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In the present study, the WRF-CHEM model is used toevaluate the contributions of trans-boundary transport to the air quality inBeijing during a persistent air pollution episode from 5 to 14 July 2015 inBeijing–Tianjin–Hebei (BTH), China. Generally, the predicted temporalvariations and spatial distributions of PM2.5 (fine particulatematter), O3 (ozone), and NO2 are in good agreement withobservations in BTH. The WRF-CHEM model also reproduces reasonably well thetemporal variations of aerosol species compared to measurements in Beijing.The factor separation approach is employed to evaluate the contributions oftrans-boundary transport of non-Beijing emissions to the PM2.5 andO3 levels in Beijing. On average, in the afternoon during thesimulation episode, the local emissions contribute 22.4 % to the O3level in Beijing, less than 36.6 % from non-Beijing emissions. The O3concentrations in Beijing are decreased by 5.1 % in the afternoon due tointeractions between local and non-Beijing emissions. The non-Beijingemissions play a dominant role in the PM2.5 level in Beijing, with acontribution of 61.5 %, much higher than 13.7 %, from Beijing localemissions. The emission interactions between local and non-Beijing emissionsenhance the PM2.5 concentrations in Beijing, with a contribution of5.9 %. Therefore, the air quality in Beijing is generally determined bythe trans-boundary transport of non-Beijing emissions during summertime,showing that the cooperation with neighboring provinces to mitigatepollutant emissions is key for Beijing to improve air quality

Contributions of trans-boundary transport to summertime air quality in Beijing, China