Origin, Maintenance and Variability of the Asian Tropopause Aerosol Layer (ATAL): The Roles of Monsoon Dynamics

William K M Lau,Cheng Yuan,Zhanqing Li

doi:10.1038/s41598-018-22267-z

Abstract

Using NASA MERRA2 daily data, we investigated the origin, maintenance and variability of the Asian Tropopause Aerosol Layer (ATAL) in relation to variations of the Asia Monsoon Anticyclone (AMA) during the summer of 2008. During May-June, abundant quantities of carbon monoxide (CO), carbonaceous aerosols (CA) and dusts are found in the mid- and upper troposphere over India and China, arising from enhanced biomass burning emissions, as well as westerly transport from the Middle East deserts. During July-August, large quantities of dusts transported from the deserts are trapped and accumulate over the southern and eastern foothills of the Tibetan Plateau. Despite strong precipitation washout, ambient CO, CA and dust are lofted by orographically forced deep convection to great elevations, 12–16 km above sea level, via two key pathways over heavily polluted regions: a) the Himalayas-Gangetic Plain, and b) the Sichuan Basin. Upon entering the upper-troposphere-lower-stratosphere, the pollutants are capped by a stable layer near the tropopause, advected and dispersed by the anticyclonic circulation of AMA, forming the ATAL resembling a planetary-scale “double-stem chimney cloud”. The development and variability of the ATAL are strongly linked to the seasonal march and intraseasonal (20–30 days and higher frequency) oscillations of the Asian monsoon.

Highlights

It is well known that aerosol species from both natural and anthropogenic sources can enter the ATAL via diverse pathways, i.e., volcanic eruptions, generation of secondary aerosols through ice cloud microphysical and chemical processes, and atmospheric transport[21,22,23]
Previous studies have shown that both slow tropical upwelling and overshooting deep convection associated with the ASM can transport water vapor, as well as surface pollutants including SO2, black carbon (BC) and organic carbon (OC) over India and China into the UTLS9,24–28
During the pre-monsoon period, atmospheric loading of CO, and CA from biomass burning and desert dusts are strongly increased from surface to the mid- and upper troposphere (~5–10 km) likely due to a deepened planetary boundary layer and intensified dry convection associated with strong land surface heating, as well as orographic uplifting

Summary

Introduction

It is well known that aerosol species from both natural and anthropogenic sources can enter the ATAL via diverse pathways, i.e., volcanic eruptions, generation of secondary aerosols through ice cloud microphysical and chemical processes, and atmospheric transport[21,22,23]. The last decade has seen a dramatic increase in studies of aerosol-climate interactions for the ASM, revealing strong evidences that monsoon meteorology can affect emissions, transport, and accumulation of aerosols but can be influenced by aerosol radiative and microphysical effects, impacting monsoon weather and climate on diverse temporal (hours to decades), and spatial (1–104 km) scales[31,32] These studies have shown that aerosol, the light absorbing types, e.g., desert dusts, black carbon and organic carbon from biomass burning are components of an intrinsic aerosol-monsoon climate system[33,34].

Methods

Results

Conclusion