Estimates of Regional Source Contributions to the Asian Tropopause Aerosol Layer Using a Chemical Transport Model

The Asian Tropopause Aerosol Layer (ATAL) represents an accumulation of aerosol in the upper troposphere and lower stratosphere associated with the Asian Summer Monsoon. Here we simulate the ATAL for summer 2013 with the GEOS‐Chem chemical transport model and explore the likely composition of ATAL aerosols and the relative contributions of regional anthropogenic sources versus those from farther afield. The model indicates significant contributions from organic aerosol, nitrate, sulfate, and ammonium aerosol, with regional anthropogenic precursor sources dominant. The model underestimates aerosol backscatter in the ATAL during summer 2013, compared with Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite instrument. Tests of a more physically based treatment of wet scavenging of SO2 in convective updrafts raise sulfate, eliminating the low bias with respect to CALIPSO backscatter in the ATAL, but lead to an unacceptable high bias of sulfate compared with in situ observations from aircraft over the United States. Source apportionment of the model results indicate the dominance of regional anthropogenic emissions from China and the Indian subcontinent to aerosol concentrations in the ATAL; ~60% of sulfate in the ATAL region in August 2013 is attributable to anthropogenic sources of SO2 from China (~30%) and from the Indian subcontinent (~30%), twice as much as in previously published estimates. Nitrate aerosol is found to be a dominant component of aerosol composition on the southern flank of the Asian Summer Monsoon anticyclone. Lightning sources of NOx are found to make a significant (10–15%) contribution to nitrate in the ATAL for the case studied.