Tropospheric ozone (O₃) has replaced PM_2.5 or PM₁₀ as the primary pollutant in the North China Plain (NCP) during summer in recent years. A comprehensive understanding of O₃ production in response to the reduction of precursor emissions over the NCP is urgently demanded for effective control policy design. In this study, the air quality modeling system RAMS-CMAQ (Regional Atmospheric Modeling System–Community Multiscale Air Quality), coupled with the ISAM (Integrated Source Apportionment Method) module is applied to investigate the O₃ regional transport and source contribution features during a heavy O₃ pollution episode in June 2015 over the NCP. The results show that emissions sources in Shandong and Hebei were the major contributors to O₃ production in the NCP. Not only the highest local contribution of O₃ mass burden but also more than 30 % contribution of O₃ mass burdens in Beijing and Tianjin came from the emissions sources in these two provinces, respectively. Conversely, the urban areas and most O₃-polluted regions of the NCP were mainly dominated by conditions sensitive to volatile organic compounds, while both control and NO_x-sensitive conditions dominated the suburban and remote areas, respectively. Then, based on the sensitivity tests, the effects of several hypothetical scenarios of emissions control on reducing the O₃ pollution were compared and discussed. The results indicated that the emissions control of industry and residential sectors was the most efficient method if the emissions reduction percentage was higher than 40 %. However, when the emissions reduction percentage dropped below 30 %, the power plant sector could make significant contributions to the decrease in O₃. The control strategies should be promptly adjusted based on the emissions reduction, and the modeling system can provide valuable information for precisely choosing the emissions sector combination to achieve better efficiency.

China is one of the largest agricultural countries in the world. Thus, NH₃ emission from agricultural activities in China considerably affects the country's regional air quality and visibility. In this study, a high-resolution agricultural NH₃ emission inventory compiled on 1 km × 1 km horizontal resolution was applied to calculate the NH₃ mass burden in China and reliably estimate the influence of NH₃ on agriculture. The key parameter emission factors of this inventory were enhanced by considering many experiment results, and the dynamic data of spatial and temporal information were updated using statistical data of 2015. In addition to fertilizers and husbandry, farmland ecosystems, livestock waste, crop residue burning, wood-based fuel combustion, and other NH₃ emission sources were included in this inventory. Furthermore, a source apportionment tool, namely, the Integrated Source Apportionment Method (ISAM) coupled with the air quality modeling system Regional Atmospheric Modeling System and Community Multiscale Air Quality, was applied to capture the contribution of NH₃ emitted from total agriculture (T_agr) in China. The aerosol mass concentration in 2015 was simulated, and results showed that the high mass concentration of NH₃ exceeded 10 µg m⁻³ and mainly appeared in the North China Plain, Central China, the Yangtze River Delta, and the Sichuan Basin. Moreover, the annual average contribution of T_agr NH₃ to PM_2.5 mass burden was 14 %–22 % in China. Specific to the PM_2.5 components, T_agr NH₃ contributed dominantly to ammonium formation (87.6 %) but trivially to sulfate formation (2.2 %). In addition, several brute-force sensitivity tests were conducted to estimate the impact of T_agr NH₃ emission reduction on PM_2.5 mass burden. In contrast to the result of ISAM, even though the T_agr NH₃ only provided 10.1 % contribution to nitrate under the current emission scenario, the reduction of nitrate could reach 95.8 % upon removal of the T_agr NH₃ emission. This deviation occurred because the contribution of NH₃ to nitrate should be small under a “rich NH₃”environment and large under a “poor NH₃” environment. Thus, the influence of NH₃ on nitrate formation would be enhanced with the decrease in ambient NH₃ mass concentration.

The unique energy structure, high intensity of coal production, and complex terrain, make Fenwei Plain a highly polluted region in China. In this study, we characterized the transport characteristic and sources of PM_2.5 (the fraction of particulate matter ≤ 2.5 μm) in Sanmenxia, a polluted city in canyon terrain. The results showed that special topography in Sanmenxia had an important role in the transport of particulates. Sanmenxia is located between two northeast-southwest facing mountains, showing a special local circulation. The local circulation was dominated by a downslope wind at nighttime, while the cross−mountain airflow and zonal wind were dominant during the daytime in the canyon terrain. PM_2.5 accumulated near Sanmenxia with the influence of downslope, zonal wind, and topography. The main regional transport paths could be summarized into an eastern path, a northern path, and a western path during the severe haze episodes. The PM_2.5 source apportionment revealed by an on-line tracer-tagged of the Nested Air Quality Prediction Model System (NAQPMS) showed that the main regional sources of Sanmenxia were Yuncheng, Sanmenxia, and Weinan. The contribution to PM_2.5 concentration in Sanmenxia was 39%, 25%, and 11%, respectively. The northern path had the most important impact on Sanmenxia. The results can provide scientific basis for the establishment of severe haze control in Sanmenxia and regional joint control.