Chemical composition of ambient PM2.5 over China and relationship to precursor emissions during 2005–2012

In this work, we presented the characteristic of PM2.5 chemical composition over China for the period of 2005– 2012, by synthesis of in-situ measurement data collected from literatures and satellite-based estimates using aerosol optical 10 depth (AOD) data and the GEOS-Chem chemical transport model. We revealed the spatiotemporal variations in PM2.5 composition during 2005–2012 and investigated the driving forces behind the variations by examining the changes in precursor emissions using a bottom-up emission inventory. Both in-situ observations and satellite-based estimates identified that secondary inorganic aerosols (i.e., sulfate, nitrate, and ammonium, SNA) ranked the highest fraction in dust-free PM2.5 concentrations, followed by organic matters (OM) and black carbon (BC). For instance, satellite-based estimates found that 15 SNA, OM and BC contributed to 59%, 33% and 8% of national population-weighted mean dust-free PM2.5 concentrations respectively during 2005–2012. National population-weighted mean PM2.5 concentration increased from 63.9 μg/m in 2005 to 75.2 μg/m in 2007, and subsequently decreased to 66.9 μg/m from 2007 to 2012. Variations in PM2.5 concentrations are mainly driven by the decrease of sulfate and the increase of nitrate. Population-weighted mean sulfate concentration decreased by 2.4% per year during 2005–2012 (from 14.4 μg/m to 12.9 μg/m), while population-weighted mean nitrate 20 concentration increased by 3.4% per year during 2005–2012 (from 9.8 μg/m to 12.2 μg/m), largely offsetting the decrease of sulfate concentrations. By examining the emission data from the MEIC emission inventory, we found that the changes in sulfate and nitrate concentrations were in line with the decrease in SO2 emission and increase in NOx emissions during the same period. The desulfurization regulation in power plants enforced around 2005 was the primary contributor to the SO2 emissions reduction since 2006. In contrast, growth of energy consumption and lack of control measures for NOx resulted in 25 persistent increase in NOx emissions until the installation of denitrification devices on power plants late in 2011, which began to take effect in 2012. The results of this work indicate that the synchronized abatement of emissions for multipollutants are necessary for reducing ambient PM2.5 concentrations over China.