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The oxidative potential (OP) of ambient particulate matter (PM) is a common metric for estimating PM toxicity and linking PM exposure to adverse health effects. Organic aerosol (OA), a dominant fraction of ambient PM worldwide, may significantly contribute to PM toxicity. Here, we investigated the source‐based OA components contributing to the OP of PM in the urban (Beijing, summer and winter) and rural (Gucheng, winter) environments of the North China Plain (NCP). Various OA components as identified by the aerosol mass spectrometer/aerosol chemical speciation monitor (AMS/ACSM), transition metals, and black carbon were compared with the OP of PM measured by dithiothreitol assays. The results consistently demonstrate the importance of OA as a contributor to PM's OP in both urban and rural NCP environments. Higher intrinsic OP was observed in winter Beijing than in summer, possibly due to OA being predominantly from anthropogenic sources in winter. Furthermore, different OA components were found to drive the response of OP in the two environments. More‐oxidized oxygenated OA (MO‐OOA), cooking OA, and oxidized primary OA (during winter) are the OA contributors to OP in the urban environment, with a dominant contribution from MO‐OOA. In contrast, biomass burning OA (BBOA) and OOA play a major role in the OP in the rural environment, with BBOA making the largest contribution. Overall, this work highlights the significance of OA in determining PM's OP and calls for more work to reveal the sources and characteristics of OA components contributing to OP across different regions. Plain Language Summary: Ambient fine particulate matter (PM2.5) is a top risk factor for human health. The oxidative potential (OP) of PM2.5 is widely used to estimate PM2.5 toxicity and link PM2.5 exposure with its adverse health effects. Organic aerosol (OA) constitutes a major fraction of PM2.5 and can contribute substantially to the OP of PM2.5. But OA is also a complex mixture comprised of thousands of compounds each with widely different health effects. Here, we reveal the key OA components driving the OP in the urban and rural environments of the NCP. We find that more‐oxidized oxygenated OA, which is highly oxidized and can be formed from multiple atmospheric processes, is a dominant contributor to the OP in the urban environment. In contrast, biomass burning OA makes the largest contribution to the OP in the rural environment. In sum, different OA components are found to be the main drivers of the OP in the two environments. More studies are warranted to further our understanding of the causes, sources, and characteristics of the PM2.5 OP over a wide range of geographical regions. Key Points: Seasonal and spatial variations of oxidative potential (OP) of ambient particulate matter are foundOrganic aerosol (OA) is an important contributor to the oxidative potential in both urban and rural environmentsMore‐oxidized oxygenated OA and biomass burning OA have a dominant contribution to the OP in urban and rural environments, respectively [ABSTRACT FROM AUTHOR]

