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Although regional haze adversely affects human health and possibly counteracts global warming from increasing levels of greenhouse gases, the formation and radiative forcing of regional haze on climate remain uncertain. By combining field measurements, laboratory experiments, and model simulations, we show a remarkable role of black carbon (BC) particles in driving the formation and trend of regional haze. Our analysis of long-term measurements in China indicates declined frequency of heavy haze events along with significantly reduced SO2, but negligibly alleviated haze severity. Also, no improving trend exists for moderate haze events. Our complementary laboratory experiments demonstrate that SO2 oxidation is efficiently catalyzed on BC particles in the presence of NO2 and NH3, even at low SO2 and intermediate relative humidity levels. Inclusion of the BC reaction accounts for about 90-100% and 30-50% of the sulfate production during moderate and heavy haze events, respectively. Calculations using a radiative transfer model and accounting for the sulfate formation on BC yield an invariant radiative forcing of nearly zero W m-2 on the top of the atmosphere throughout haze development, indicating small net climatic cooling/warming but large surface cooling, atmospheric heating, and air stagnation. This BC catalytic chemistry facilitates haze development and explains the observed trends of regional haze in China. Our results imply that reduction of SO2 alone is insufficient in mitigating haze occurrence and highlight the necessity of accurate representation of the BC chemical and radiative properties in predicting the formation and assessing the impacts of regional haze.

Unexpectedly frequent severe haze episodes were observed in Beijing during February–March in 2021 after two phases of clean air action plan (2013–2020), yet the causes remained unclear. Here, we conducted real‐time fine particle (PM2.5) composition measurements during January–March in 2021 using a time‐of‐flight aerosol chemical speciation monitor and an aethalometer and compared with those during the coronavirus disease (COVID‐19) period in 2020. Our results showed ubiquitously elevated concentrations of chloride, black carbon (BC), and primary organic aerosol (POA) in 2021, suggesting increased primary emissions during the post‐COVID‐19 period. By using the machine learning‐based random forest (RF) algorithm, we found largely different responses of aerosol changes to meteorology in different months. After decoupling the effects of meteorology, the PM2.5 changes from 2020 to 2021 were reduced from −35.6% to −29.0% in January, −24.1% to −4.5% in February, and +92.6% to +34.2% in March, respectively. Our results demonstrate the dominant roles of stagnant meteorology and secondary production in the formation of severe haze episodes in March 2021. In particular, we found that the compositions of observed and deweathered PM2.5 were fairly similar between 2020 and 2021, and the ratios of secondary OA to secondary inorganic aerosols were close. Our study indicates that decoupling the influence of meteorological conditions is of great importance for better evaluation of mitigating strategies of air pollution due to the large impact of meteorology on the changes in PM2.5 species particularly in a short period. Key Points: Unexpectedly severely haze episodes were observed in March 2021 in BeijingClear increases in primary emissions during the post‐COVID‐19 period in BeijingLargely different responses of aerosol species to meteorology in different months [ABSTRACT FROM AUTHOR]

The formation mechanism of aerosol sulfate during wintertime haze events in China is still largely unknown. As companions, SO2 and transition metals are mainly emitted from coal combustion. Here, we argue that the transition metal-catalyzed oxidation of SO2 on aerosol surfaces could be the dominant sulfate formation pathway and investigate this hypothesis by integrating chamber experiments, numerical simulations and in-field observations. Our analysis shows that the contribution of the manganese-catalyzed oxidation of SO2 on aerosol surfaces is approximately one to two orders of magnitude larger than previously known routes, and contributes 69.2% ± 5.0% of the particulate sulfur production during haze events. This formation pathway could explain the missing source of sulfate and improve the understanding of atmospheric chemistry and climate change. Sulfate aerosols are an important component of wintertime haze events in China, but their production mechanisms are not well known. Here, the authors show that transition metal-catalyzed oxidation of SO2 on aerosol surfaces could be the dominant sulfate formation pathway in Northern China. [ABSTRACT FROM AUTHOR]

Severe haze episodes have occurred frequently in the Seoul metropolitan area (SMA) and throughout East Asian countries, especially during winter and early spring. Although notable progress has been attained in understanding these issues, the causes of severe haze formation have not yet been fully investigated. SMA haze is especially difficult to understand, because the area is impacted by both local emissions from anthropogenic and biogenic activities and emissions transported from upwind sources. Here, we investigated the emission sources and formation processes of particulate matter (PM) during three haze episodes measured in early spring of 2019, from 22 February to 2 April, using a high-resolution aerosol mass spectrometer (HR-AMS). Overall, the average concentration of nonrefractory submicron aerosol (NR-PM 1) + BC (black carbon) was 35.1 µgm-3 , which was composed of 38 % organics, 12 % SO4 , 30 % NO3 , 13 % NH4 , and 5 % BC. The organics had an average oxygen-to-carbon ratio (O/C) of 0.52 and an average organic mass to organic carbon ratio (OM/OC) of 1.86. Seven distinct sources of organic aerosols (OAs) were identified via positive matrix factorization (PMF) analysis of the HR-AMS data: vehicle-emitted hydrocarbon-like OA (HOA), cooking OA (COA), solid-fuel-burning emitted OA (SFOA), and four different types of oxidized secondary OA with varying oxidation degrees and temporal trends. Of the 40 d of the measurement period, 23 were identified as haze days (daily average: >35 µgm-3), during which three severe haze episodes were recorded. In particular, PM 1 concentration exceeded 100 µgm-3 during the first episode when an alert was issued, and strict emission controls were implemented in the SMA. Our results showed that nitrate dominated during the three haze episodes and accounted for 39 %–43 % of the PM 1 concentration on average (vs. 21 %–24 % during the low-loading period), for which there were indications of regional-transport influences. Two regional-transport-influenced oxidized organic aerosols (OOAs), i.e., less oxidized OOA2 (LO-OOA2) and more oxidized OOA2 (MO-OOA2), contributed substantially to the total PM 1 during the haze period (12 %–14 % vs. 7 % during the low-loading period), as well. In contrast, HOA and COA only contributed little (4 %–8 % vs. 4 %–6 % during the low-loading period) to the PM 1 concentration during the haze days, indicating that local emissions were likely not the main reason for the severe haze issues. Hence, from simultaneous downwind (SMA) and upwind (Beijing) measurements using HR-AMS and ACSM (aerosol chemical speciation monitor) over the same period, the temporal variations in PM 1 and each chemical species showed peak values on the order of Beijing (upwind) to the SMA for approximately 2 d. Furthermore, lead (Pb) derived from HR-AMS measurements was observed to increase significantly during the haze period and showed good correlations with MO-OOA2 and LO-OOA2, which is consistent with regional sources. A multiple linear regression model indicated that the transported regionally processed air masses contributed significantly to Pb in the SMA (31 %), especially during the haze period, although local burning was also important by contributing 38 %. The above results suggest that regional transport of polluted air masses might have played an important role in the formation of the haze episodes in the SMA during early spring. [ABSTRACT FROM AUTHOR]

