Over the past decade, China has achieved remarkable progress in mitigating aerosol pollution. However, ozone (O3) pollution still shows a worsening trend, implying that China's control measures may have been less effective in tackling O3 pollution. Herein, we conduct synchronized observations of O3 and its precursors across 37 cities in the heavily polluted North China Plain (NCP) during the summer of 2021 and apply a unified observation-based model to diagnose O3 formation mechanisms. Our results reveal a significant transition in the urban O3 formation regime, shifting from being primarily volatile organic compound (VOC) limited to VOC-nitrogen oxides (NOx) co-limited across the NCP between the 2010s and the 2020s. Notably, the primary VOC species, their respective sources, and the optimal VOCs/NOx reduction ratios exhibit remarkable regional consistency. Modeling analyses further indicate that a long-term national 'carbon neutrality' strategy could effectively alleviate O3 pollution, with targeted VOC emission reductions from major anthropogenic sources offering the greatest mitigation potential. These findings underscore the efficacy of China's endeavors in mitigating O3 pollution, although the effects are not immediately evident from ambient O3 concentrations. O3 pollution control in Chinese cities has reached a critical inflection point, offering considerable flexibility and feasibility in formulating future control policies.

Assimilating observational data into numerical simulation is crucial for accurately estimating the spatiotemporal distribution of PM2.5 chemical components (NH4+, NO3-, SO42-, OC, and BC), which is beneficial to quantifying the impact of aerosols on the environment, climate change and human health. However, chemical transport model (CTM)-based data assimilation (DA) is computationally inefficient for large ensemble sizes and offers limited improvements in simulation skill, as it solely provides optimal initial conditions. This paper introduces an incrementally updatable machine learning-based data assimilation system (Optimized Incremental Random Forest coupled with Localized Ensemble Kalman Filter, OIRF-LEnKF v1.0) that achieves high efficiency and high quality in generating background and analysis fields for chemical components. Computational efficiency tests indicate that the total time consumed by OIRF-LEnKF v1.0 constitutes only 11.41 %-16.60 % of that of CTM-based DA, primarily because the simulation process requires only 0.13 %-0.20 % of the CTM computation time. Sensitivity tests demonstrate that the incremental learning during the simulation process enhances the percentage change of the Pearson correlation coefficient relative to its minimum value (Delta CORR) by 2.43 %-11.75 % and reduces the percentage change of the RMSE relative to its maximum value (Delta RMSE) by 32.55 %-40.36 %, compared to the stationary training mechanism. A 2-month DA experiment reveals that the RMSE values of chemical components after DA are less than 7.80 and 2.36 & micro;g m-3 during the simulation and analysis processes, respectively, indicating reductions of at least 26.38 % and 68.99 % compared to values without DA. Notably, the RMSE values of our system during the simulation process exhibit a significant reduction of 33.16 %-90.10 % compared to those of the CTM-based DA, highlighting the superior simulation capability of our system. Furthermore, the spatial overestimation and underestimation of chemical components have been significantly mitigated following DA. Compared to multiple reanalysis datasets of inorganic salt aerosols (CORR: 0.56-0.89, RMSE: 2.55-8.52 & micro;g m-3), the dataset generated by OIRF-LEnKF v1.0 (CORR: 0.97, RMSE: 1.12 & micro;g m-3) demonstrates higher data quality.

Reducing fine particulate nitrate (pNO3-) is critical for further mitigating PM2.5 pollution in China. However, previous NOx emission reductions have failed to achieve the expected pNO3- decreases. The present study reports that pNO3- concentration in summer increased by 55.8% and 5.6% at North China Peak (1534 m a.s.l.) from 2007 to 2014 and 2014 to 2021, respectively. pNO3- formation enhancement was caused mainly by decreased aerosol acidity due to notable SO42- reduction. pNO3- formation changed from a process limited by NH4+ to one colimited by NO2 and NH4+, suggesting an increased effect of NOx reduction on decreasing pNO3- production. Vertical transport represents a significant source of pNO3- near the surface, illustrating a percentage as high as 98% recorded during daytime hours and a proportion of 34% in the dark over North China in the simulation scenario during summer 2020. The scheme to reduce NOx emissions by 10% from 2020 to 2025 is predicted to slowly decrease aloft pNO3- over North China, which may facilitate further reductions in pNO3- concentrations near the surface via vertical transport. The inflection of nitrate chemistry in the top boundary layer suggests an opportunity to accelerate PM2.5 reduction under projected further emission reductions.

