Carbonaceous aerosols in China: top-down constraints on primary sources and estimation of secondary contribution
We simulated elemental carbon (EC) and organic carbon (OC) aerosols in China and compared model results to surface measurements at Chinese rural and background sites, with the goal of deriving 'top-down' emission estimates of EC and OC, as well as better quantifying the secondary sources of OC. We included in the model state-of-the-science Chinese 'bottom-up' emission inventories for EC (1.92 TgC yr(-1)) and OC (3.95 TgC yr(-1)), as well as updated secondary OC formation pathways. The average simulated annual mean EC concentration at rural and background sites was 1.1 mu gC m(-3), 56% lower than the observed 2.5 mu gC m(-3). The average simulated annual mean OC concentration at rural and background sites was 3.4 mu gC m(-3), 76% lower than the observed 14 mu gC m(-3). Multiple regression to fit surface monthly mean EC observations at rural and background sites yielded the best estimate of Chinese EC source of 3.05 +/- 0.78 TgC yr(-1). Based on the top-down EC emission estimate and observed seasonal primary OC/EC ratios, we estimated Chinese OC emissions to be 6.67 +/- 1.30 TgC yr(-1). Using these top-down estimates, the simulated average annual mean EC concentration at rural and background sites was significantly improved to 1.9 mu gC m(-3). However, the model still significantly underestimated observed OC in all seasons (simulated average annual mean OC at rural and background sites was 5.4 mu gC m(-3)), with little skill in capturing the spatiotemporal variability. Secondary formation accounts for 21% of Chinese annual mean surface OC in the model, with isoprene being the most important precursor. In summer, as high as 62% of the observed surface OC may be due to secondary formation in eastern China. Our analysis points to four shortcomings in the current bottom-up inventories of Chinese carbonaceous aerosols: (1) the anthropogenic source is underestimated on a national scale, particularly for OC; (2) the spatiotemporal distributions of emissions are misrepresented; (3) there is a missing source in western China, likely associated with the use of biofuels or other low-quality fuels for heating; and (4) sources in fall are not well represented, either because the seasonal shifting of emissions and/or secondary formation are poorly captured or because specific fall emission events are missing. In addition, secondary production of OC in China is severely underestimated. More regional measurements with better spatiotemporal coverage are needed to resolve these shortcomings.