Most previous studies treat regional transport of aerosols as a whole, without distinguishing the transport of secondary aerosols and that of their precursors. A new method of quantifying the transport forms of secondary inorganic aerosols (SIA) using the Nested Air Quality Prediction Modeling System was proposed. The contribution of nonlocal emissions to SIA in the receptor region was divided into three parts: (1) SIA chemically formed by nonlocal emissions in their source regions; (2) SIA chemically formed by nonlocal emissions during transport; and (3) SIA chemically formed by nonlocal emissions in the receptor region, representing transport of precursors. In the North China Plain, the transport of precursors and SIA produced during transport are the two main transport forms. Furthermore, the contribution from transport of precursors increased under polluted conditions in most cities. The results indicate that joint control of precursors is paramount for mitigating air pollution. Plain Language Summary: Uncertainties still exist about the sources of secondary inorganic aerosols (SIA), which are one of the crucial drivers of haze pollution. The spatial distribution of precursors is not exactly consistent with that of aerosols, which is puzzling to policymakers. Most existing studies only treat contribution from regional transport of nonlocal emissions as a whole but do not distinguish the transport of secondary aerosols themselves and that of their precursors. Therefore, it is necessary to quantify the contribution from transport of precursors and secondary aerosols. The Nested Air Quality Prediction Model System with an online tracer‐tagging module was used to investigate the regional sources of SIA in Beijing and surrounding cities. The regional contribution of nonlocal emissions to SIA in the receptor region was divided into three parts: (1) SIA chemically formed by nonlocal emissions in their source region; (2) SIA chemically formed by nonlocal emissions in transport pathway; and (3) SIA chemically formed by nonlocal emissions in the receptor region, indicating transport of precursors. The results showed that transport of precursors and the SIA produced during transport are two main transport forms. The results strongly suggest that the joint control of precursors is important for mitigating air pollution. Key Points: A new method of quantifying the transport forms of secondary inorganic aerosols (SIA) using an air quality model was proposedThe transported SIA in the receptor region is from conversion of nonlocal emissions in the source region, transport path, and receptor regionThe chemical conversion of nonlocal emissions after they leave their source region is crucial in the North China Plain [ABSTRACT FROM AUTHOR]

Black carbon aerosols play an important role in climate change because they directly absorb solar radiation. In this study, the mixing state of refractory black carbon (rBC) at an urban site in Beijing in the early summer of 2018 was studied with a single-particle soot photometer (SP2) as well as a tandem observation system with a centrifugal particle mass analyzer (CPMA) and a differential mobility analyzer (DMA). The results demonstrated that the mass-equivalent size distribution of rBC exhibited an approximately lognormal distribution with a mass median diameter (MMD) of 171 nm. When the site experienced prevailing southerly winds, the MMD of rBC increased notably, by 19 %. During the observational period, the ratio of the diameter of rBC-containing particles (Dp) to the rBC core (Dc) was 1.20 on average for Dc=180 nm, indicating that the majority of rBC particles were thinly coated. The Dp/Dc value exhibited a clear diurnal pattern, with a maximum at 14:00 LST and a Dp growth rate of 2.3 nm h -1 ; higher Ox conditions increased the coating growth rate. The microphysical properties of rBC were also studied. Bare rBC particles were mostly found in fractal structures with a mass fractal dimensions (Dfm) of 2.35, with limited variation during both clean and polluted periods. The morphology of rBC changed with coating thickness increasing. When the mass ratio of nonrefractory matter to rBC (MR) was <1.5, rBC-containing particles were primarily found in external fractal structures, and they changed to a core–shell structure when MR>6 , at which point the measured scattering cross section of rBC-containing particles was consistent with that based on the Mie-scattering simulation. We found that only 28 % of the rBC-containing particles were in core–shell structures with a particle mass of 10 fg in the clean period but that proportion increased considerably, to 45 %, in the polluted period. Due to the morphology change, the absorption enhancement (Eabs) was 12 % lower than that predicted for core–shell structures. [ABSTRACT FROM AUTHOR]

Refractory black carbon (rBC) in the atmosphere is known for its significant impacts on climate. The relationship between the microphysical and optical properties of rBC remains poorly understood and is influenced by its size and mixing state. Mixing state also influences its cloud scavenging potential and thus atmospheric lifetime. This study presents a coupling of a centrifugal particle mass analyser (CPMA) and a single-particle soot photometer (SP2) for the morphology-independent quantification of the mixing state of rBC-containing particles, used in the urban site of Beijing as part of the Air Pollution and Human Health–Beijing (APHH-Beijing) project during winter (10 November–10 December 2016) and summer (18 May–25 June 2017). This represents a highly dynamic polluted environment with a wide variety of conditions that could be considered representative of megacity area sources in Asia. An inversion method (used for the first time on atmospheric aerosols) is applied to the measurements to present two-variable distributions of both rBC mass and total mass of rBC-containing particles and calculate the mass-resolved mixing state of rBC-containing particles, using previously published metrics. The mass ratio between non-rBC material and rBC material (MR) is calculated to determine the thickness of a hypothetical coating if the rBC and other material followed a concentric sphere model (the equivalent coating thickness). The bulk MR (MR bulk) was found to vary between 2 and 12 in winter and between 2 and 3 in summer. This mass-resolved mixing state is used to derive the mass-weighted mixing state index for the rBC-containing particles (χrBC). χrBC quantifies how uniformly the non-rBC material is distributed across the rBC-containing-particle population, with 100 % representing uniform mixing. The χrBC in Beijing varied between 55 % and 70 % in winter depending on the dominant air masses, and χrBC was highly correlated with increased MR bulk and PM 1 mass concentration in winter, whereas χrBC in summer varied significantly (ranging 60 %–75 %) within the narrowly distributed MR bulk and was found to be independent of air mass sources. In some model treatments, it is assumed that more atmospheric ageing causes the BC to tend towards a more homogeneous mixture, but this leads to the conclusion that the MR bulk may only act as a predictor of χrBC in winter. The particle morphology-independent and mass-based information on BC mixing used in this and future studies can be applied to mixing-state-aware models investigating atmospheric rBC ageing. [ABSTRACT FROM AUTHOR]

Biomass burning and primary biological aerosol particles account for an important part of urban aerosols. Floods of studies have been conducted on the chemical compositions of fine aerosols (PM 2.5) in megacities where the haze pollution is one of the severe environmental issues in China. However, little is known about their size distributions in atmospheric aerosols in the urban boundary layer. Here, size-segregated aerosol samples were collected in Beijing during haze and clear days from April 2017 to January 2018. Three anhydrosugars, six primary saccharides and four sugar alcohols in these samples were identified and quantified by gas chromatography/mass spectrometry (GC/MS). Higher concentrations of a biomass burning tracer, levoglucosan, were detected in autumn and winter than in other seasons. Sucrose, glucose, fructose, mannitol and arabitol were more abundant in the bloom and glowing seasons. A particularly high level of trehalose was found in spring, which was largely associated with the Asian dust outflows. Anhydrosugars, xylose, maltose, inositol and erythritol are mainly present in the fine mode (<2.1 µ m), while the others showed the coarse-mode preference. The concentrations of measured tracers of biomass burning particles and primary biological particles in the haze events were higher than those in the non-hazy days, with enrichment factors of 2–10. Geometric mean diameters (GMDs) of molecular markers of biomass burning and primary biological aerosols showed that there was no significant difference in the coarse mode (>2.1 µ m) between the haze and non-haze samples, while a size shift towards large particles and large GMDs in the fine fraction (<2.1 µ m) was detected during the hazy days, which highlights that the stable meteorological conditions with high relative humidity in urban Beijing may favor the condensation of organics onto coarse particles.The contributions of reconstructed primary organic carbon (POC) by tracer-based methods from plant debris, fungal spores and biomass burning to aerosol OC in the total-mode particles were in the ranges of 0.09 %–0.30 % (on average 0.21 %), 0.13 %–1.0 % (0.38 %) and 1.2 %–7.5 % (4.5 %), respectively. This study demonstrates that the contribution of biomass burning was significant in Beijing throughout the whole year with the predominance in the fine mode, while the contributions of plant debris and fungal spores dominated in spring and summer in the coarse mode, especially in sizes >5.8 µ m. Our observations demonstrate that the sources, abundance and chemical composition of urban aerosol particles are strongly size dependent in Beijing, which is important to better understand the environmental and health effects of urban aerosols and should be considered in air quality and climate models. [ABSTRACT FROM AUTHOR]