Coal-to-gas (CTG) policies are important energy transformation strategies for addressing air pollution issues, but how well they improve atmospheric lead (Pb) pollution remains poorly understood. By the end of 2018, Beijing had achieved coal-free status in urban and plain areas. The mixing state and atmospheric chemical processes of Pb-rich particles in Beijing were monitored using single-particle aerosol mass spectrometry (SPAMS) in 2019. Based on a large dataset of mass spectra, this study finds that the number fractions of Pb-rich particles, as well as two specific types of Pb-rich particles (K-Na-EC and K-OC, where EC and OC denote elemental and organic carbon, respectively) related to coal combustion during the official Beijing heating period, show lower number fractions than those after the heating period. Based on concentration-weighted trajectory plots, the results indicate that lead aerosols mainly derive from transmission from surrounding provinces. Lead nitrate is one of the important forms of lead in aerosol particles, particularly as a result of photo-chemical reactions in the spring, fall, and winter. Due to nitrate decomposition during high temperatures, the aqueous reaction mechanism contributes more to lead nitrate during the summer. These results improve our understanding of the seasonal distribution, formation mechanisms, and influencing factors of toxic Pb-containing particles after CTG.

Abstract. Assimilating observational data into numerical forecasts is crucial for accurately estimating the spatiotemporal distribution of PM2.5 chemical components (NH4+, NO3-, SO42-, OC, and BC), which is beneficial to quantifying the impact of aerosols on the environment, climate change and human health. However, chemical transport model (CTM)-based data assimilation (DA) is computationally inefficient for large ensemble sizes and offers limited improvements in forecasting, as it solely provides optimal initial conditions. This paper introduces a machine learning (ML)-based self-evolving data assimilation system (OIRF-LEnKF v1.0) that achieves high efficiency and high quality in the forecast and analysis fields of chemical components. Computational efficiency tests indicate that the total time consumed by OIRF-LEnKF v1.0 constitutes only 11.41–16.60 % of that of CTM-based DA, particularly during the forecasting process (0.13–0.20 %). Sensitivity tests demonstrate that the self-evolution mechanism in our system enhances the Pearson correlation coefficient (CORR) and reduces the RMSE during the forecasting process by 2.28–11.75 % and 32.94–40.98 %, respectively, compared to the stationary training mechanism. A 2-month DA experiment reveals that the RMSE values of chemical components after DA are less than 7.80 µg m-3 and 2.36 µg m-3 during the forecasting and analysis processes, respectively, indicating reductions of at least 26.38 % and 68.99 % compared to values without DA. Notably, the RMSE values of our system during the forecasting process exhibit a significant reduction of 33.16–90.10 % compared to those of the CTM-based DA, highlighting the superior forecasting capability of our system. Furthermore, the spatial overestimation and underestimation of chemical components have been significantly mitigated following DA. Compared to multiple reanalysis datasets of inorganic salt aerosols (CORR: 0.56–0.89, RMSE: 2.55–8.52 μg m-3), the dataset generated by OIRF-LEnKF v1.0 (CORR: 0.97, RMSE: 1.12 μg m-3) demonstrates higher data quality.

