Global modelling studies of composition and decadal trends of the Asian Tropopause Aerosol Layer

Abstract. The Asian Summer Monsoon (ASM) traps convectively-lifted boundary layer pollutants inside its upper-tropospheric lower-stratospheric Asian monsoon anticyclone (AMA). It is associated with a seasonal and spatially-confined enhanced aerosol layer, called the Asian Tropopause Aerosol Layer (ATAL). The knowledge of the ATAL properties in terms of aerosol budget, chemical composition, as well as its variability and temporal trend is still largely uncertain, due to the dynamical variability of the AMA, the dearth of in situ observations in this region, the complex transport pathways of pollutants and its atmospheric chemical processes. In this work, we use the Community Earth System Model (CESM 1.2 version) based on the coupling of the Community Atmosphere Model (CAM5) and the MAM7 (Modal Aerosol Model) aerosol module to simulate the composition of the ATAL and its decadal trends. Our simulations cover a long-term period of 16 years from 2000 to 2015. We identify a double-peak aerosols vertical profile for the ATAL. We attribute the upper peak (around 100 hPa, predominant during early ATAL in June) to dry aerosols, possibly from nucleation processes and the lower peak (around 250 hPa, predominant for a well-developed and late ATAL, in July and August) to cloud-borne aerosols associated with convective clouds. We find that mineral dust is the dominant aerosol by mass in the ATAL showing a large interannual variability, but no long-term trend, due to its natural variation. The results between 120–80 hPa (dry aerosol peak) suggest that for aerosols other than dust the ATAL is composed of around 40 % of sulfate, 30% of secondary and 15 % of primary organic aerosols, 14 % of ammonium aerosols and less than 3 % of black carbon. The analysis of the anthropogenic and biomass burning aerosols shows a positive trend for all aerosols simulated by CESM-MAM7.