The effects of aerosols on visibility through scattering and absorption of light and on climate through altering cloud droplet concentration are closely associated with their hygroscopic properties. Here, based on field campaigns in winter and summer in Beijing, we compare the size-resolved hygroscopic parameter (κgf) of ambient fine particles derived by an HTDMA (hygroscopic tandem differential mobility analyzer) to that (denoted as κchem) calculated by an HR-ToF-AMS (high-resolution time-of-flight aerosol mass spectrometer) measurements using a simple rule with the hypothesis of uniform internal mixing of aerosol particles. We mainly focus on contrasting the disparity of κgf and κchem between summer and winter to reveal the impact of atmospheric processes/emission sources on aerosol hygroscopicity and to evaluate the uncertainty in estimating particle hygroscopicity with the hypothesis. We show that, in summer, the κchem for 110, 150, and 200 nm particles was on average ∼10 %–12 % lower than κgf , with the greatest difference between the values observed around noontime when aerosols experience rapid photochemical aging. In winter, no apparent disparity between κchem and κgf is observed for those >100 nm particles around noontime, but the κchem is much higher than κgf in the late afternoon when ambient aerosols are greatly influenced by local traffic and cooking sources. By comparing with the observation from the other two sites (Xingtai, Hebei and Xinzhou, Shanxi) of north China, we verify that atmospheric photochemical aging of aerosols enhances their hygroscopicity and leads to 10 %–20 % underestimation in κchem if using the uniform internal mixing assumption. The effect is found more significant for these >100 nm particles observed in remote or clean regions. The lower κchem likely resulted from multiple impacts of inappropriate application of the density and hygroscopic parameter of organic aerosols in the calculation, as well as influences from chemical interaction between organic and inorganic compounds on the overall hygroscopicity of mixed particles. We also find that local/regional primary emissions, which result in a large number of externally mixed BC (black carbon) and POA (primary organic aerosol) in urban Beijing during traffic rush hour time, cause a 20 %–40 % overestimation of the hygroscopic parameter. This is largely due to an inappropriate use of density of the BC particles that is closely associated with its morphology or the degree of its aging. The results show that the calculation can be improved by applying an effective density of fresh BC (0.25–0.45 g cm -3) in the mixing rule assumption. Our study suggests that it is critical to measure the effective density and morphology of ambient BC, in particular in those regions with influences of rapid secondary conversion/aging processes and local sources, so as to accurately parameterize the effect of BC aging on particle hygroscopicity. [ABSTRACT FROM AUTHOR]

The aerosol liquid water (ALW) content (ALWC), an important component of atmospheric particles, has a significant effect on atmospheric optical properties, visibility and multiphase chemical reactions. In this study, ALWC is determined from aerosol hygroscopic growth factor (GF) and particle number size distribution (PNSD) measurements and is also simulated by ISORROPIA II, a thermodynamic equilibrium model, with measured aerosol chemical composition data taken at an urban site in Beijing from 8 November to 15 December 2017. Rich measurements made during the experiment concerning virtually all aerosol properties allow us not only to derive the ALWC but also to study the contributions by various species for which little has been done in this region. The simulated ALWC including the contribution of organics and the calculated ALWC are highly correlated (coefficient of determination R2=0.92). The ALWC contributed by organics (ALWC Org) accounts for 30%±22% of the total ALWC during the sampling period. These results suggest a significant contribution of organics to ALWC, which is rather different from previous studies that showed negligible contributions by organics. Our results also show that ALWC correlates well with the mass concentrations of sulfate, nitrate, and secondary organic aerosols (SOAs) (R2=0.66 , 0.56 and 0.60, respectively). We further noted that accumulation mode particles play a key role in determining ALWC, dominating among all the aerosol modes. ALWC is an exponential function of ambient relative humidity (RH), whose strong diurnal variation influence the diurnal variation of ALWC. However, there is a 3 h lag between the extremes of ALWC and RH values, due to the diurnal variations in PNSD and aerosol chemical composition. Finally, a case study reveals that ALWC Org plays an important role in the formation of secondary aerosols through multiphase reactions at the initial stage of a heavy-haze episode. [ABSTRACT FROM AUTHOR]

The evolution of haze, involving multiple processes such as nucleation, coagulation, and condensation, may exert complex effects on aerosols' cloud condensation nuclei (CCN) activity and number concentration (NCCN). Based on field campaigns carried out in the winters of 2014 and 2016 in Beijing, we show that NCCN was significantly enhanced by the evolution of haze, substantially driven by the nucleation process (or new particle formation). The enhancement factor of NCCN by such nucleation‐initiated haze episodes, E_NCCN, defined as the ratio of NCCN after haze events to NCCN prior to haze events, ranged from 2.2 to 6.5 at a supersaturation (S) = 0.76% and from 4.2 to 17.3 at S = 0.23%, the magnitude of which partially depends on the severity of the haze event. The enhancements are much greater than those previously observed and those from model simulations of contribution from new particle formation. This suggests that CCN sources from new particle formation may be underestimated, needing reevaluation in polluted environments where the subsequent growth of newly formed particles can last 2–3 days, yielding more CCN‐sized particles. We further quantified that the changes in particle size and composition during the nucleation‐initiated evolution of haze are responsible for > 80% and 12–20%, respectively, of the enhancement in CCN activity. The changes in particle composition had a limited impact because most of the ambient particles were already hydrophilic, with hygroscopic parameters of 0.2–0.65. Key Points: Nucleation‐initiated haze events lead to more CCN‐sized particlesThe effects of changes in particle size and composition on CCN activity during the evolution of haze events are quantifiedThe source of CCN from new particle formation needs reevaluation in polluted regions where the subsequent growth of nucleated particles may last 2–3 days [ABSTRACT FROM AUTHOR]

Wet scavenging is one of the most efficient processes for removing aerosols from the atmosphere. This process is not well constrained in chemical transport models (CTMs) due to a paucity of localized parameterization regarding the below-cloud wet scavenging coefficient (BWSC). Here we conducted field measurements of the BWSC during the Atmospheric Pollution and Human Health Beijing (APHH-Beijing) campaign of 2016. Notably, the observed BWSC values based on the updated aerosol mass balance agree well with another estimation technique, and they fall in a range of 10 -5 s-1. The measurement in this winter campaign, combined with that in summer of 2014, supported an exponential power distribution of BWSCs with rainfall intensity. The observed parameters were also compared with both the theoretical calculations and modeling results. We found that the theoretical estimations can effectively characterize the observed BWSCs of aerosols with sizes smaller than 0.2 µm and larger than 2.5 µm. However, the theoretical estimations were an order of magnitude lower than observed BWSCs within 0.2–2.5 µm , a domain size range of urban aerosols. Such an underestimation of BWSC through a theoretical method has been confirmed not only in APHH-Beijing campaign but also in all the rainfall events in summer of 2014. Since the model calculations usually originated from the theoretical estimations with simplified scheme, the significantly lower BWSC could well explain the underprediction of wet depositions in polluted regions as reported by the Model Inter-Comparison Study for Asia (MICS-Asia) and the global assessment of the Task Force on Hemispheric Transport of Atmospheric Pollutants (TF-HTAP). The findings highlighted that the wet deposition module in the CTMs requires improvement based on field measurement estimation to construct a more reasonable simulation scheme for BWSC, especially in polluted regions. [ABSTRACT FROM AUTHOR]