High concentrations of organic aerosol (OA) occur in Asian countries, leading to great health burdens. Clean air actions have resulted in significant emission reductions of air pollutants in China. However, long-term nation-wide trends in OA and their causes remain unknown. Here, we present both observational and model evidence demonstrating widespread decreases with a greater reduction in primary OA than in secondary OA (SOA) in China during the period of 2013 to 2020. Most of the decline is attributed to reduced residential fuel burning while the interannual variability in SOA may have been driven by meteorological variations. We find contrasting effects of reducing NOx and SO2 on SOA production which may have led to slight overall increases in SOA. Our findings highlight the importance of clean energy replacements in multiple sectors on achieving air-quality targets because of high OA precursor emissions and fluctuating chemical and meteorological conditions. Clean air actions affect air quality greatly. Here, the authors report widespread decreases in organic aerosol (OA) in China from 2013 to 2020 with primary OA decreasing more than secondary OA. However, further reductions are challenging.

Identifying PM2.5 chemical components is crucial for formulating emission strategies, estimating radiative forcing, and assessing human health effects. However, accurately describing spatiotemporal variations in PM2.5 chemical components remains a challenge. In our earlier work, we developed an aerosol extinction coefficient data assimilation (DA) system (Nested Air Quality Prediction Model System with the Parallel Data Assimilation Framework (NAQPMS-PDAF) v1.0) that was suboptimal for chemical components. This paper introduces a novel hybrid nonlinear chemical DA system (NAQPMS-PDAF v2.0) to accurately interpret key chemical components (SO42-, NO3-, NH4+, OC, and EC). NAQPMS-PDAF v2.0 improves upon v1.0 by effectively handling and balancing stability and nonlinearity in chemical DA, which is achieved by incorporating the non-Gaussian distribution ensemble perturbation and hybrid localized Kalman–nonlinear ensemble transform filter with an adaptive forgetting factor for the first time. The dependence tests demonstrate that NAQPMS-PDAF v2.0 provides excellent DA results with a minimal ensemble size of 10, surpassing previous reports and v1.0. A 1-month DA experiment shows that the analysis field generated by NAQPMS-PDAF v2.0 is in good agreement with observations, especially in reducing the underestimation of NH4+ and NO3- and the overestimation of SO42-, OC, and EC. In particular, the Pearson correlation coefficient (CORR) values for NO3-, OC, and EC are above 0.96, and the R2 values are above 0.93. NAQPMS-PDAF v2.0 also demonstrates superior spatiotemporal interpretation, with most DA sites showing improvements of over 50 %–200 % in CORR and over 50 %–90 % in RMSE for the five chemical components. Compared to the poor performance in the global reanalysis dataset (CORR: 0.42–0.55, RMSE: 4.51–12.27 µg m−3) and NAQPMS-PDAF v1.0 (CORR: 0.35–0.98, RMSE: 2.46–15.50 µg m−3), NAQPMS-PDAF v2.0 has the highest CORR of 0.86–0.99 and the lowest RMSE of 0.14–3.18 µg m−3. The uncertainties in ensemble DA are also examined, further highlighting the potential of NAQPMS-PDAF v2.0 for advancing aerosol chemical component studies.

Abstract. Since non-methane hydrocarbons (NMHCs) include numerous species of volatile organic compounds (VOCs), they are the only indicators that can characterize the total amount of VOCs in ambient air. More than 90 % of NMHC instruments in the market use the indirect method for NMHC determination, utilizing a gas chromatograph to determine the concentrations of total hydrocarbons (THCs) and methane in air. However, we found indirect NMHC measurements incorrectly characterized the low NMHC concentrations in the urban air. These measurements were hindered by the uncertain changes in the errors associated with low THC concentrations, humidity, and macroscopic substances (oxygen) in the atmosphere. In this study, we conducted intercomparisons between 10 instruments in the market using different operation principles; among these, two used the direct method and eight used the indirect method. For the indirect method, experiments showed that when the NMHC concentration was 50 ppbC, the errors in the measurement results were 32 % (CH4) and 98 % (THC) when compared to theoretical values. The oxygen content in the sample gas directly affected the response of the THC. The effect of increased oxygen content on the analysis of oxygen-containing substances was more significant than that of the increase in hydrocarbon substances. The response of dry zero was blank, and the response value increased non-linearly with relative humidity (RH). RH had a great impact on low NMHC concentrations of nearly 50 %. For the direct method, although various flow designs were applied for different instruments, the results indicated that they generally showed lower interference and higher accuracy for the ambient NMHC measurements. With the online direct methods, we obtained more reliable data and characteristics for low-concentration NMHCs in downtown Shanghai. The median of hourly data was 163.1 ppbC, and the highest ratio of the maximum and minimum concentrations of NMHCs reached 9.56 in a single day.

