In this work, we presented the characteristics 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 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 as the highest fraction of dust-free PM2.?5 concentrations, followed by organic matter (OM) and black carbon (BC). For instance, satellite-based estimates found that SNA, OM, and BC contributed to 59, 33, and 8?%, respectively, of national population-weighted mean dust-free PM2.?5 concentrations during 2005–2012. National population-weighted mean PM2.?5 concentration increased from 63.9?μg?m?3 in 2005 to 75.2?μg?m?3 in 2007 and subsequently decreased to 66.9?μg?m?3 from 2007 to 2012. Variations in PM2.?5 concentrations are mainly driven by the decrease in sulfate and the increase in nitrate. Population-weighted mean sulfate concentration decreased by 2.4?%?yr?1 during 2005–2012 (from 14.4 to 12.9?μg?m?3), while population-weighted mean nitrate concentration increased by 3.4?%?yr?1 during 2005–2012 (from 9.8 to 12.2?μg?m?3), largely offsetting the decrease in sulfate concentrations. By examining the emission data from the Multi-resolution Emission Inventory for China (MEIC), we found that the changes in sulfate and nitrate concentrations were in line with the decrease in SO2 emissions and the increase in NOx emissions during the same period. The desulfurization regulation in power plants enforced around 2005 has been the primary contributor to the SO2 emission reduction since 2006. In contrast, growth of energy consumption and lack of control measures for NOx resulted in a 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 is necessary for reducing ambient PM2.?5 concentrations over China.
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