Rapid economic growth in China over the past 30 years has resulted in significant increases in the concentrations of small particulates (PM 2.5) over the city of Beijing. In addition to health problems, high aerosol loading can impact visibility and thus reduce photolysis rates over the city, leading to potential implications for photochemistry. Photolysis rates are highly sensitive not only to the vertical distribution of aerosols but also to their composition, as this can impact how the incoming solar radiation is scattered or absorbed. This study, for the first time, uses aerosol composition measurements and lidar optical depth to drive the Fast-JX photolysis scheme and quantify the photochemical impacts of different aerosol species during the Air Pollution and Human Health (APHH) measurement campaigns in Beijing in November–December 2016 and May–June 2017. This work demonstrates that severe haze pollution events (PM 2.5 > 75 µg m -3) occur during both winter and summer, leading to reductions in O3 photolysis rates of 27 %–34 % (greatest in winter) and reductions in NO2 photolysis of 40 %–66 % (greatest in summer) at the surface. It also shows that in spite of much lower PM 2.5 concentrations in the summer months, the absolute changes in photolysis rates are larger for both O3 and NO2. In the winter, absorbing species such as black carbon dominate the photolysis response to aerosols, leading to mean reductions in J [O 1 D] and J [ NO2 ] in the lowest 1 km of 24 % and 23 %, respectively. In contrast, in the summer, scattering aerosol such as organic matter dominate the response, leading to mean decreases of 2 %–3 % at the surface and increases of 8 %–10 % at higher altitudes (3–4 km). During these haze events in both campaigns, the influence of aerosol on photolysis rates dominates over that from clouds. These large impacts on photochemistry can have significant implications for concentrations of important atmospheric oxidants such as the hydroxyl radical. Idealized photochemical box model studies show that such large impacts on photochemistry could lead to a 12 % reduction in surface O3 (3 % for OH) due to haze pollution. This highlights that PM 2.5 mitigation strategies could have important implications for the oxidation capacity of the atmosphere both at the surface and in the free troposphere. [ABSTRACT FROM AUTHOR]

Regional transport and heterogeneous reactions have been shown to play crucial roles in haze formation over a megacity cluster centered on Beijing. In this study, the updated Nested Air Quality Prediction Model System (NAQPMS) and the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model were employed to investigate the evolution of aerosols – in terms of the number concentration, size distribution, and degree of aging – in Beijing during six haze episodes between 15 November and 15 December 2016, as part of the Air Pollution and Human Health–Beijing (APHH-Beijing) winter campaign of 2016. The model exhibited reasonable performance not only with respect to mass concentrations of PM 2.5 and its components in Beijing but also regarding the number concentration, size distribution, and degree of aging. We revealed that regional transport played a non-negligible role in haze episodes, with contributions of 14 %–31 % to the surface PM 2.5 mass concentration. The contribution of regional transport to secondary inorganic aerosols was larger than that to primary aerosols (30 %–63 % and 3 %–12 %, respectively). The chemical transformation of SO2 along the transport pathway from source regions to Beijing was the major source of SO42-. We also found that sulfate formed outside Beijing from SO2 emitted in Beijing; this sulfate was then blown back to Beijing and considerably influenced haze formation. Along the transport pathway, aerosols underwent aging, which altered the mass ratio of the coating of black carbon to black carbon (RBC) and the size distribution of number concentrations. During the episodes, the geometric mean diameter (GMD) increased from less than 100 nm at the initial site to approximately 120 nm at the final site (Beijing), and the RBC increased from 2–4 to 4–8. During haze episodes with high humidity, the average contributions of gas and aqueous chemistry, heterogeneous chemistry, and primary emission to sulfate were comparable. However, their relative contributions varied with pollution levels. Primary emissions had the greatest impact under light to moderate pollution levels, whereas heterogeneous chemistry had a stronger effect under high pollution levels. [ABSTRACT FROM AUTHOR]

We explore the impacts of short-term emission controls on haze events in Beijing in October–November 2014 using high-resolution Weather Research and Forecasting model with Chemistry (WRF-Chem) simulations. The model reproduces surface temperature and relative humidity profiles over the period well and captures the observed variations in key atmospheric pollutants. We highlight the sensitivity of simulated pollutant levels to meteorological variables and model resolution and in particular to treatment of turbulent mixing in the planetary boundary layer. We note that simulating particle composition in the region remains a challenge, and we overpredict NH4 and NO3 at the expense of SO4. We find that the emission controls implemented for the Asia-Pacific Economic Cooperation (APEC) summit period made a relatively small contribution to improved air quality (20 %–26 %), highlighting the important role played by favourable meteorological conditions over this period. We demonstrate that the same controls applied under less favourable meteorological conditions would have been insufficient in reducing pollutant levels to meet the required standards. Continued application of these controls over the 6-week period considered would only have reduced the number of haze days when daily mean fine particulate matter exceeds 75 µgm-3 from 15 to 13 d (days). Our study highlights the limitations of current emission controls and the need for more stringent measures over a wider region during meteorologically stagnant weather. [ABSTRACT FROM AUTHOR]

Although nitrogen oxide (NOx) emission controls have been implemented for several years, northern China is still facing high particulate nitrate (NO3-) pollution during severe haze events in winter. In this study, the thermodynamic equilibrium model (ISORROPIA-II) and the Weather Research and Forecast model coupled with chemistry (WRF-Chem) were used to study the efficiency of NH3 emission controls on alleviating particulate NO3- during a severe winter haze episode. We found that particulate- NO3- formation is almost NH3 -limited in extremely high pollution but HNO3 -limited on the other days. The improvements in manure management of livestock husbandry could reduce 40 % of total NH3 emissions (currently 100 kt month -1) in northern China in winter. Consequently, particulate NO3- was reduced by approximately 40 % (on average from 40.8 to 25.7 µ g m -3). Our results indicate that reducing livestock NH3 emissions would be highly effective in reducing particulate NO3- during severe winter haze events. [ABSTRACT FROM AUTHOR]

The North China Plain (NCP) has experienced frequent severe haze pollution events in recent years. While extensive measurements have been made in megacities, aerosol sources, processes, and particle growth at urban downwind sites remain less understood. Here, an aerosol chemical speciation monitor and a scanning mobility particle sizer, along with a suite of collocated instruments, were deployed at the downwind site of Xingtai, a highly polluted city in the NCP, for real-time measurements of submicron aerosol (PM1) species and particle number size distributions during May and June 2016. The average mass concentration of PM1 was 30.5 (±19:4) µgm-3, which is significantly lower than that during wintertime. Organic aerosols (OAs) constituted the major fraction of PM1 (38 %), followed by sulfate (25 %) and nitrate (14 %). Positive matrix factorization with the multilinear engine version 2 showed that oxygenated OA (OOA) was the dominant species in OA throughout the study, on average accounting for 78% of OA, while traffic and cooking emissions both accounted for 11% of OA. Our results highlight that aerosol particles at the urban downwind site were highly aged and mainly from secondary formation. However, the diurnal cycle also illustrated the substantial influence of urban emissions on downwind sites, which are characterized by similar pronounced early morning peaks for most aerosol species. New particle formation and growth events were also frequently observed (58% of the time) on both clean and polluted days. Particle growth rates varied from 1.2 to 4.9 nm h-1 and our results showed that sulfate and OOA played important roles in particle growth during clean periods, while OOA was more important than sulfate during polluted events. Further analyses showed that particle growth rates have no clear dependence on air mass trajectories. [ABSTRACT FROM AUTHOR]

pH is an important property of aerosol particles but is difficult to measure directly. Several studies have estimated the pH values for fine particles in northern China winter haze using thermodynamic models (i.e., E-AIM and ISORROPIA) and ambient measurements. The reported pH values differ widely, ranging from close to 0 (highly acidic) to as high as 7 (neutral). In order to understand the reason for this discrepancy, we calculated pH values using these models with different assumptions with regard to model inputs and particle phase states.We find that the large discrepancy is due primarily to differences in the model assumptions adopted in previous studies. Calculations using only aerosol-phase composition as inputs (i.e., reverse mode) are sensitive to the measurement errors of ionic species, and inferred pH values exhibit a bimodal distribution, with peaks between -2 and 2 and between 7 and 10, depending on whether anions or cations are in excess. Calculations using total (gas plus aerosol phase) measurements as inputs (i.e., forward mode) are affected much less by these measurement errors. In future studies, the reverse mode should be avoided whereas the forward mode should be used. Forward-mode calculations in this and previous studies collectively indicate a moderately acidic condition (pH from about 4 to about 5) for fine particles in northern China winter haze, indicating further that ammonia plays an important role in determining this property. The assumed particle phase state, either stable (solid plus liquid) or metastable (only liquid), does not significantly impact pH predictions. The unrealistic pH values of about 7 in a few previous studies (using the standard ISORROPIA model and stable state assumption) resulted from coding errors in the model, which have been identified and fixed in this study. [ABSTRACT FROM AUTHOR]