Benefiting from a series of pollution prevention initiatives, fine particle (PM2.5) pollution was effectively controlled in 2018–2020, but ground-level ozone pollution during warm season has become a major issue for the continuous air quality improvement in Beijing. As a key source of hydroxyl (OH) radical, nitrous acid (HONO) has attracted much attention for its important role in the atmospheric oxidant capacity (AOC) increase; the elucidation of the pollution characteristics, unknown sources and the contribution to secondary pollution of HONO has become a research hotspot. In this study, we made a comparative study on the ambient levels, variation patterns, sources and formation pathway in warm season (from June to October in 2021) on a basis of a continuous intensive observation in an urban site of Beijing. The monthly average mixing ratio of HONO were 1.26 ppb, 1.28 ppb, 1.01 ppb, 0.96 ppb, 0.89 ppb, respectively, showing a larger contribution to OH radical relative to ozone at daytime with a mean OH production rate of 2.70, 2.91, 2.00, 2.25, and 0.93 ppb/h, respectively. The emission factor from the vehicle emissions, Pemis, was estimated to be 0.017, higher than most studies conducted in Beijing, the observation site and traffic control policies could affect this phenomenon. The homogeneous production of HONO via reaction of NO + OH, PnetOH+NO, in each month were 0.050, 0.045, 0.033, 0.052, and 0.17 ppb/h, respectively. The average nocturnal NO2 to HONO conversion frequency CHONO in each month were 0.011 h−1, 0.0096 h−1, 0.013% h−1, 0.0081 h−1, and 0.0017 h−1, respectively. In warm seasons, the missing source of HONO, Punknown, around noontimewere 0.29–2.37 ppb/hr. Punknown in each month might be various. According to the observation results, relatively low humidity and strong solar illumination were conductive to HONO formation in June, which might be due to light-induced heterogeneous reactions of NO2. In July in Beijing, high humidity condition was beneficial to the heterogeneous reaction of NO2, and due to the increase of precipitation, more HONO would enter the liquid phase (the high Henry coefficient of HONO). For days with high humidity and strong sunlight in June and August, photolysis of nitrate was also one important HONO source. For August and September, light-induced reactions of NO2 on non-aerosol surfaces under relatively low humidity and strong light conditions could be an important HONO source. In addition, the presence of Cl ions and sulfate could enhance the photolysis of nitrate, and this was obvious in July and October; the presence of organic compounds also could have this effect, which was obvious in June and October. Not only the HONO concentration but also the HONO source has temporal patterns, even within a season, it varies from month to month. This work highlights the importance of HONO for AOC in warm season, while encouraging long-term HONO observation to assess the contribution of HONO sources over time compared to the capture of pollution processes. [ABSTRACT FROM AUTHOR]

Water-soluble and water-insoluble organic aerosol (WSOA and WIOA) constitute a large fraction of fine particles in winter in northern China, yet our understanding of their sources and processes are still limited. Here we have a comprehensive characterization of WSOA in cold season in Beijing. Particularly, we present the first mass spectral characterization of WIOA by integrating online and offline organic aerosol measurements from high-resolution aerosol mass spectrometer. Our results showed that WSOA on average accounted for 59 % of the total OA and comprised dominantly secondary OA (SOA, 69 %). The WSOA composition showed significant changes during the transition season from autumn to winter. While the photochemical-related SOA dominated WSOA (51 %) in early November, the oxidized SOA from biomass burning increased substantially from 8 % to 29 % during the heating season. Comparatively, local primary OA dominantly from cooking aerosol contributed the major fraction of WSOA during clean periods. WIOA showed largely different spectral patterns from WSOA which were characterized by prominent hydrocarbon ions series and low oxygen-to-carbon (O/C = 0.19) and organic mass-to-organic carbon (OM/OC = 1.39) ratios. The nighttime WIOA showed less oxidized properties (O/C = 0.16 vs. 0.24) with more pronounced polycyclic aromatic hydrocarbons (PAHs) signals than daytime, indicating the impacts of enhanced coal combustion emissions on WIOA. The evolution process of WSOA and WIOA was further demonstrated by the triangle plot of f44 (fraction of m/z 44 in OA) vs. f43 , f44 vs. f60 , and the Van Krevelen diagram (H/C vs. O/C). We also found more oxidized WSOA and an increased contribution of SOA in WSOA compared with previous winter studies in Beijing, indicating that the changes in OA composition due to clean air act have affected the sources and properties of WSOA. [ABSTRACT FROM AUTHOR]

The radiative forcing of black carbon (BC) depends strongly on its mixing state in different chemical environments. Here we analyzed the chemical composition and mixing state of BC-containing particles by using a single-particle aerosol mass spectrometer and investigated their impact on light absorption enhancement (Eabs) at an urban (Beijing) and a rural site (Gucheng) in the North China Plain. While the BC was dominantly mixed with organic carbon (OC), nitrate, and sulfate at both the urban and rural sites, the rural site showed a much higher fraction of BC coated with OC and nitrate (36 % vs. 15 %–20 %). Moreover, the BC mixing state evolved significantly as a function of relative humidity (RH), with largely increased coatings of OC–nitrate and nitrate at high RH levels. By linking with an organic aerosol (OA) composition, we found that the OC coated on BC comprised dominantly secondary OA in Beijing, while primary and secondary OA were similarly important in Gucheng. Furthermore, Eabs was highly dependent on secondary inorganic aerosol coated on BC at both sites, while the coated primary OC also resulted in an Eabs of ∼ 1.2 for relatively fresh BC particles at the rural site. A positive matrix factorization analysis was performed to quantify the impact of different mixing states on Eabs. Our results showed a small Eabs (1.06–1.11) for BC particles from fresh primary emissions, while the Eabs increased significantly above 1.3 when BC was aged rapidly with increased coatings of OC–nitrate or nitrate; it can reach above 1.4 as sulfate was involved in BC aging. [ABSTRACT FROM AUTHOR]

