Evolution of secondary inorganic aerosols amidst improving PM2.5 air quality in the North China plain

The Clean Air Action implemented by the Chinese government in 2013 has greatly improved air quality in the North China Plain (NCP). In this work, we report changes in the chemical components of atmospheric fine particulate matter (PM2.5) at four NCP sampling sites from 2012/2013 to 2017 to investigate the impacts and drivers of the Clean Air Action on aerosol chemistry, especially for secondary inorganic aerosols (SIA). During the observation period, the concentrations of PM2.5 and its chemical components (especially SIA, organic carbon (OC), and elemental carbon (EC)) and the frequency of polluted days (daily PM2.5 concentration
75 mg m-3) in the NCP, declined significantly at all four sites. Asynchronized reduction in SIA components (large decreases in SO4 2 with stable or even increased NO3 and NH4) was observed in urban Beijing, revealing a shift of the primary form of SIA, which suggested the fractions of NO3 increased more rapidly than SO4 during PM2.5 pollution episodes, especially in 2016 and 2017. In addition, unexpected increases in the sulfur oxidation ratio (SOR) and the nitrogen oxidation ratio (NOR) were observed among sites and across years in the substantially decreased PM2.5 levels. They were largely determined by secondary aerosol precursors (i.e. decreased SO2 and NO2), photochemical oxidants (e.g. increased O3), temperature, and relative humidity via gas-phase and heterogeneous reactions. Our results not only highlight the effectiveness of the Action Plan for improving air quality in the NCP, but also suggest an increasing importance of SIA in determining PM2.5 concentration and composition.