Significant reduction of PM.sub.2.5 in eastern China due to regional-scale emission control: evidence from SORPES in 2011-2018

Haze pollution caused by PM.sub.2.5 is the largest air quality concern in China in recent years. Long-term measurements of PM.sub.2.5 and the precursors and chemical speciation are crucially important for evaluating the efficiency of emission control, understanding formation and transport of PM.sub.2.5 associated with the change of meteorology, and accessing the impact of human activities on regional climate change. Here we reported long-term continuous measurements of PM.sub.2.5, chemical components, and their precursors at a regional background station, the Station for Observing Regional Processes of the Earth System (SORPES), in Nanjing, eastern China, since 2011. We found that PM.sub.2.5 at the station has experienced a substantial decrease (-9.1 % yr.sup.-1 ), accompanied by even a very significant reduction of SO.sub.2 (-16.7 % yr.sup.-1 ), since the national "Ten Measures of Air" took action in 2013. Control of open biomass burning and fossil-fuel combustion are the two dominant factors that influence the PM.sub.2.5 reduction in early summer and winter, respectively. In the cold season (November-January), the nitrate fraction was significantly increased, especially when air masses were transported from the north. More NH.sub.3 available from a substantial reduction of SO.sub.2 and increased oxidization capacity are the main factors for the enhanced nitrate formation. The changes of year-to-year meteorology have contributed to 24 % of the PM.sub.2.5 decrease since 2013. This study highlights several important implications on air pollution control policy in China.