Atmospheric aerosols play an important role in the radiation balance of the earth–atmosphere system. However, our knowledge of the long-term changes in equivalent black carbon (eBC) and aerosol optical properties in China is very limited. Here we analyze the 9-year measurements of eBC and aerosol optical properties from 2012 to 2020 in Beijing, China. Our results showed large reductions in eBC by 71 % from 6.25 ± 5.73 µ g m -3 in 2012 to 1.80 ± 1.54 µ g m -3 in 2020 and 47 % decreases in the light extinction coefficient (bext , λ = 630 nm) of fine particles due to the Clean Air Action Plan that was implemented in 2013. The seasonal and diurnal variations of eBC illustrated the most significant reductions in the fall and at nighttime, respectively. Δ eBC / Δ CO also showed an annual decrease from ∼ 7 to 4 ng m -3 ppbv -1 and presented strong seasonal variations with high values in spring and fall, indicating that primary emissions in Beijing have changed significantly. As a response to the Clean Air Action Plan, single-scattering albedo (SSA) showed a considerable increase from 0.79 ± 0.11 to 0.88 ± 0.06, and mass extinction efficiency (MEE) increased from 3.2 to 3.8 m 2 g -1. These results highlight the increasing importance of scattering aerosols in radiative forcing and a future challenge in visibility improvement due to enhanced MEE. Brown carbon (BrC) showed similar changes and seasonal variations to eBC during 2018–2020. However, we found a large increase of secondary BrC in the total BrC in most seasons, particularly in summer with the contribution up to 50 %, demonstrating an enhanced role of secondary formation in BrC in recent years. The long-term changes in eBC and BrC have also affected the radiative forcing effect. The direct radiative forcing (ΔFR) of BC decreased by 67 % from + 3.36 W m -2 in 2012 to + 1.09 W m -2 in 2020, and that of BrC decreased from + 0.30 to + 0.17 W m -2 during 2018–2020. Such changes might have important implications for affecting aerosol–boundary layer interactions and the improvement of future air quality. [ABSTRACT FROM AUTHOR]

Particulate organic nitrates (pON) are an important component of secondary organic aerosol in biogenic-emission-dominant environments and play a critical role in NO x cycles. However, estimation of pON has been a challenge in polluted environments, e.g., North China Plain, with high concentrations of inorganic nitrate and NO x. Here we developed a method for estimation of pON from the measurements of high-resolution aerosol mass spectrometer coupled with a thermodenuder based on the volatility differences between inorganic nitrate and pON. The results generally correlated well with those estimated from positive matrix factorization of combined organic and inorganic mass spectra and from the ratio of NO + to NO 2+ (NO x+ ratio), yet they had improvements in reducing negative values due to the influences of high concentration of inorganic nitrate and constant NO x+ ratio of organic nitrates (RON). By applying this approach to the measurements at an urban (Beijing) and a rural site (Gucheng) in summer and winter in the North China Plain, we estimated that the average mass concentrations of NO 3,org (1.8 µ g m -3 vs. 1.0 µ g m -3) and pON to OA (27.5 % vs. 14.8 %) were higher in summer than in winter in Beijing, indicating more pON formation in biogenically and anthropogenically mixed environments. In addition, the average NO 3,org loading in Gucheng was 1.9 µ g m -3 , and the pON at the rural site also showed higher contribution to OA than that in Beijing during wintertime due to higher primary emissions and gaseous precursors in Gucheng. In addition, RON was determined and showed considerable differences between day–night and clean–polluted periods, highlighting the complexity of pON compounds from different chemical pathways (e.g., OH and NO 3 oxidation) and sources. [ABSTRACT FROM AUTHOR]

