Study on the contribution of transport to PM2.5 in typical regions of China using the regional air quality model RAMS-CMAQ

Regional transport plays a significant role in the air pollution in China, which is characterized by diverse emission sources and distinct distributions. To improve the understanding of factors determining the PM2.5 level in China, a source apportionment tool coupled with the RAMS-CMAQ model (CMAQ-ISAM) was used to quantify the contribution of transport to PM2.5 and its major components during 2017 in the North China Plain (NCP), Yangtze River Delta (YRD), Pearl River Delta (PRD), and Chengyu area. It is found that transport accounts for a predominant fraction of the PM2.5 in Beijing, Tianjin, and Shanghai with relatively low PM2.5 levels. Transport in the NCP is mainly at intraregional scale and comparable to local emissions. In contrast, the contributions of interregional transport from the NCP to the YRD (similar to 10-25%) and from NCP and YRD to PRD and Chengyu (similar to 5-25%) is at similar level to those of intraregional transport and local emissions in winter and fall, but are lower in spring and summer. It is worth noting that particle components have very different transport capabilities. Nitrate exhibits much stronger intraregional transport than other components in the NCP, and much higher concentration than other components during winter. In contrast, the concentration of sulfate is higher than that of nitrate during spring and summer in most provinces. In addition, the transport potential of primary OC, EC, and ammonia are relatively weaker, but these compounds can still have considerable contributions. Our results reveal that the notable contributions of regional transport to PM2.5 should be addressed according to. targeted emission sources in order to improve air quality efficiently.