters in the polluted Yangtze River Delta in East China during four seasons and developed a physical proxy based on the budget analysis of gaseous H2SO4. Besides the photo-oxidation of SO2, we found that primary emissions can contribute considerably, particularly at night. Dry deposition has the potential to be a non-negligible sink, in addition to condensation onto particle surfaces. Compared with the empirical proxies, the newly developed physical proxy demonstrates extraordinary stability in all the seasons and has the potential to be widely used to improve the understanding of global NPF fundamentally.

Secondary Formation and Impacts of Gaseous Nitro-Phenolic Compounds in the Continental Outflow Observed at a Background Site in South China.

Abstract: Nitro-phenolic compounds (NPs) have attracted increasing attention because of their health risks and impacts on visibility, climate, and atmospheric chemistry. Despite many measurements of particulate NPs, the knowledge of their gaseous abundances, sources, atmospheric fates, and impacts remains incomplete. Here, 18 gaseous NPs were continuously measured with a time-of-flight chemical ionization mass spectrometer at a background site in South China in autumn and winter. Abundant NPs were observed in the continental outflows from East Asia, with a total concentration up to 122.1 pptv. Secondary formation from the transported aromatics dominated the observed NPs, with mono-NPs exhibiting photochemical daytime peaks and nighttime enrichments of di-NPs and Cl-substituted NPs. The budget analysis indicates that besides the •OH oxidation of aromatics, the NO3• oxidation also contributed significantly to the daytime mono-NPs, while the further oxidation of mono-NPs by NO3• dominated the nocturnal formation of di-NPs. Photolysis was the main daytime sink of NPs and produced substantial HONO, which would influence atmospheric oxidation capacity in downwind and background regions. This study provides quantitative insights on the formation and impacts of gaseous NPs in the continental outflow and highlights the role of NO3• chemistry in the secondary nitro-aromatics production that may facilitate regional pollution.

Atmospheric gaseous hydrochloric and hydrobromic acid in urban Beijing, China: detection, source identification and potential atmospheric impacts

Abstract: . Gaseous hydrochloric (HCl) and hydrobromic acid (HBr) are vital halogen species that play essential roles in tropospheric physicochemical processes. Yet, the majority of the current studies on these halogen species were conducted in marine or coastal areas. Detection and source identification of HCl and HBr in inland urban areas remain scarce, thus limiting the full understanding of halogen chemistry and potential atmospheric impacts in the environments with limited influence from the marine sources. Here, both gaseous HCl and HBr were concurrently measured in urban Beijing, China, during winter and early spring of 2019. We observed significant HCl and HBr concentrations ranging from a minimum value at 1 × 10 8 molecules cm −3 (4 ppt) and 4 × 10 7 molecules cm −3 (1 ppt) up to 6 × 10 9 molecules cm −3 (222 ppt) and 1 × 10 9 molecules cm −3 (37 ppt), respectively. The HCl and HBr concentrations are enhanced along with the increase of atmospheric temperature, UVB and levels of gaseous HNO 3 . Based on the air mass analysis and high correlations of HCl and HBr with the burning indicators (HCN and HCNO), gaseous HCl and HBr are found to be related to anthropogenic burning aerosols. The gas–particle partitioning may also play a dominant role in the elevated daytime HCl and HBr. During the daytime, the reactions of HCl and HBr with OH radicals lead to significant production of atomic Cl and Br, up to 2 × 10 4 molecules cm −3 s −1 and 8 × 10 4 molecules cm −3 s −1 , respectively. The production rate of atomic Br (via HBr + OH) is 2–3 times higher than that of atomic Cl (via HCl + OH), highlighting the potential importance of bromine chemistry in the urban area. On polluted days, the production rates of atomic Cl and Br are faster than those on clean days. Furthermore, our observations of elevated HCl and HBr may suggest an important recycling pathway of halogen species in inland megacities and may provide a plausible explanation for the widespread halogen chemistry, which could affect the atmospheric oxidation in China.

Rapid mass growth and enhanced light extinction of atmospheric aerosols during the heating season haze episodes in Beijing revealed by aerosol–chemistry–radiation–boundary layer interaction