We conducted the first real-time continuous vertical measurements of particle extinction (bext), gaseous NO2, and black carbon (BC) from ground level to 260 m during two severe winter haze episodes at an urban site in Beijing, China. Our results illustrated four distinct types of vertical profiles: (1) uniform vertical distributions (37% of the time) with vertical differences less than 5%, (2) higher values at lower altitudes (29%), (3) higher values at higher altitudes (16%), and (4) significant decreases at the heights of ~100-150 m (14%). Further analysis demonstrated that vertical convection as indicated by mixing layer height, temperature inversion, and local emissions are three major factors affecting the changes in vertical profiles. Particularly, the formation of type 4 was strongly associated with the stratified layer that was formed due to the interactions of different air masses and temperature inversions. Aerosol composition was substantially different below and above the transition heights with ~20-30% higher contributions of local sources (e.g., biomass burning and cooking) at lower altitudes. A more detailed evolution of vertical profiles and their relationship with the changes in source emissions, mixing layer height, and aerosol chemistry was illustrated by a case study. BC showed overall similar vertical profiles as those of bext (R² = 0.92 and 0.69 in November and January, respectively). While NO2 was correlated with bext for most of the time, the vertical profiles of bext/NO2 varied differently for different profiles, indicating the impact of chemical transformation on vertical profiles. Our results also showed that more comprehensive vertical measurements (e.g., more aerosol and gaseous species) at higher altitudes in the megacities are needed for a better understanding of the formation mechanisms and evolution of severe haze episodes in China. [ABSTRACT FROM AUTHOR]

Rare and consecutive high-nitrate haze pollution episodes were observed in Beijing in spring 2012. We present detailed characterization of the sources and evolutionary mechanisms of this haze pollution, and focus on an episode that occurred between 15 and 26 April. Submicron aerosol species were found to be substantially elevated during haze episodes, and nitrates showed the largest increase and occupation (average: 32.2%) in non-refractory submicron particles (NR-PM 1 ), which did not occur in other seasons as previously reported. The haze episode (HE) was divided into three sub-episodes, HEa, HEb, and HEc. During HEa and HEc, a shallow boundary layer, stagnant meteorological conditions, and high humidity favored the formation of high-nitrate concentrations, which were mainly produced by three different processes — daytime photochemical production, gas-particle partitioning, and nighttime heterogeneous reactions — and the decline in visibility was mainly induced by NR-PM 1. However, unlike HEa and HEc, during HEb, the contribution of high nitrates was partly from the transport of haze from the southeast of Beijing — the transport pathway was observed at ~ 800–1000 m by aerosol Lidar — and the decline in visibility during HEb was primarily caused by PM 2.5 . Our results provide useful information for air quality improvement strategies in Beijing during Spring. [ABSTRACT FROM AUTHOR]

The Nested Air Quality Prediction Model System (NAQPMS) was used to investigate an extreme regional haze episode persisting over the Beijing-Tianjin-Hebei megacity cluster from November 26 to December 1, 2015. During this extreme haze event, the regional daily mean PM 2.5 exceeded 500 μg/m3. We found that local emissions were the main source of haze over Beijing and Hebei in the early formational stage of this episode. The accumulation of regionally transported, highly aged secondary inorganic aerosols (SIA) along the foot of the mountains was responsible (60%) for the rapid increase of surface PM 2.5 in Beijing between November 30 and December 1, although PM 2.5 concentrations in the source regions of Hebei province were lower. The height of regional transport ranged from 200 to 700 m above ground level, with a slow increase with increasing distance of the source regions from Beijing. This indicates that more attention should be given to point sources at heights of 200–500 m in order to reduce the contribution of transport. The contribution of local emissions to haze in Beijing was mostly concentrated below 300 m above ground level, and was more significant for black carbon (BC) and organic matter (OM) than SIA. Tagging of pollutants by emission time showed that PM 2.5 had been aged before it arrived at Beijing, and PM 2.5 formed one or more days prior to arrival was twice that formed on the arrival day. This suggests that control measures would be more effective if they were implemented two days prior to haze episodes. In contrast to Beijing, haze in Tianjin was governed by transport from outside sources, whereas in cities located in Hebei province this episode resulted from local emissions. [ABSTRACT FROM AUTHOR]

Black carbon (BC) exerts a large impact on climate radiative forcing and public health, and such impacts depend strongly on chemical composition and mixing state. Here a single particle aerosol mass spectrometry (SPA-MS) along with an aerosol chemical speciation monitor was employed to characterize the composition and mixing state of BC-containing particles in summer and winter in Beijing. Approximately 2 million BC-containing particles were chemically analyzed, and the particles were classified into nine and eight different types in summer and winter, respectively, according to mass spectral signatures and composition. The BC-containing particles in summer were dominated by the type of nitrate-related BC (BC-N, 56.7%), while in winter the BC mixed with organic carbon (OC) and sulfate (BCOC-S), and OC and nitrate (BCOC-N) were two dominant types accounting for 44.9% and 16.6%, respectively. The number fractions of BC-N in summer, and BCOC-N and BC-SN in winter increased largely during periods with severe air pollution, suggesting the enhanced secondary formation on BC-containing particles. We also found that the primary emissions of the biomass burning and coal combustion can affect BC mixing state substaintially as indicated by the considerable fraction of BC mixed with levoglucosan and polycyclic aromatic hydrocarbons in winter. Bivariate polar plots and back trajectory analysis indicated that the sulfate-associated BC-containing particles were mostly from regional transport while the nitrate-related type was more from local production. The optical parameter of absorbing Ångström exponents (AAE) of BC was 1.2 and 1.5 in summer and winter, respectively, and the AAE dependence of BC mixing state was also different in the two seasons. While higher fractions of BC-N were observed during lower AAE periods in summer, the variations of dominant OC-related BC-containing particles in winter were fairly stable as a function of AAE. Image 1 • Mixing state and composition of BC-containing particles in summer and winter in Beijing were characterized. • BC mixed with nitrate and organic carbon were dominant types in summer and winter, respectively. • Largely different AAE dependence of BC mixing state between summer and winter were observed. We characterized the mixing state and composition of BC-containing particles in a polluted urban environment, and found that BC was dominantly mixed with nitrate in summer and organic carbon in winter in Beijing. [ABSTRACT FROM AUTHOR]

The accurate derivation of the proportion and absorption enhancement of black carbon (BC)-containing aerosols in the atmosphere is critical to assess their effect on air quality and climate. Here, using the field measured size-resolved volatility shrink factor, BC bulk mass concentration and the BC mass fraction in BC-containing particles in winter Beijing, we retrieved and quantified both the number and mass concentration of (1) non-BC, (2) internally mixed BC and (3) externally mixed BC of ambient fine aerosol particles. The reliability of the retrieval method has been evaluated by comparing with the simultaneously measured data. The number fraction of BC-containing particles accounts for 60–78% of ambient fine particles, with internally (both BC core and coating materials) and externally mixed BC of 51–64% and 9–23%, respectively. Only for nucleated particles on clean days, when nucleation is a major source of aerosol particles, did the non-BC component dominate (54%). A large amount of aerosols are BC-containing particles, with mass fraction of 32–52%, suggesting the dominant role of BC in elevating mass concentration of particulate matter (PM) in a polluted urban area. We also show that the BC particles are thickly coated with coating thickness (characterized by D p /D c , ratio of the BC diameter before and after heating at 300 °C) of 1.6–2.2, implying efficient aging of BC particles in polluted urban area. Our results imply a large proportion of BC-containing particles in the atmosphere, which could help towards understanding the role of BC on regional haze formation and climate forcing. Image 1 • Mixing state of BC was retrieved using a new method. • BC-containing particles account for large proportions of ambient fine aerosols. • The BC-containing particles are thickly coated. Main findings of this work: 1. BC-containing particles account for large proportion. 2. The BC-containing particles are thickly coated. [ABSTRACT FROM AUTHOR]

