Patterns in atmospheric carbonaceous aerosols in China: emission estimates and observed concentrations
Abstract. China is experiencing severe carbonaceous aerosol pollution driven mainly by large emissions resulting from intensive use of solid fuels. To gain a better understanding of the levels and trends of carbonaceous aerosol emissions and the resulting ambient concentrations at the national scale, we update an emission inventory of anthropogenic organic carbon (OC) and elemental carbon (EC) and employ existing observational studies to analyze characteristics of these aerosols including temporal, spatial, and size distributions, and the levels and shares of secondary organic carbon (SOC) in total OC. We further use ground observations to test the levels and inter-annual trends of the calculated national and provincial emissions of carbonaceous aerosols, and propose possible improvements in emission estimation for the future. The national OC emissions are estimated to have increased 29 % from 2000 (2127 Gg) to 2012 (2749 Gg) and EC by 37 % (from 1356 to 1857 Gg). The residential, industrial, and transportation sectors contributed an estimated 74–78, 17–21, and 4–6 % of the total emissions of OC, respectively, and 49–55, 30–34, and 14–18 % of EC. Updated emission factors (EFs) based on the most recent local field measurements, particularly for biofuel stoves, led to considerably lower emissions of OC compared to previous inventories. Compiling observational data across the country, higher concentrations of OC and EC are found in northern and inland cities, while higher OC / EC ratios are found in southern sites, due to the joint effects of primary emissions and meteorology. Higher OC / EC ratios are estimated at rural and remote sites compared to urban ones, attributed to more emissions of OC from biofuel use, more biogenic emissions of volatile organic compound (VOC) precursors to SOC, and/or transport of aged aerosols. For most sites, higher concentrations of OC, EC, and SOC are observed in colder seasons, while SOC / OC is reduced, particularly at rural and remote sites, attributed partly to weaker atmospheric oxidation and SOC formation compared to summer. Enhanced SOC formation from oxidization and anthropogenic activities like biomass combustion is judged to have crucial effects on severe haze events characterized by high particle concentrations. Several observational studies indicate an increasing trend in ambient OC / EC (but not in OC or EC individually) from 2000 to 2010, confirming increased atmospheric oxidation of OC across the country. Combining the results of emission estimation and observations, the improvement over prior emission inventories is indicated by inter-annual comparisons and correlation analysis. It is also indicated, however, that the estimated growth in emissions might be faster than observed growth, and that some sources with high primary OC / EC, such as burning of biomass, are still underestimated. Further studies to determine changing EFs over time in the residential sector and to compare to other measurements, such as satellite observations, are thus suggested to improve understanding of the levels and trends of primary carbonaceous aerosol emissions in China.