Emission or atmospheric processes? An attempt to attribute the source of large bias of aerosols in eastern China simulated by global climate models

摘要

Global climate models often underestimate aerosol loadings in China, andthese biases can have significant implications for anthropogenic aerosolradiative forcing and climate effects. The biases may be caused by either theemission inventory or the treatment of aerosol processes in the models, orboth, but so far no consensus has been reached. In this study, a relativelynew emission inventory based on energy statistics and technology,Multi-resolution Emission Inventory for China (MEIC), is used to drive theCommunity Atmosphere Model version 5 (CAM5) to evaluate aerosol distributionand radiative effects against observations in China. The model results arecompared with the model simulations with the widely used IntergovernmentalPanel on Climate Change Fifth Assessment Report (IPCC AR5) emissioninventory. We find that the new MEIC emission improves the aerosol opticaldepth (AOD) simulations in eastern China and explains 22–28 % of the AODlow bias simulated with the AR5 emission. However, AOD is still biased low ineastern China. Seasonal variation of the MEIC emission leads to a betteragreement with the observed seasonal variation of primary aerosols than theAR5 emission, but the concentrations are still underestimated. This impliesthat the atmospheric loadings of primary aerosols are closely related to theemission, which may still be underestimated over eastern China. In contrast,the seasonal variations of secondary aerosols depend more on aerosolprocesses (e.g., gas- and aqueous-phase production from precursor gases) thatare associated with meteorological conditions and to a lesser extent on theemission. It indicates that the emissions of precursor gases for thesecondary aerosols alone cannot explain the low bias in the model.Aerosol secondary production processes in CAM5should also be revisited. The simulation using MEIC estimates theannual-average aerosol direct radiative effects (ADREs) at the top of theatmosphere (TOA), at the surface, and in the atmosphere to be −5.02,−18.47, and 13.45 W m−2, respectively, over eastern China, which areenhanced by −0.91, −3.48, and 2.57 W m−2 compared with the AR5emission. The differences of ADREs by using MEIC and AR5 emissions are largerthan the decadal changes of the modeled ADREs, indicating the uncertainty ofthe emission inventories. This study highlights the importance of improvingboth the emission and aerosol secondary production processes in modeling theatmospheric aerosols and their radiative effects. Yet, if the estimations ofMEIC emissions in trace gases do not suffer similar biases to those in theAOD, our findings will help affirm a fundamental error in the conversion fromprecursor gases to secondary aerosols as hinted in other recent studiesfollowing different approaches.