Ambient carbonyls are important precursors of radicals and ground-level ozone (O-3). In this study, sources, precursors, and impacts on radicals and O-3 of carbonyls were investigated based on online observations of volatile organic compounds (VOCs) at an urban site in Beijing during June 2021. Carbonyls accounted for 36% and 42% of mixing ratios and OH reactivity for total measured VOCs, respectively. Formaldehyde was the most abundant carbonyl, with the mean level of 4.13 +/- 2.28 ppb. Source apportionment results based on the multi linear regression (MLR) method suggested that secondary production contributed 41%, 25%, 36%, and 30% of formaldehyde, acetaldehyde, propanal, and acetone, respectively. Key precursors of carbonyls were then identified based on the calculation of their production rates. It was found that alkenes contributed 59%-80% of aldehydes production. Impacts of carbonyls on HOx radicals (OH and HO2) and O-3 production were explored using a box model based on observations (OBM). Photolysis of HONO, formaldehyde, and O-3 were the dominant primary sources of HOx radicals during daytime of O-3 pollution days, with average relative contributions of 52%, 28%, and 19% to the total primary production rate of HOx, respectively. Aldehydes accounted for 32% (20% from formaldehyde) of average HOx removal rates. The relative incremental reactivity (RIR) values of NOx determined by the OBM were negative, suggesting that the O-3-VOCs-NOx sensitivity was in the VOCs-limited regime. Using the observed concentrations of carbonyls as constraints of OBM, the absolute values of RIR for NOx tended to increase but those for anthropogenic VOCs tended to decrease. Formaldehyde showed the largest RIR value for anthropogenic VOCs during O-3 pollution days. These findings indicated the important impacts of carbonyls on O-3 production and O-3-VOCs-NOx sensitivity.

Coal-to-gas (replacing coal with natural gas) is an important energy transformation strategy for addressing environmental, but the improvement of atmospheric Pb pollution from coal-to-gas remains poorly understood. Beijing has basically realized coal-free in urban and plain areas by the end of 2018. A single particle aerosol mass spectrometry (SPAMS) was deployed to investigate the mixing state and chemical processing of Pb-rich particles in urban Beijing in 2019. Based on a large dataset of mass spectra, this study find K-Na-EC and K-OC particles associated with coal combustion. The contribution of coal combustion to atmospheric Pb decreased significantly, and iron/steel industries are the most important source of Pb after coal-to-gas. The atmospheric Pb in Beijing urban area is mainly transmitted from surrounding provinces such as Mongolia Province, Hebei Province, Liaoning Province, Anhui Province and Shandong Province. These Pb in Pb-rich particles are mainly in the form of Pb(NO3)2, through photochemical and/or aqueous phase reactions of PbCl2 and HNO3 and/or NO2 in the gas phase. These results assess the effect of coal-to-gas on the source structure and mixing state of Pb particles, and indicate that Pb pollution still needs to be paid attention to because Pb(NO3)2 abundance increases with improved control quality.

