Comment on acp-2021-910

doi:10.5194/acp-2021-910-rc2

Abstract

Satellite observations have revealed an enhanced aerosol layer near the tropopause over Asia during the summer monsoon, called the Asian Tropopause Aerosol Layer (ATAL). In this work, aerosol particles in the ATAL were collected with a balloon-borne impactor near the tropopause region over India, using extended duration balloon flights, in summer 2017 and winter 2018. Their chemical composition was further investigated by quantitative analysis using offline ion chromatography. Nitrate (NO3−) and nitrite (NO2−) were found to be the dominant ions in the collected aerosols with values ranging between 87–343 ng/m3 STP during the summer campaign. In contrast, sulfate (SO42−) levels were found above the detection limit (> 10 ng/m3 STP) only in winter. In addition, we determined the origin of the air masses sampled during the flights through analysis of back trajectories along with convective influence. The results obtained therein were put into a context of large-scale transport and aerosol distribution with GEOS-Chem chemical transport model simulations. The first flight of summer 2017 which sampled air mass within the Asian monsoon anticyclone (AMA), influenced by convection over Western China, was associated with particle size radius (0.05–2 μm). In contrast, the second flight sampled air mass at the edge of the AMA associated with larger particle size radius (> 2 μm) with higher nitrite concentration. The sampled air masses in winter 2018 were likely affected by smoke from the Pacific Northwest fire event in Canada, which occurred 7 months prior to our campaign, leading to concentration enhancements of SO42− and Ca2+. Overall, our results suggest that nitrogen-containing particles represent a large fraction of aerosols populating the ATAL, in agreement with the results from aircraft measurements during the StratoClim campaign. Furthermore, GEOS-Chem model simulations suggest that lightning NOx emissions had a significant impact on the production of nitrate aerosols sampled during the summer 2017.

Highlights

1.1 Asian Summer Monsoon and the transport of pollutionRapid economic growth in Asia over the past two decades have led to serious environmental threats on water and air qualities
K+ was seen on S3 with a concentration of 26ng/m3 STP that could have originated from biomass burning
In situ measurements revealed the presence of NO3, and NO2- aerosols (60-200 ng/m3 STP) of size ranging between 0.05-2μm

Summary

Introduction

1.1 Asian Summer Monsoon and the transport of pollutionRapid economic growth in Asia over the past two decades have led to serious environmental threats on water and air qualities. Polluted air masses transported from the boundary layer to higher altitudes are confined within the Asian Monsoon Anticyclone (AMA) (Ploeger et al, 2017). Deep convective clouds represent conduits for air pollution to reach the Upper Troposphere and Lower Stratosphere (UTLS) region. Aerosols in the UTLS have longer residence times than those in the lower troposphere, influencing the chemistry of the atmosphere and the Earth’s climate (Rasch et al, 2008). They affect the concentration of chemical species through changes in photolysis rates and heterogenous reactions (Pitari et al, 2014). It has been further reported that aerosols in the UTLS can impact climate by altering properties of cirrus clouds via homogeneous or heterogeneous ice nucleation (Li et al., 2005; Liu et al, 2009; Yin et al, 2012; Fadnavis et al, 2013)

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