Yearly changes of the sulfate-nitrate-ammonium aerosols and the relationship with their precursors from 1999 to 2016 in Beijing

The change in particulate matter (PM)_2.5 composition in relation to precursors over recent decades has not been elucidated clearly in Beijing. Using ground-based measurements from the literature, this study investigated the yearly time series of PM_2.5 and its chemical composition over Beijing from 1999 to 2016 to identify the driving forces underlying these changes. The PM_2.5 concentration declined slightly, due to the organic carbon, elemental carbon, and dust rather than to either sulfate–nitrate–ammonium (SNA) aerosols. Before 2013, the trend of SNA aerosols was opposite to that of PM_2.5; however, subsequently, SNA aerosols have represented the major contribution to the reduction of PM_2.5, coinciding with a large decline of regional precursor gases. The yearly time series of SNA aerosols can be explained better by regional precursor gases than by local ones. Generally, precursor gases emissions over the region of North China Plain can be controlled if Beijing's air quality is to be improved.