Abstract: . Despite the numerous studies investigating haze formation mechanism in China, it is still puzzling that intensive haze episodes could form within hours directly following relatively clean periods. Haze has been suggested to be initiated by the variation of meteorological parameters and then to be substantially enhanced by aerosol-radiation-boundary layer feedback. However, knowledge on the detailed chemical processes and the driving factors for extensive aerosol mass accumulation during the feedback is still scarce. Here, the dependency of the aerosol number size distribution, mass concentration and chemical composition on the daytime mixing layer height (MLH) in urban Beijing is investigated. The size distribution and chemical composition-resolved dry aerosol light extinction is also explored. The results indicate that the aerosol mass concentration and fraction of nitrate increased dramatically when the MLH decreased from high to low conditions, corresponding to relatively clean and polluted conditions, respectively. Particles having their dry diameters in the size of ~ 400–700 nm, and especially particle-phase ammonium nitrate and liquid water, contributed greatly to visibility degradation during the winter haze periods. The dependency of aerosol composition on the MLH revealed that ammonium nitrate and aerosol water content increased the most during low MLH conditions, which may have further triggered enhanced formation of sulphate and organic aerosol via heterogeneous reactions. As a result, more sulphate, nitrate and water soluble organics were formed, leading to an enhanced water uptake ability and increased light extinction by the aerosols. The results of this study contribute towards a more detailed understanding of the aerosol-chemistry-radiation-boundary layer feedback that is likely to be responsible for explosive aerosol mass growth events in urban Beijing.

Modeling the effect of reduced traffic due to COVID-19 measures on air quality using a chemical transport model: impacts on the Po Valley and the Swiss Plateau regions

Abstract: The spread of COVID-19 has posed serious challenges for the global communities. To reduce the circulation of the infection, governmental bodies have imposed different lockdown measures at various levels of complexity and duration. As a result, a substantial reduction in mobility might have important, yet unknown, implications for air quality. In this study, we applied the Comprehensive Air quality Model with eXtensions (CAMx) to investigate potential changes in air quality and its chemical composition over northern Italy and Switzerland during periods when lockdown measures were enforced. Our results indicated that lockdown measures reduced nitrogen dioxide (NO2) air concentrations by up to 46% and 25% in the Po Valley and Swiss Plateau regions, respectively, whereas fine particulate matter (PM2.5) air concentrations were reduced only by up to 10% and 6%. This highlights the importance of other emission categories other than traffic for the total PM2.5 levels. The analysis of the PM2.5 components indicated that elemental carbon (EC) and particulate nitrate (NO3−) were the species most affected by the lockdown measures, whereas a mild increase in the secondary organic aerosol (SOA) concentrations occurred in the Po Valley, and specifically over the metropolitan area of Milan. Our results indicated that an increase in the oxidation capacity of the atmosphere, i.e. in the ˙OH and ˙NO3 radicals, was mainly responsible for the mild increase in SOA concentrations.

Molecular Composition of Oxygenated Organic Molecules and Their Contributions to Organic Aerosol in Beijing.

Abstract: The understanding at a molecular level of ambient secondary organic aerosol (SOA) formation is hampered by poorly constrained formation mechanisms and insufficient analytical methods. Especially in developing countries, SOA related haze is a great concern due to its significant effects on climate and human health. We present simultaneous measurements of gas-phase volatile organic compounds (VOCs), oxygenated organic molecules (OOMs), and particle-phase SOA in Beijing. We show that condensation of the measured OOMs explains 26-39% of the organic aerosol mass growth, with the contribution of OOMs to SOA enhanced during severe haze episodes. Our novel results provide a quantitative molecular connection from anthropogenic emissions to condensable organic oxidation product vapors, their concentration in particle-phase SOA, and ultimately to haze formation.

Impacts of coagulation on the appearance time method for new particle growth rate evaluation and their corrections

Abstract: . The growth rate of atmospheric new particles is a key parameter that determines their survival probability to become cloud condensation nuclei and hence their impact on the climate. There have been several methods to estimate the new particle growth rate. However, due to the impact of coagulation and measurement uncertainties, it is still challenging to estimate the initial growth rate of sub-3 nm particles, especially in polluted environments with high background aerosol concentrations. In this study, we explore the feasibility of the appearance time method to estimate the growth rate of sub-3 nm particles. The principle of the appearance time method and the impacts of coagulation on the retrieved growth rate are clarified. New formulae in both discrete and continuous spaces are proposed to correct the impacts of coagulation. Aerosol dynamic models are used to test the new formulae. New particle formation in urban Beijing is used to illustrate the importance to consider the impacts of coagulation on sub-3 nm particle growth rate and its calculation. We show that the conventional appearance time method needs to be corrected when the impacts of coagulation sink, coagulation source, and particle coagulation growth are non-negligible compared to the condensation growth. Under the simulation conditions with a constant vapor concentration, the corrected growth rate agrees with the theoretical growth rates. The variation of vapor concentration is found to impact growth rate obtained with the appearance time method. Under the simulation conditions with a varying vapor concentration, the average bias of the corrected 1.5–3 nm particle growth rate range from 6–44 %. During the test new particle formation event in urban Beijing, the corrected condensation growth rate of sub-3 nm particles was in accordance with the growth rate contributed by sulfuric acid condensation, whereas the conventional appearance time method overestimated the condensation growth rate of 1.5 nm particles by 80 %.