Despite substantial mitigation of particulate matter (PM) pollution during the past decade in Beijing, the response of aerosol chemistry to clean air action and meteorology remains less understood. Here we characterized the changes in aerosol composition as responses to emission reductions by using two-year long-term measurements in 2011/2012 and 2017/2018, and WRF-Chem model. Our results showed substantial decreases for all aerosol species except nitrate from 2011/2012 to 2017/2018. Chloride exhibited the largest decrease by 65–89% followed by organics (37–70%), mainly due to reductions in coal combustion emissions in winter and agriculture burning in June. Primary and secondary organic aerosol (SOA) showed comparable decreases by 61–70% in fall and winter, and 34–63% in spring and summer, suggesting that reductions in primary emissions might also suppress SOA formation. The changes in nitrate were negligible and even showed increases due to less reductions in NO x emissions and increased formation potential from N 2 O 5 heterogeneous reactions. As a result, nitrate exceeded sulfate and became the major secondary inorganic aerosol species in PM with the contribution increasing from 14–21% to 22–32%. Further analysis indicated that the reductions in aerosol species from 2011/2012 to 2017/2018 were mainly caused by the decreases of severely polluted events (PM 1 > 100 μg m−3). WRF-Chem simulations suggested that the decreases in OA and sulfate in fall and winter were mainly resulted from emission reductions (27–36% and 25–43%) and favorable meteorology (4–10% and 19–30%), while they were dominantly contributed by emission changes in spring and summer. Comparatively, the changes in nitrate were mainly associated with meteorological variations while the contributions of emissions changes were relatively small. Our results highlight different chemical responses of aerosol species to emission changes and meteorology, suggesting that future mitigation of air pollution in China needs species-targeted control policy. Image 1 • Substantial decreases for organics and chloride from 2011/2012 to 2017/2018. • Small changes and even increases for nitrate in Beijing during the past six years. • Decreases in aerosol species were driven by the reductions of severely haze events. • Different chemical responses of aerosol species to emission change and meteorology. [ABSTRACT FROM AUTHOR]

Abstract: A 4-year campaign from 2001 to 2004 monitoring PM2.5 and TSP in the spring season in urban Beijing, China was performed to study the variation of characteristics and the different formation mechanisms of aerosols in dust, haze, and clear days. A total of 315 aerosol samples were collected and used in this study. The aerosols were more basic in dust days and more acidic in haze days. The ions presented in the order of >Ca2+≫>Cl−>>Na+ in dust days, and of >>≫Cl−>Ca2+>K+ in haze days. Ions has been classified into three groups, “Na+, Mg2+, Ca2+”, “K+, , Cl−”, and “, ”, representing crust, pollution-crust, and pollution species, respectively. Crust and pollution ions were the main ion fractions in dust and haze days, respectively. The variation of Ca2+/Al showed that the increase of dust in dust and haze days was from soil and construction, respectively. “CaCO3, CaSO4, and (NH4)2SO4” and “(NH4)2SO4, NH4NO3, and Ca(NO3)2” were the major species in dust and haze days, respectively. The formation of CaSO4 on airborne soil particles and the formation of (NH4)2SO4 and NH4NO3 were the predominant pathways of sulfate and nitrate formations in dust and haze days, respectively. Sulfate might be mainly formed through heterogeneous reactions in the aqueous surface layer on the pre-existing particles, while nitrate mainly through homogeneous gas-phase reactions in the spring season in Beijing. The formation of sulfate and nitrate was accelerated in dust and haze days. [Copyright &y& Elsevier]

The light absorption enhancement (E abs) of black carbon (BC) caused by non-BC materials is an important source of uncertainty in radiative forcing estimate, yet remains poorly understood in relatively polluted environment such as the megacity Beijing. Here BC absorption enhancement at 630 nm was in-situ measured using a thermodenuder coupled with a soot particle aerosol mass spectrometer and a single scattering albedo monitor in Beijing in summer. The project average (±1σ) E abs was 1.59 (±0.26), suggesting a significant amplification of BC absorption due to coating materials. E abs presented a clear daytime increase due to enhanced photochemical processing, and a strong dependence on the mass ratios of non-BC coatings to BC (R BC). Our results showed that the increase in E abs as a function of R BC was mainly caused by the increased contributions of secondary aerosol. Further analysis showed that the BC absorption enhancement in summer in Beijing was mainly associated with secondary formation of nitrate, sulfate and highly oxidized secondary organic aerosol (SOA), while the formation of freshly and less oxidized SOA appeared not to play an important role. Image 1 • A significant BC absorption enhancement (∼1.6) in summer in Beijing. • Light absorption enhancement depends strongly on the amount of BC coatings. • SIA and highly oxidized SOA play the most impacts on absorption enhancement. [ABSTRACT FROM AUTHOR]

Abstract HONO is an important precursor of OH radical and plays a key role in atmospheric chemistry, but its source and formation mechanism remain uncertain, especially during complex atmospheric pollution processes. In this study, HONO mixing ratios were measured by a custom-made instrument during a severe pollution event from 16 to 23 December 2016, at an urban area of Beijing. The measurement was divided into three periods: I (haze), II (severe haze) and III (clean), according to the levels of PM 2.5. This pollution episode was characterized by high levels of NO (75 ± 39 and 94 ± 40 ppbV during periods I and II, respectively) and HONO (up to 10.7 ppbV). During the nighttime, the average heterogeneous conversion frequency during the two haze periods were estimated to be 0.0058 and 0.0146 h−1, and it was not the important way to form HONO. Vehicle emissions contributed 52% (±16)% and 40% (±18)% to ambient HONO at nighttime during periods I and II. The contribution of homogeneous reaction of NO with OH should be reconsidered under high-NO x conditions and could be noticeable to HONO sources during this pollution event. Furthermore, HONO was positively correlated with PM 2.5 during periods I and II, suggesting a potential chemical link between HONO and haze particles. Graphical abstract Unlabelled Image Highlights • High HONO concentrations were observed in a severe pollution episode in Beijing. • The NO 2 conversion was not an important pathway to form HONO at nighttime. • Direct emissions could be critical to the observed HONO. • The homogeneous reaction could be a noticeable HONO source. • Good relation of HONO with PM 2.5 implied a chemical link between HONO and PM 2.5. [ABSTRACT FROM AUTHOR]

To investigate the composition, sources and evolution processes of submicron aerosol during wintertime, a field experiment was conducted during December 1–31, 2013 in urban Nanjing, a megacity in Yangtze River Delta of China. Non-refractory submicron aerosol (NR-PM 1 ) species were measured with an Aerodyne Aerosol Chemical Speciation Monitor. NR-PM 1 is dominated by secondary inorganic aerosol (55%) and organic aerosol (OA, 42%) during haze periods. Six OA components were identified by positive matrix factorization of the OA mass spectra. The hydrocarbon-like OA and cooking-related OA represent the local traffic and cooking sources, respectively. A highly oxidized factor related to biomass burning OA accounted for 15% of the total OA mass during haze periods. Three types of oxygenated OA (OOA), i.e., a less-oxidized OOA (LO-OOA), a more-oxidized OOA (MO-OOA), and a low-volatility OOA (LV-OOA), were identified. LO-OOA is likely associated with fresh urban secondary OA. MO-OOA likely represents photochemical products showing a similar diurnal cycle to nitrate with a pronounced noon peak. LV-OOA appears to be a more oxidized factor with a pronounced noon peak. The OA composition is dominated by secondary species, especially during haze events. LO-OOA, MO-OOA and LV-OOA on average account for 11%, (18%), 24% (21%) and 23% (18%) of the total OA mass for the haze (clean) periods respectively. Analysis of meteorological influence suggested that regional transport from the northern and southeastern areas of the city is responsible for large secondary and low-volatility aerosol formation. [ABSTRACT FROM AUTHOR]