Volatility and viscosity have substantial impacts on gas–particle partitioning, formation and evolution of aerosol and hence the predictions of aerosol-related air quality and climate effects. Here aerosol volatility and viscosity at a rural site (Gucheng) and an urban site (Beijing) in the North China Plain (NCP) in summer and winter were investigated by using a thermodenuder coupled with a high-resolution aerosol mass spectrometer. The effective saturation concentration (C*) of organic aerosol (OA) in summer was smaller than that in winter (0.55 µ g m -3 vs. 0.71–0.75 µ g m -3), indicating that OA in winter in the NCP is more volatile due to enhanced primary emissions from coal combustion and biomass burning. The volatility distributions varied and were largely different among different OA factors. In particular, we found that hydrocarbon-like OA (HOA) contained more nonvolatile compounds compared to coal-combustion-related OA. The more oxidized oxygenated OA (MO-OOA) showed overall lower volatility than less oxidized OOA (LO-OOA) in both summer and winter, yet the volatility of MO-OOA was found to be relative humidity (RH) dependent showing more volatile properties at higher RH. Our results demonstrated the different composition and chemical formation pathways of MO-OOA under different RH levels. The glass transition temperature (Tg) and viscosity of OA in summer and winter are estimated using the recently developed parameterization formula. Our results showed that the Tg of OA in summer in Beijing (291.5 K) was higher than that in winter (289.7–290.0 K), while it varied greatly among different OA factors. The viscosity suggested that OA existed mainly as solid in winter in Beijing (RH = 29 ± 17 %), but as semisolids in Beijing in summer (RH = 48 ± 25 %) and Gucheng in winter (RH = 68 ± 24 %). These results have the important implication that kinetically limited gas–particle partitioning may need to be considered when simulating secondary OA formation in the NCP. [ABSTRACT FROM AUTHOR]

Nitrogen dioxide (NO 2) is an important substance in atmospheric photochemical processes and can also be absorbed by plants. NO 2 fluxes between the atmosphere and P. nigra seedlings were investigated by a double dynamic chambers method in Beijing from June 15 to September 3, 2017. The range of NO 2 exchange fluxes between P. nigra seedlings and the atmosphere was from − 14.6 to 0.8 nmol/(m2·sec) (the positive data represent NO 2 emission from trees, while the negative values indicate absorption). Under ambient concentrations, the mean NO 2 flux during the fast-growing stage (Jun. 15–Aug. 4) was − 3.0 nmol/(m2·sec), greater than the flux of − 1.5 nmol/(m2·sec) during the later growth stage (Aug. 8–Sept. 3). The daily exchange fluxes of NO 2 obviously fluctuated. The fluxes were largest in the morning and decreased gradually over time. Additionally, the NO 2 fluxes were larger under high light intensities than under low light intensities during the whole growth period. The effects of temperature on NO 2 fluxes were different under two growth periods. The NO 2 exchange fluxes were larger in a range of temperatures close to 44°C in the fast-growing stage, whereas there were no evident differences in NO 2 exchange fluxes under widely differing temperatures in the later growth stage. Under polluted conditions, the uptake ability of NO 2 was weakened. Additionally, the compensation point of NO 2 was 5.6 ppb in the fast-growing stage, whereas it was 1.4 ppb in the later growth stage. The deposition velocities of NO 2 were between 0.3 and 2.4 mm/sec. Unlabelled Image [ABSTRACT FROM AUTHOR]

