Constraining Black Carbon Aerosol over Asia using OMI Aerosol Absorption Optical Depth and the Adjoint of GEOS-Chem

Accurate estimates of the emissions and distribution of Asian black carbon (BC) are critical to atmospheric environment and climate change studies. Constraining BC from observations is crucial to understanding emission uncertainties and improving model simulations. We present a top-down estimate that spatially constrains the BC emissions using OMI observation of aerosol absorption optical depth (AAOD) with the GEOS-Chem model and its adjoint for April and October 2006. The analysis indicates underestimation of anthropogenic emissions over most of the Asian area when using any of four different emission inventories. Overwhelming enhancements of anthropogenic emissions for BC are shown after optimization over broad areas of Asia in April, up to 500%. In October, the optimization of anthropogenic emissions yields a slight reduction (1~5%) over India and parts of south China while emissions increase by 10~50% over eastern China. Sensitivity experiments based on different prior anthropogenic emissions inventories are conducted to explore the emission uncertainties and annul variability. Observational data from in situ measurements and remote sensing (AERONET) are used to evaluate the BC inversions and assess the AAOD bias between OMI and AERONET. Low biases of BC concentrations are improved or corrected in most of eastern and central sites over China after optimization, while the constrained model still underestimates concentration in Indian sites both in April and October. In addition, the impacts of different observation operators and a priori constraints on the optimizations are investigated. We also use WRF-Chem to assess the impacts of model resolution and dynamic feedbacks on the discrepancies between model and observation.