Since non-methane hydrocarbons (NMHCs) include numerous species of volatile organic compounds (VOCs), they are the only indicators that can characterize the total amount of VOCs in ambient air. More than 90 % of NMHC instruments in the market use the indirect method for NMHC determination, utilizing a gas chromatograph to determine the concentrations of total hydrocarbons (THCs) and methane in air. However, we found indirect NMHC measurements incorrectly characterized the low NMHC concentrations in the urban air. These measurements were hindered by the uncertain changes in the errors associated with low THC concentrations, humidity and macroscopic substances (oxygen) in the atmosphere. In this study, we conducted intercomparisons between 10 instruments in the market using different operation principles; among these, two used the direct method, and eight used the indirect method. For the indirect method, experiments showed that when the NMHC concentration was 50 ppb C, the errors in the measurement results were 32 % (CH4) and 98 % (THC) when compared to theoretical values. The oxygen content in the sample gas directly affected the response of the THC. The effect of increased oxygen content on the analysis of oxygen-containing substances was more significant than that of the increase in hydrocarbon substances. The response of dry zero was blank, and the response value increased non-linearly with relative humidity (RH). RH had a great impact on low NMHC concentrations of nearly 50 %. For the direct method, although various flow designs were applied for different instruments, the results indicated that they generally showed lower interference and higher accuracy for the ambient NMHC measurements. With the online direct methods, we obtained more reliable data and characteristics for low-concentration NMHCs in downtown Shanghai. The median of hourly data was 163.1 ppb C, and the highest ratio of the maximum and minimum concentrations of NMHCs reached 9.56 in a single day.

Scientific monitoring of Volatile organic compounds （VOCs） in the atmosphere is the key basis for the formation mechanism and prevention and control of the combined pollution of O 3 and PM 2.5 . It is of great significance to continuously improve the air quality in China. In order to optimize and improve the VOCs monitoring system and efficiency in China， and put forward the development direction and path of photochemical monitoring in the future， the key elements of VOCs monitoring system design， monitoring sites， monitoring items and periods， monitoring technology and quality control technology at home and abroad are evaluated through research， data mining and other methods. This research is carried out from the development history， monitoring points， monitoring items and time periods， monitoring technology and quality of VOCs monitoring at home and abroad. The results show that: （1） The photochemical monitoring network covering key areas has been initially established in China， but in the VOCs monitoring network， key points such as regions， transportation and high emission areas are lacking. It is an important path to optimize and improve them to improve the VOCs monitoring performance in our country. （2） May to September is an important period of O 3 pollution in China. The components involved in Photochemical Assessment Monitoring Stations （PAMS） belonging to the Environmental Protection Agency USA （EPA） are important components affecting O 3 ， especially ethylene， benzene series， formaldehyde and acetaldehyde. And some branched chain olefins and natural source olefins such as α- Pinene β- The impact of pinene on O 3 or particulate matters is also noteworthy. Finally， the monitoring project should be optimized according to the localization characteristics. （3） In terms of monitoring technology， manual monitoring and automatic monitoring have their own advantages， showing the development trend of automatic monitoring as the main and manual monitoring as the auxiliary. The monitoring technology system should be developed in the direction of high precision， standardization， miniaturization， modularization， intelligence， high time resolution and low monitoring cost. At the same time， the quality system standard research should be continued to fill the gaps and improve the quality of monitoring data. The research shows the monitoring points of environmental atmospheric VOCs in China should be optimized， the standardization of monitoring technology system and quality system should be strengthened， and the localized monitoring projects in China should be formed focusing on key periods， so as to scientifically optimize and improve the photochemical monitoring program in China.