Molecular characterization of ultrafine particles using extractive electrospray time-of-flight mass spectrometry.

Abstract: Aerosol particles negatively affect human health while also having climatic relevance due to, for example, their ability to act as cloud condensation nuclei. Ultrafine particles (diameter Dp < 100 nm) typically comprise the largest fraction of the total number concentration, however, their chemical characterization is difficult because of their low mass. Using an extractive electrospray time-of-flight mass spectrometer (EESI-TOF), we characterize the molecular composition of freshly nucleated particles from naphthalene and β-caryophyllene oxidation products at the CLOUD chamber at CERN. We perform a detailed intercomparison of the organic aerosol chemical composition measured by the EESI-TOF and an iodide adduct chemical ionization mass spectrometer equipped with a filter inlet for gases and aerosols (FIGAERO-I-CIMS). We also use an aerosol growth model based on the condensation of organic vapors to show that the chemical composition measured by the EESI-TOF is consistent with the expected condensed oxidation products. This agreement could be further improved by constraining the EESI-TOF compound-specific sensitivity or considering condensed-phase processes. Our results show that the EESI-TOF can obtain the chemical composition of particles as small as 20 nm in diameter with mass loadings as low as hundreds of ng m−3 in real time. This was until now difficult to achieve, as other online instruments are often limited by size cutoffs, ionization/thermal fragmentation and/or semi-continuous sampling. Using real-time simultaneous gas- and particle-phase data, we discuss the condensation of naphthalene oxidation products on a molecular level.

Measurement Report: A Multi-Year Study on the Impacts of Chinese New Year Celebrations on Air Quality in Beijing, China

Abstract: . We investigated the influence of the Chinese New Year (CNY) celebrations on local air quality in Beijing from 2013 through 2019, bringing together comprehensive observations at the newly-constructed Aerosol and Haze Laboratory at Beijing University of Chemical Technology – West Campus (BUCT-AHL) and data from Chinese government air quality measurement stations. In this study, these datasets are used together to provide a detailed analysis of air quality during the CNY over multiple years. Before CNY in 2018, the city of Beijing prohibited the use of fireworks and firecrackers in an effort to reduce air pollution. In 2018 air pollutant concentrations still showed a significant peak during the CNY night, even though not as strong as in previous years, but in 2019, the pollution levels were notably lower. During the studied 7-year study period, it appears that there has been a long-term decrease in CNY related emissions since 2016. Based on our analysis, the pollutants with the most notable spike during CNY were sulfur dioxide and particulate matter, including black carbon. Sulfuric acid concentration followed the sulfur dioxide concentration and showed elevated overnight concentration in CNY 2018, but not notably in 2019. Additionally, spectrometer data and analysis of aerosol particle number size distribution shows direct emissions of particles with diameters around 20 nm during CNY in 2018 and 2019. Meteorological conditions were comparable between the latest two years, indicating that air quality associated with the CNY may be improving, perhaps a positive effect of the restrictions. The longer observations in the future will provide confirmation for these trends.

Measurement Report: New particle formation characteristics at an urban and a mountain station in Northern China

Abstract: . Atmospheric new particle formation (NPF) events have attracted increasing attention for their contribution to the global aerosol number budget, and therefore their effects on climate, air quality, and human health. NPF events are regarded as a regional phenomenon, occurring over a large area. However, the spatial variation of NPF intensity has not been investigated in detail by incorporating both urban and regional measurements. Urban environments have more heterogeneous and freshly emitted NPF precursors as compared to environments with less anthropogenic activity. Here, we provide a comparison of NPF event characteristics – NPF event frequency, particle formation rate, and growth rate – by comparing an urban Beijing site and a background mountain site separated by ~80 km from June 14 to July 14, 2019 as well as give insights into the connection between both locations. During the measurement period, 12 and 13 NPF events were observed at the urban and background mountain sites, respectively, with 9 NPF events observed on the same day at both sites. Although the median condensation sink during the first two hours of the common NPF events was around 0.01 s−1 at both sites, there were notable differences in particle formation rates between the two locations (median of 5.42 cm−3 s−1 at the urban site and 1.13 cm−3 s−1 at the mountain site during the first two hours of common NPF events). Yet, the particle growth rates in the 7–15 nm range for common NPF events were comparable (median of 7.6 nm.h−1 at the urban site and 6.5 nm.h−1 at the mountain site as median values). To understand whether the observed events were connected, we compared air mass trajectories as well as meteorological conditions at both stations. Favorable conditions for the occurrence of regional NPF events were largely affected by air mass transport. Overall, our results demonstrate a clear inhomogeneity of regional NPF within a distance of ~100 km, which should be considered in regional-scale aerosol models when estimating the budget of aerosol load and cloud condensation nuclei.