In recent decade the ambient fine particle (PM2.5) levels have shown a trend of distinct dropping in China, while ground-level ozone concentrations have been increasing in Beijing and many other Chinese mega-cities. The variation pattern in Los Angeles was markedly different, with PM2.5 and ozone decreasing together over past decades. In this study, we utilize observation-based methods to establish the parametric relationship between PM2.5 concentration and key aerosol physical properties (including aerosol optical depth and aerosol surface concentration), and an observation-based 1-D photochemical model to quantify the response of PM2.5 decline in enhancing ground-level ozone pollution over a large PM2.5 concentration range (10–120 μg m−3). We find that the significance of ozone enhancement due to PM2.5 dropping depends on both the PM2.5 levels and optical properties of particles. Ozone formation increased by 37% in 2006–2016 due to PM2.5 dropping in Beijing, while it becomes less important (7%) as PM2.5 reaches below 40 μg/m3, similar to Los Angeles since 1980s. Therefore, the two cities show the convergence of air pollutant characteristics. Hence a control strategy prioritizing reactive volatile organic compound abatement is projected to yield simultaneous ozone and PM2.5 reductions in Beijing, as experienced in Los Angeles.

Abstract Despite the recent decrease in pollution events in Chinese urban areas, the World Health Organization air quality guideline values are still exceeded. Observations from monitoring networks show a stronger decrease of organic aerosol directly emitted to the atmosphere relative to secondary organic aerosol (SOA) generated from oxidation processes. Here, the uptake of water-soluble gas-phase oxidation products is reported as a major SOA contribution to particulate pollution in Beijing, triggered by the increase of aerosol liquid water. In pollution episodes, this pathway is enough to explain the increase in SOA mass, with formaldehyde, acetaldehyde, glycolaldehyde, formic acid, and acetic acid alone explaining 15%–25% of the SOA increase. Future mitigation strategies to reduce non-methane volatile organic compound emissions should be considered to reduce organic particulate pollution in China.

As an important reactive trace gases in the troposphere, nitryl chloride (ClNO2) has significant impacts on atmospheric oxidation capacity , the degradation of primary pollutants and the formation of secondary pollutants, and plays indispensable roles in global cycles of both nitrogen and chlorine. In this paper, we introduce basic properties of ClNO2 as well as its formation and removal mechanisms in the troposphere, and describe in brief techniques currently used in laboratory and field work to measure ClNO2. In addition , we review spatial and temporal distributions of tropospheric ClNO2 over the globe as reported in the last 10 similar to 20 years , discuss in a systematical manner chemical mechanisms and environmental factors which determine its heterogeneous formation in the atmosphere via critical analysis of important results from laboratory studies and field measurements, and summarize impacts of ClNO(2 )on chlorine radicals, atmospheric oxidation capacity as well as the formation of O-3 and nitrate aerosol. We emphasize that ClNO2 couples gas phase chemistry and heterogeneous chemistry , and also couples nocturnal atmospheric chemistry with daytime photochemistry , thus very likely playing an important role in the formation of air pollution complex in China. Important questions which remain to be answered to better understand atmospheric chemistry of ClNO2 are outlined at the end, and we also discuss in brief how these questions can be addressed in future work.

High concentrations of PM2.5 (particulate matter with an aerodynamic diameter less than 2.5 mu m) in China have caused severe visibility degradation. Accurate simulations of PM2.5 and its chemical components are essential for evaluating the effectiveness of pollution control strategies and the health and climate impacts of air pollution. In this study, we compared the GEOS-Chem model simulations with comprehensive datasets for organic aerosol (OA), sulfate, nitrate, and ammonium in China. Model results are evaluated spatially and temporally against observations. The new OA scheme with a simplified secondary organic aerosol (SOA) parameterization significantly improves the OA simulations in polluted urban areas, highlighting the important contributions of anthropogenic SOA from semivolatile and intermediate-volatility organic compounds. The model underestimates sulfate and overestimates nitrate for most of the sites throughout the year. More significant underestimation of sulfate occurs in winter, while the overestimation of nitrate is extremely large in summer. The model is unable to capture some of the main features in the diurnal pattern of the PM2.5 chemical components, suggesting inaccuracies in the presented processes. Potential model adjustments that may lead to a better representation of the boundary layer height, the precursor emissions, hydroxyl radical concentrations, the heterogeneous formation of sulfate and nitrate, and the wet deposition of nitric acid and nitrate have been tested in the sensitivity analysis. The results show that uncertainties in chemistry perhaps dominate the model biases. The proper implementation of heterogeneous sulfate formation and the good estimates of the concentrations of sulfur dioxide, hydroxyl radical, and aerosol liquid water are essential for the improvement of the sulfate simulation. The update of the heterogeneous uptake coefficient of nitrogen dioxide significantly reduces the modeled concentrations of nitrate. However, the large overestimation of nitrate concentrations remains in summer for all tested cases. The possible bias in the chemical production and the wet deposition of nitrate cannot fully explain the model overestimation of nitrate, suggesting issues related to the atmospheric removal of nitric acid and nitrate. A better understanding of the atmospheric nitrogen budget, in particular, the role of the photolysis of particulate nitrate, is needed for future model developments. Moreover, the results suggest that the remaining underestimation of OA in the model is associated with the underrepresented production of SOA.