Continuous measurements of volatile organic compounds (VOCs), ozone (O3), fine particulate matter (PM2.5), and related parameters were conducted between April 2020 and March 2021 in Beijing, China, to characterize potential sources of VOCs and their impacts on secondary organic aerosols (SOAs) and O3 levels. The annual average mixing ratio of VOCs was 17.4 ± 10.1 ppbv, with monthly averages ranging from 11.6 to 25.2 ppbv. According to the empirical kinetic modeling approach (EKMA), O3 formation during O3 season was "VOCs-limited", while it was in a "transition" regime during O3 pollution episodes. In the O3 season, higher ozone formation potential (OFP) of m/p-xylene, o-xylene, toluene, isopentane, and n-butane were evident during O3 pollution episodes, in line with the increasing contributions of solvent usage and coating, as well as gasoline evaporation to OFP obtained through a matrix factorization model (PMF). Aromatics contributed the most to the secondary organic aerosol formation potential (SOAFP). In the non-O3 season, the contribution of vehicle exhaust to SOAFP elevated on hazy days, thereby revealing the importance of traffic-derived VOCs for PM2.5 pollution. Our results indicate that the prior control of different VOC sources should vary by season, thereby facilitating the synergistic control of O3 and PM2.5 in Beijing.

To better understand the change characteristics and reduction in organic carbon (OC) and elemental carbon (EC) in particulate matter (PM) with a diameter of ≤2.5 μm (PM2.5) driven by the most stringent clean air policies and pandemic-related lockdown measures in China, a comprehensive field campaign was performed to measure the carbonaceous components in PM2.5 on an hourly basis via harmonized analytical methods in the Beijing-Tianjin-Hebei and its surrounding region (including 2 + 26 cities) from January 1 to December 31, 2020. The results indicated that the annual average concentrations of OC and EC reached as low as 6.6 ± 5.7 and 1.8 ± 1.9 μg/m3, respectively, lower than those obtained in previous studies, which could be attributed to the effectiveness of the Clean Air Action Plan and the impact of the COVID-19-related lockdown measures implemented in China. Marked seasonal and diurnal variations in OC and EC were observed in the 2 + 26 cities. Significant correlations (p < 0.001) between OC and EC were found. The annual average secondary OC levels level ranged from 1.8-5.4 μg/m3, accounting for 37.7-73.0% of the OC concentration in the 2 + 26 cities estimated with the minimum R squared method. Based on Interagency Monitoring of Protected Visual Environments (IMPROVE) algorithms, the light extinction contribution of carbonaceous PM to the total amount reached 21.1% and 26.0% on average, suggesting that carbonaceous PM played a less important role in visibility impairment than did the other chemical components in PM2.5. This study is expected to provide an important real-time dataset and in-depth analysis of the significant reduction in OC and EC in PM2.5 driven by both the Clean Air Action Plan and COVID-19-related lockdown policies over the past few years, which could represent an insightful comparative case study for other developing countries/regions facing similar carbonaceous PM pollution.

我国排污单位自行监测起步较晚,基础薄弱,相关研究较少.针对排污单位自行监测频次确定方法中仅考虑排放源或污染物单因素的不足,提出排放源分级分类叠加污染物分级分类的监测频次确定方法.对该方法中的核心内容废气排放源、 污染物分级分类方法,废水排放口、 污染物分级分类方法进行了研究.