Organic aerosol (OA) generally accounts for a large fraction of fine particulate matter (PM 2.5) in the urban atmosphere. Despite significant advances in the understanding their emission sources, transformation processes and optical properties in the submicron aerosol fraction (PM 1), larger size fractions - e.g., PM 2.5 - still deserve complementary investigations. In this study, we conducted a comprehensive analysis on sources, formation process and optical properties of OA in PM 1 and PM 2.5 under haze and foggy environments in the Yangtze River Delta (eastern China), using two aerosol chemical speciation monitors, as well as a photoacoustic extinctiometer at 870 nm. Positive matrix factorization analysis - using multilinear engine (ME2) algorithm - was conducted on PM 1 and PM 2.5 organic mass spectra. Four OA factors were identified, including three primary OA (POA) factors, i.e., hydrocarbon-like OA (HOA), cooking OA (COA), and biomass burning OA (BBOA), and a secondary OA (SOA) factor, i.e., oxidized oxygenated OA (OOA). An enhanced PM 1-2.5 COA concentration was clearly observed during cooking peak hours, suggesting important contribution of fresh cooking emissions on large-sized particles (i.e., PM 1-2.5). The oxidation state and concentration of PM 2.5 HOA were higher than that in PM 1 , suggesting that large-sized HOA particles might be linked to oxidized POA. High contribution (44%) of large-sized OOA to non-refractory PM 2.5 mass was observed during haze episodes. During foggy episodes, PM 1 and PM 2.5 OOA concentrations increased as a positive relationship over time, along with an exponential increase in the PM 2.5 -OOA to PM 1 -OOA ratio. Meanwhile, OOA loadings increased with the aerosol liquid water content (ALWC) during foggy episodes. Random forest cross-validation analysis also supported the important influence of ALWC on OOA variations, supporting substantial impact of aqueous process on SOA formation during haze and/or foggy episodes. Obtained results also indicated high OOA contributions (21%–36%) and low POA contributions (6%–14%) to the PM 2.5 scattering coefficient during haze and foggy episodes, respectively. Finally, we could illustrate that atmospheric vertical diffusion and horizontal transport have important but different effects on the concentrations of different primary and secondary OA factors in different particle size fractions. • Sources of OA in non-refractory PM1 and PM2.5 were quantified with a ME-2 model. • Evident increase of large-sized COA during cooking peak hours was observed. • Enhanced OOA formation and the OOA2.5 to OOA1 ratio were observed during foggy episodes. • OOA formation linked to aqueous process during foggy episodes was investigated. [ABSTRACT FROM AUTHOR]

The abundance and behaviour of metals and water-soluble metals (V, Cr, Mn, Fe, Cu, Zn, As, Sr, Ag, Cd, Sn, Sb, Ba and Pb) in size-fractionated aerosols were investigated during two typical episodes in Beijing. Water-soluble inorganic ions (Na + , K + , Mg 2+ , Ca 2+ , N H 4 + , F − , Cl − , SO 4 2 − and NO 3 − ) were also measured. Atmospheric metals and water-soluble metals were both found at high levels; for PM 2.5 , average As, Cr, Cd, Cu, Mn and Pb concentrations were 14.8, 203.3, 2.5, 18.5, 42.6 and 135.3 ng/m 3 , respectively, and their water-soluble components were 11.1, 1.7, 2.4, 14.5, 19.8 and 97.8 ng/m 3 , respectively. Daily concentrations of atmospheric metals and water-soluble metals were generally in accordance with particle mass. The highest concentrations of metals and water-soluble metals were generally located in coarse mode and droplet mode, respectively. The lowest mass of metals and water-soluble metals was mostly in Aitken mode. The water solubility of all metals was low in Aitken and coarse modes, indicating that freshly emitted metals have low solubility. Metal water solubility generally increased with the decrease in particle size in the range of 0.26–10 μm. The water solubility of metals for PM 10 was: 50% ≤ Cd, As, Sb, Pb; 26% < V, Mn, Cu, Zn and Sr ≤ 50%; others ≤20%. Most metals, water-soluble metals and their water solubility increased when polluted air mass came from the near west, near north-west, south-west and south-east of the mainland, and decreased when clean air mass came from the far north-west and far due south. The influence of dust-storms and clean days on water-soluble metals and size distribution was significant; however, the influence of rainfall was negligible. Aerosols with high concentrations of SO 4 2 − , K + and N H 4 + might indicate increased potential for human health effects because of their high correlation with water-soluble metals. Industrial emissions contribute substantially to water-soluble metal pollution as water-soluble metals show higher correlation with Cd, Sn, Sb and Pb that are mainly derived from industrial sources. [ABSTRACT FROM AUTHOR]

A variety of legislative actions for air quality improvement have been conducted in China since 2013, and the emission control measures have achieved remarkable reduction in severe haze frequency. The composition of the fine particles may change along with the improved air quality, and thus may induce distinct environmental and climate effects in future. In this study, a very recent long-term dataset (2018–2019) of non-refractory chemical composition measured by a quadrupole aerosol chemical speciation monitor (Q-ACSM) observed in urban Beijing is applied to investigate the changes in ratio of nitrate (NO 3 −) to sulfate (SO 4 2−) in PM 2.5 (particulate matter with diameters of less than 2.5 μm). We show that the ratio of NO 3 − to SO 4 2− varies seasonally, with a maximum in winter (1.6 ± 1.2) and a minimum in summer (0.7 ± 1.0). Compared with results from earlier studies showing a continuous increase in the ratio of NO 3 − to SO 4 2− since 1999, a decline in the ratio is found during the period of 2018–2019. This is partially associated with an attenuated nitrate formation likely due to reduced nitrogen oxides emissions since 2016 in China. Our results suggest that the strict reduction control measures in place serve only to improve SO 4 2− pollution in winter but not in summer when high SO 4 2− levels are still observed. SO 4 2− and NO 3 − concentrations during study periods together comprise 37–53% of PM 2.5 , presenting significant role in dominating the levels of PM 2.5. In addition, we show that the ratio of NO 3 −/SO 4 2− in warm seasons generally increases with increasing relative humidity (RH) due to enhanced NO 3 − hydrolysis formation, but with a maximum value of only ~1.0 that is pulled down by the high levels of sulfate in summer, while the dependence of the ratios on RH is more pronounced in cold seasons when multiple factors (regional transportation, planetary boundary layer, PBL and sources of sulfate, etc.) can impact nitrate levels. We finally characterize two typical processes that lead to the rapid accumulation of nitrate in the atmosphere over urban Beijing: the regional transportation and PBL variations, which is found driving heavy haze in cold seasons, and the hydrolysis formation and partitioning of NO 3 − that tends to impact the diurnal patterns of nitrate in warm seasons. • The ratio of NO 3 − to SO 4 2− in 2018–2019 declined. • A continuous increase in the ratio of NO 3 − to SO 4 2− of 1999–2018 is presented. • The reduction measures in place serve only to improve SO 4 2− pollution in winter. • High SO 4 2− levels are still observed in summer. [ABSTRACT FROM AUTHOR]

