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Fine particulate matter (PM2.5) is closely linked to human health, with its sources generally divided into local emissions and regional transport. This study combined concentration-weighted trajectory (CWT) analysis with the HYSPLIT trajectory ensemble to obtain hourly-resolution pollutant source results. The Extreme Gradient Boosting (XGBoost) model was then employed to simulate local emissions and ambient PM2.5 in Beijing from 2013 to 2020. The results revealed that clean air masses influencing the Beijing area mainly originated from the north and east regions, exhibiting a strong winter and weak summer pattern. Following the implementation of the Air Pollution Prevention and Control Action Plan (Action Plan) by the Chinese government in 2017, pollution in Beijing decreased significantly, with the most substantial reduction in regional transport pollution events occurring in the west region during summer. Regional transport pollution events were most frequent in spring, up to 1.8 times higher than in winter. Pollutants mainly originated from the west and south regions, while polluted air masses from the east showed the least reduction, and the proportion of pollution sources from this region is gradually increasing. From 2013 to 2020, local emissions were the main contributors of pollution events in Beijing. The Action Plan has more effectively reduced pollution caused by regional transport, particularly during autumn and winter. This finding underscores the importance of Beijing prioritizing local emission reduction while also considering potential contributions from the east region to effectively mitigate pollution events. [ABSTRACT FROM AUTHOR]

China's sustained air quality improvement is hindered by unregulated ammonia (NH3) emissions from inefficient nitrogen management in smallholder farming. Although the Chinese government is promoting a policy shift to large-scale farming, the benefits of this, when integrated with nitrogen management, remain unclear. Here we fill this gap using an integrated assessment, by combining geostatistical analysis, high-resolution emission inventories, farm surveys and air quality modeling. Smallholder-dominated farming allows only 13%–31% NH3 reduction, leading to limited PM2.5 decreases nationally due to non-linear PM2.5 chemistry. Conversely, large-scale farming would double nitrogen management adoption rates, increasing NH3 reduction potential to 48%–58% and decreasing PM2.5 by 9.4–14.0 μg·m−3 in polluted regions. The estimated PM2.5 reduction is conservative due to localized NH3-rich conditions under large-scale livestock farming. This strategy could prevent over 300 000 premature deaths and achieve a net benefit of US $68.4–86.8 billion annually, unlocking immense benefits for air quality and agricultural sustainability. [ABSTRACT FROM AUTHOR]

A record-breaking East Asian dust storm over recent years occurred in March 2021. Ångström Exponent (AE) can resolve the particle size and is significantly sensitive to large aerosol such as dust. Due to lack of observation during dust storm and high uncertainty of satellite retrieved AE, it is crucial to estimate the dust storm emission using the lagged ground-based AE observations. In this study, the Aerosol Robotic Network (AERONET) observed hourly AEs are assimilated with the fixed-lag ensemble Kalman smoother and Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) to optimize the simulated dust emission from 14 to 16 March 2021. The emission inversion results reveal that the dust emissions from the Gobi desert in the official WRF-Chem are significantly underestimated. Not only the temporal variation of simulated AE but also that of simulated aerosol optical thickness (AOT) can be improved through assimilating AE information. Compared to the assimilation with only AOT, the additional inclusion of AE doubles the dust emission and induces the extra 46.8 % improvement of root mean square error (RMSE) between the simulated AOTs and the AERONET and independent Skynet Observation NETwork (SONET) observations. The optimized dust emission from Mongolia Gobi and China Gobi reach the peak value about 441.65 kt/hour and 346.87 kt/hour at 08:00 UTC on 14 March and at 19:00 UTC on 15 March, respectively. The additional inclusion of AE also best captures the magnitude and variations of aerosol vertical extinctions both in the westward and eastward pathways of dust transport. [ABSTRACT FROM AUTHOR]