The mass concentrations of fine particles (PM2.5) have decreased significantly in China in recent years, while surface ozone pollution shows an opposite trend. To better understand the combined PM2.5 and O-3 pollution in China's urban atmosphere, the Spearman correlation between PM2.5 mass concentration and daily maximum 8-hour average ozone concentration (MDA8 O-3) was analyzed in this study, as well as the correlation between the concentrations of PM2.5 and O-x (O-x=O-3 +NO2). Since 2015, the number of days when both O-3 and PM2.5 concentrations exceed the national ambient air quality standards has decreased significantly with the decrease in PM2.5 concentrations. The pollution combined O-3 and PM2.5 usually occurs in April and May in the Beijing-Tianjin-Hebei (BTH) area. It is worth noting that the correlations between PM2.5 and MDA8 O-3 concentrations depend on regions and seasons on south of 40 degrees N in China. A stronger positive correlation between the concentrations of PM2.5 and MDA8 O-3 in the Pearl River Delta (PRD) area was obtained throughout the year (R>0.6). In the BTH area, this type of relationship occurs only in summer (R similar to 0.5) whereas in winter, a weak negative correlation between PM2.5 and MDA8 O-3 concentrations was observed (R<-0.2). This may be due to a higher primary contribution of PM2.5 and a low concentration of O-3 due to low photochemical production compared to summer in wintertime. For the case of O-3 concentration exceeds national ambient air quality standards in Beijing and Xuzhou, MDA8 O-3 and PM2.5 concentrations correlate positively when PM2.5 = 50 mu g/m(3). In contrast, when PM2.5 > 50 mu g/m(3), a weak negative correlation was observed (R similar to-0.1), suggesting that high concentrations of particulate matter may inhibit O-3 formation on days when is polluted. However, the mechanism of such phenomenon remains confusing due to the complex relationship between the production of O-3 and PM2.5. Meanwhile, PM2.5 and MDA8 O-3 concentrations show a stronger positive correlation during daytime when O-3 concentration exceeds the national ambient air quality standards, which is due to the strong photochemical formation of O-3 as well as secondary aerosols. Furthermore, the positive correlation coefficients between Ox and PM2.5 are significantly higher than those between PM2.5 and MDA8 O-3 in most cities. This result is consistent with the predominant contribution of secondary aerosol to PM2.5 mass concentrations, both in wintertime and in summertime, after the stringent control of primary source emissions. In addition, the chemical concentration of water-soluble inorganic composition in PM2.5 samples collected in urban Beijing was analyzed in this work using ion chromatography in 2020. Comparing the correlation coefficients between PM2.5 and MDA8 O-3 concentrations, there are stronger positive correlations in the concentrations between MDA8 O-3 and the ratio between PM2.5 mass concentration and secondary inorganic aerosols (SNA= sulfate + nitrate + ammonium). This result may be due to the fact that a high concentration of O-3 can promote the formation of secondary aerosols.

Imidazoles (IMs) are potential contributors to brown carbon; they may notably contribute to climate radiative forcing. However, only a few studies have assessed the mixing state, seasonal and spatial distributions of IMs, and influencing factors for IM formation in urban aerosols. In this study, two single-particle aerosol mass spectrometers were employed to investigate the IM-containing particles in the urban areas of Beijing and Guangzhou, China. IM-containing particles were identified in the size range (dva) of 0.2-2.0 μm, accounting for 0.7-21.7 % of all the detected particles. The number fractions of IM-containing particles in both cities were the lowest in winter and the highest in spring, probably owing to the difference in the abundance of precursors and the particle acidity. Majority of (60-80 % by number) the IM-containing particles were mixed with organic carbon (OC), with the lowest fractions found in summer. Although the number fractions of IM-containing particles in Beijing were generally higher (~1.5-3 times) than those in Guangzhou, the mixing states of the IM-containing particles at these two sites were only slightly different. Potassium-rich (K-rich) and potassium-sodium (KNa) particles were rarely found in Guangzhou; they accounted for ~15 % of the IM-containing particles in Beijing. Additionally, our results indicate that particles with higher acidity are favorable for IM formation. These findings help improving our knowledge of the mixing state, seasonal variation, and spatial distribution of IMs in urban aerosols, and the insights in influencing factors into IM formation provide valuable information for future studies of the atmospheric chemical processes associated with IMs.

