Abstract
Abstract. Enhanced aerosol abundance in the upper troposphere and lower stratosphere (UTLS) associated with the Asian summer monsoon (ASM) is referred to as the Asian Tropopause Aerosol Layer (ATAL). The chemical composition, microphysical properties, and climate effects of aerosols in the ATAL have been the subject of discussion over the past decade. In this work, we use the ECHAM/MESSy Atmospheric Chemistry (EMAC) general circulation model at a relatively fine grid resolution (about 1.1×1.1∘) to numerically simulate the emissions, chemistry, and transport of aerosols and their precursors in the UTLS within the ASM anticyclone during the years 2010–2012. We find a pronounced maximum of aerosol extinction in the UTLS over the Tibetan Plateau, which to a large extent is caused by mineral dust emitted from the northern Tibetan Plateau and slope areas, lofted to an altitude of at least 10 km, and accumulating within the anticyclonic circulation. We also find that the emissions and convection of ammonia in the central main body of the Tibetan Plateau make a great contribution to the enhancement of gas-phase NH3 in the UTLS over the Tibetan Plateau and ASM anticyclone region. Our simulations show that mineral dust, water-soluble compounds, such as nitrate and sulfate, and associated liquid water dominate aerosol extinction in the UTLS within the ASM anticyclone. Due to shielding of high background sulfate concentrations outside the anticyclone from volcanoes, a relative minimum of aerosol extinction within the anticyclone in the lower stratosphere is simulated, being most pronounced in 2011, when the Nabro eruption occurred. In contrast to mineral dust and nitrate concentrations, sulfate increases with increasing altitude due to the larger volcano effects in the lower stratosphere compared to the upper troposphere. Our study indicates that the UTLS over the Tibetan Plateau can act as a well-defined conduit for natural and anthropogenic gases and aerosols into the stratosphere.
Highlights
The upper troposphere and lower stratosphere (UTLS) is a transition region between the troposphere and stratosphere, ranging about ±5 km around the tropopause (Gettelman et al, 2011), and the region plays an important role in the stratosphere–troposphere exchange (STE) and influences the Published by Copernicus Publications on behalf of the European Geosciences Union.J
The Asian summer monsoon (ASM) circulation is characterized by cyclonic flow and convergence in the lower troposphere and a strong anticyclone and divergence in the UTLS (Krishnamurti and Bhalme, 1976); its structure is primarily a response to diabatic heating associated with deep convection (Hoskins and Rodwell, 1995)
We have investigated the chemical components and their sources of aerosols in the UTLS over the Tibetan Plateau during summer, within the ASM anticyclone, using the ECHAM/Modular Earth Submodel System (MESSy) Atmospheric Chemistry (EMAC) atmospheric chemistry general circulation model at T106L90 resolution
Summary
The upper troposphere and lower stratosphere (UTLS) is a transition region between the troposphere and stratosphere, ranging about ±5 km around the tropopause (Gettelman et al, 2011), and the region plays an important role in the stratosphere–troposphere exchange (STE) and influences the Published by Copernicus Publications on behalf of the European Geosciences Union. While most studies have focused on the transport of pollutants originating from southern and southeastern Asia to the UTLS over the Tibetan Plateau and within the ASM anticyclone, the influence of natural sources such as mineral dust and their interactions with pollution on the formation of ATAL has not been well addressed. A 3 d case study using the WRF mesoscale model coupled with a dust module showed that deep convection in early summer over the Tibetan Plateau can inject dust aerosols into the stratosphere (Yang et al, 2014) The contribution of these dust plumes to the total aerosols in the UTLS over the Tibetan Plateau and associated transport and transformation processes needs to be thoroughly investigated.
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