Modeling study of PM2.5 pollutant transport across cities in China's Jing-Jin-Ji region during a severe haze episode in December 2013

To study the influence of particulate matter (PM) transported from surrounding regions on the high PM2.5 pollution levels in Beijing, the GRAPES-CUACE model was used to simulate a serious haze episode that occurred on 67 December 2013. The results demonstrate the model's suitability for describing haze episodes throughout China, especially in the Beijing-Tianjin-Hebei (Jing-Jin-Ji) region. A very close positive correlation was found between the southerly wind speed over the plain to the south of Beijing and changes in PM2.5 in Beijing, both reaching maximum values at about 900 hPa, suggesting that the lower atmosphere was the principal layer for pollutant PM transport from its southern neighbouring region to Beijing. During haze episodes, and dependent upon the period, Beijing was either a pollution source or sink for its surrounding area. PM input from Beijing's environs was much higher than the output from the city, resulting in the most serious pollution episode, with the highest PM2.5 values occurring from 00:00 to 10:00 UTC (08:00 to 18:00 LT), 7 December 2013. PM pollutants from the environs of the city accounted for over 50% of the maximum PM2.5 values reached in Beijing. At other times, the Beijing area was a net contributor to pollution in its environs.