Fine particulate matter (PM2.5) and tropospheric ozone (O-3) are currently the two air pollutants of the greatest concern in China, affecting air quality and human health. Since 2013, Beijing has implemented multiple measures such as the "Air Pollution Prevention and Control Action Plan" and the "Three-Year Action Plan for Winning the Blue Sky Defense War", which have significantly reduced primary emissions and improved air quality. This study focuses on the two major pollutants: PM2.5 and O-3, which are tightly related to the secondary formation chemistry in ambient air. The statistical data on the number of days during the period from 2013 to 2020 on which the concentration of PM2.5 and O-3 in Beijing exceeds the standard were obtained from the China National Environmental Monitoring Center, as well as the concentration data of PM2.5 and O-3 from 2015 to 2020. The pollutant concentrations and meteorological parameters of the typical pollution process are the observation results of the typical urban site "PKUERS" located on top of the Science Building No.1 of Peking University campus in Haidian District, Beijing. In addition, the characteristics of the annual evolution of PM2.5 and O-3 pollution, as well as the "generation-development-elimination" law of the typical pollution processes are revealed. Studies have shown that the frequency, duration, and peak concentration of the PM2.5 pollution process are all decreasing year by year. Compared with 155 d of PM2.5 exceeding the standard in 2013, there are only 35 d of PM2.5 exceeding the standard in 2020 (a decrease of 77%), indicating that Beijing's atmospheric PM2.5 control has made remarkable progress. However, in the months of October, November, January and February of 2020, there are still 6-8 d of PM2.5 exceedance every month, or 1.5-2 d of exceedance per week, which implies that PM2.5 pollution always exists in the cold season. In contrast, the trend of interannual change of O-3 pollution process is not evident, about 1/3 of the time from May to July of each year is exposed to O-3 pollution. In 2018, the number of days when O-3 exceeded the standard surpassed PM2.5 for the first time, suggesting that O-3 may gradually replace PM2.5 as the most important pollutant in Beijing. Moreover, this paper summarizes three PM2.5 pollution processes, three O-3 pollution processes, and three PM2.5 and O-3 dual pollution processes. It is shown that high NOx concentrations are often associated with the occurrence of PM2.5 and O-3. PM2.5 episodes mainly occur in winter, and a pollution process lasts for a long time, the change of PM2.5 concentration shows a peak or a step pattern; while O-3 episodes mainly occur in summer, the O-3 concentration shows an obvious daily variation, high concentration during the day and low concentration at night. When O-3 pollution lasts for a long time, it can turn into a double episode of O-3 and PM2.5 pollution. In this case, the concentration of both pollutants may show an alternating peakand-valley phenomenon, and this is consistent with the daily variation in temperature and relative humidity. In terms of dominant PM2.5 components, organic matter (including primary and secondary organic matter) accounted for the highest proportion of the three types of pollution processes, followed by nitrate and sulfate. The proportion of nitrate has increased over the years, while the proportion of sulfate has declined. In summary, this study could provide a theoretical basis for relevant departments to clarify and coordinate the control of PM2.5 and O-3, and further improve the air quality based on the substantial progress made in air pollution prevention and control at the current stage.

Ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) instruments were used to carry out observation of aerosol in the urban and suburban areas of Shanghai from October 17 to November 21, 2019. Fudan University (FDU) site is a typical urban environment, surrounded by residential areas, commercial areas and arterial roads, while Dianshan Lake (DSL) site is a suburban environment with high vegetation coverage and no pollutant emission sources. The aerosol retrieved by MAX-DOAS was in good correlation with the observation of sun photometer and the PM2. 5 concentration of the corresponding site, which demonstrates that the aerosol retrieved by MAX-DOAS is reliable and feasible. Comparing the mean aerosol extinction coefficient (AEC) profiles during the observation period between urban and suburban areas, it was found that the occurrence of high aerosol concentration at FDU was nearly 3 h later than that of DSL at suburban site. And the aerosol at DSL was concentrated at an altitude of 0.3- 0.5 km, with a mean peak value of 0.486 km-1, which was slightly higher than the peak AEC of 0.453 km-1 at FDU of 0.2- 0.4 km. The difference in aerosol characteristics between the two sites may be due to the fact that the influences of aerosol transport and boundary layer dynamics are different between the two sites. The backward trajectories analysis also presents that there were mutual transports of aerosol between urban and suburban areas, which affect the optical properties of the aerosol in these two sites. In a case of aerosol pollution, we visualized the transport pathway of aerosol from the western part of the North China Plain to Shanghai using AEC profiles and backward trajectories, providing the evidence that the local aerosol pollution in Shanghai was affected by long-distance transport. (c) 2021 Published by Elsevier B.V.