Abstract: This paper proposes a method for forecasting the ionospheric critical frequency, f 0F2, 1h in advance, using the support vector machine (SVM) approach. The inputs to the SVM network are the time of day, seasonal information, 2 month running mean sunspot number (R2), 3 day running mean of the 3h planetary magnetic ap index, the solar zenith angle, the present value f 0F2(t) and its first and second increments, the observation of f 0F2 at t−23h, the 30-day mean value at time, t, f mF2 (t) and the previous 30 day running mean of f 0F2 at t−23h f mF2(t−23). The output is the predicted f 0F2 1h ahead. The network is trained to use the ionospheric sounding data at Haikou, Guangzhou, Chongqing, Lanzhou, Beijing, Changchun and Manzhouli stations at high and low solar activities. The performance of the SVM model was verified with observed data. It is shown that the predicted f 0F2 has good agreement with the observed f 0F2. The performance of the SVM model is superior to that of the autocorrelation and persistence models, and that it is comparable to that of the neural network model. [ABSTRACT FROM AUTHOR]

Smart control of window behavior is a means of effectively reducing concentrations of indoor PM 2.5 (particulate matter with aerodynamic diameter less than 2.5 μm) in naturally ventilated residential buildings without indoor air cleaning devices. This study aimed to develop a reinforcement learning approach to automatically control window behavior in real time for mitigation of indoor PM 2.5 pollution. The proposed method trains the window controller with the use of a deep Q-network (DQN) in a specific naturally ventilated apartment in the course of a month. The trained controller can then be employed to control window behavior in order to reduce the indoor PM 2.5 concentrations in that apartment. The required input data for the controller are the real-time indoor and outdoor PM 2.5 concentrations with a 1-min resolution, which can easily be obtained with low-cost sensors available on the market. A series of simulations were conducted in a virtual typical apartment in Beijing and a real apartment in Tianjin. The results show that, compared with the baseline I/O ratio algorithm, the proposed reinforcement learning window-control algorithm reduced the average indoor PM 2.5 concentration by 12.80% in a one-year period. Furthermore, the proposed algorithm reduced the indoor PM 2.5 concentrations in the real apartment by 9.11% when compared with the I/O ratio algorithm and by 7.40% when compared with real window behavior. • A reinforcement learning method was developed to automatically control window behavior. • The proposed algorithm trained the window controller with the use of deep Q-network. • The proposed algorithm reduced the average indoor PM 2.5 concentration effectively. [ABSTRACT FROM AUTHOR]

Severe haze episodes in cold season in Beijing have been mitigated greatly during the last decade. However, the changes in aerosol chemistry as responses to the large reductions in gaseous precursors during the two phases of clean air action, i.e., phase Ⅰ (2013–2017) and phase Ⅱ (2018–2020), are less understood. Here we characterized such changes in cold season (January–March) by using five-year real-time aerosol particle composition measurements. Our results showed consistently large reductions for all chemical species from 2013 to 2020 with the largest decreases being chloride (95%) and organics (74%) followed by sulfate (69%), while the decreases in nitrate were comparatively small (44%). However, the contributions of sulfate were fairly stable despite the increased nitrate contributions from 18% in 2013 to 30% in 2020. Organic aerosol (OA) composition also changed significantly since 2018 with large increases in the contributions of secondary OA and corresponding decreases in primary OA from fossil fuel combustion and cooking emissions. The changes in aerosol chemistry were closely related to the different reductions in gaseous precursors, e.g., SO 2 vs. NO 2 , and the enhanced secondary processes, e.g., the increases in O 3 , sulfur and nitrogen oxidation efficiency. Further, we found that the changes in aerosol chemistry in cold season during the phase Ⅱ of clean air action (2018–2020) started to slow down with relatively small changes in PM 2.5 and secondary inorganic species. Our results point towards a future challenge in mitigating air pollution in cold season, and the need of more stringent and scientific strategies to control secondary aerosol pollution in an environment with enhanced oxidation capacity and high precursors. Image 1 • All chemical species except nitrate showed large reductions from 2013 to 2020. • Changes in aerosol chemistry during two phases of clean air action were different. • Increased photochemical processing was important in driving chemical changes. [ABSTRACT FROM AUTHOR]