Water-soluble organic aerosol (WSOA) constitutes a large fraction of OA and plays an important role in formation of secondary OA (SOA). Here we characterized the sources and molecular composition of WSOA in summer in Beijing using high-resolution aerosol mass spectrometer and orbitrap mass spectrometer equipped with electrospray ionization. Our results showed that WSOA was the major fraction of OA on average accounting for 69% in summer, which is much higher than that (47%) in winter. However, the oxidation degree of WSOA was comparable between summer and winter (O/C = 0.62 vs. 0.63). Positive matrix factorization analysis showed that SOA contributed dominantly to WSOA (72%) indicating that WSOA was mainly from secondary formation. The two water-soluble SOA factors that are associated with regional processing (OOA-1) and photochemical production (OOA-2), respectively, showed very different behaviors throughout the study. OOA-2 showed much enhanced contribution during polluted periods with low relative humidity (RH), while OOA-1 played a more important role during high RH periods. Molecular composition analysis of WSOA revealed a high diversity of CHO (compounds only containing carbon, hydrogen and oxygen) and CHOS (sulfur-containing organics) in WSOA in summer. Particularly, the relative intensity fraction of CHOS- compounds was increased by 42% from clean to polluted days which was associated with large increases (20%) in organosulfates (OSs) with lower O∗/C (0.1–0.4), and OOA-1. These results suggest the formation of more unsaturated OSs in OOA-1 during polluted days in summer. Comparatively, the biogenic-derived OSs remained relatively stable (24–31%) for the entire study highlighting the ubiquitous importance of biogenic SOA in summer. Image 1 • Water-soluble organic aerosol was the major fraction of OA in summer in Beijing. • SOA from regional and photochemical processing contributed dominantly to WSOA. • A high diversity of CHO and CHOS molecular compounds in WSOA was observed. • Formation of more organosulfates during polluted days than clean days in summer. [ABSTRACT FROM AUTHOR]

A cavity attenuated phase shift single scattering albedo monitor was set up on a tethered airship platform to study the vertical profiles of particle light extinction coefficient (b ext) in the lower troposphere (<1000 m) in Shanghai during 12–29 December 2015. Clear transition heights (THs) for vertical profiles of b ext during the polluted days (PM 2.5 > 75 μg m−3) were observed below 1000 m. The vertical differences of b ext were highly dynamic as the vertical variation in b ext was significant by as much as 605 Mm−1. The TH was observed mostly at about 100–200 m, and 450–650 m during night and daytime, respectively, and was in a wide range of ∼50–900 m during 15:00–22:00 due to the low boundary layer and/or the transport of pollutants. In particular, the TH was consistently below 500 m throughout the day during highly polluted haze episodes, highlighting the important role of a stagnant atmosphere situation for high concentrations of PM 2.5. The vertical distribution of b ext did not have a constant rule with respect to relative humidity and wind. Sometimes, peak values of b ext at ∼350 m and 500 m during daytime were caused by enhanced regional transport. During stagnant and highly polluted situations or well-mixed clean days, b ext was usually uniformly distributed below and above the TH, respectively, although b ext was much smaller above the TH. For other situations, local emissions, pollutant transport, and the physical and chemical characteristics of aerosols resulted in highly dynamic vertical profiles of b ext. • The vertical variations of b ext were dynamic within 1000 m. • Transition heights (THs) for b ext were below 1000 m during the polluted days. • b ext was usually uniform below and above the TH during clean and heavy haze episodes. • The TH was consistently below 500 m throughout the day during heavy haze episodes. • Flexible rules were found between b ext and relative humidity/wind within 1000 m. [ABSTRACT FROM AUTHOR]

In Beijing poor urban air quality has a demonstrable effect on human health. During thewintertime, anthropogenic emissions from fossil fuel combustion can lead to high aerosolloadings and haze events. A high oxidation capacity on hazy days has previously beeninferred from the significant contribution secondary organic aerosol (SOA) make to totalPM2.5 (Huang et al., 2014). The hydroxyl radical (OH) mediates virtually all of the oxidativechemistry in the atmosphere, being responsible for the transformation of primaryemissions into secondary pollutants such as NO2, O3 and SOA. Understanding thesources and sinks of OH in the atmosphere is essential in improving predictions of thelifetimes and chemical processing of pollutants and their transport within urbanareas. We will present OH and HO2 measurements made in central Beijing during the recent'An Integrated Study of AIR Pollution PROcesses in Beijing (AIRPRO)&#x2019; project which tookplace in November and December 2016. OH measurements were made using the FAGE(Fluorescence Assay by Gas Expansion) technique, with the use of an inlet preinjector (IPI) which provides an alternative method to determine the background byinjecting a scavenger (propane) to remove ambient OH. The OH measurementswere made over a range of meteorological conditions including a number of hazedays, with the average maximum OH concentration measured for the campaignbeing ∼2.5 x 106 cm−3 and for haze days the OH concentration reached levels of∼3.5 x 106 cm−3 which is comparable to OH levels in non-haze days. We willcompare the OH observations to steady state calculations constrained to the total OHreactivity and key OH precursors that were measured alongside OH. Through thiscomparison we will identify the major OH sources which sustain the wintertime oxidationcapacity. The current understanding is that gas-phase oxidation via the OH radical becomes lessimportant in haze events due to lower light and ozone levels, making photochemistry lessimportant (Wang et al.,(2016). However, we will present evidence by comparing thedependence of the OH radical concentrations with PM2.5 levels that the OH concentrationpersists throughout haze events, and that there may be a missing source of HOx under hazeconditions. This finding implies an uncertainty in oxidation capacity during haze events. [ABSTRACT FROM AUTHOR]

Chemical mechanisms responsible for rapid sulfate production, an important driver of winter haze formation in northern China, remain unclear. We propose a potentially important heterogeneous hydroxymethanesulfonate (HMS) chemical mechanism. Through analyzing field measurements with aerosol mass spectrometry, we show evidence for a possible significant existence in haze aerosols of organosulfur primarily as HMS, misidentified as sulfate in previous observations. We estimate that HMS can account for up to about one-third of the sulfate concentrations unexplained by current air quality models. In addition, HMS in the presence of hydroxyl radicals can trigger rapid sulfate production in aerosol water. Heterogeneous production of HMS by SO2 and formaldehyde is favored under northern China winter haze conditions due to high aerosol water content, moderately acidic pH values, high gaseous precursor levels, and low temperature. These analyses identify an unappreciated importance of formaldehyde in secondary aerosol formation and calls for more research on sources and on the chemistry of formaldehyde in northern China winter. [ABSTRACT FROM AUTHOR]

Atmospheric aerosol concentrations have increased globally mainly due to anthropogenic activities such as industry, heating, transport and cooking. Increased aerosol concentrations can result in pollution events, particularly in urban mega-cities. The severity of pollution events is often defined by concentrations of PM2.5 (particulate matter with a diameter of 2.5 μm or less), which has an impact on both the climate and human health. Beijing, is a megacity which is well-known for poor air quality, often subjected to heavy pollution events termed 'haze', of which PM2.5 is a major contributor. The extent to which aerosols absorb or scatter radiation is dependent on their size and composition; the impact of this interaction can lead to atmospheric warming or cooling. Meteorological measurements in Beijing show that polluted periods are accompanied by a decrease in temperature in the lower atmosphere and an increase in the upper atmosphere, reducing turbulent mixing due to buoyancy and lowering the planetary boundary layer height. Water vapour and aerosols become more concentrated in a smaller volume and so the aerosols swell and interact with radiation to a greater extent. These interactions between aerosols, radiation and meteorology thus form a positive feedback loop, exacerbating pollution episodes. Observations in several cities have established the existence of this feedback loop and several regional modelling studies have examined the importance of aerosol-radiation interactions in enhancing polluted episodes. Through the use of a Large Eddy Scale model coupled to a Sectional Aerosol Module (UCLALES-SALSA), this work allows for much higher sensitivity studies on the impact of aerosol size and composition upon meteorology. Through the use of both meteorological and aerosol measurements taken in November-December 2016, I will present case studies quantifying the impact of changing aerosol characteristics on meteorology for a polluted multi-day period. This work will also identify whether aerosol size or composition is of more importance in prolonging and enhancing polluted episodes. [ABSTRACT FROM AUTHOR]