Influence of convection over East Asia on the chemical composition of the Asian Tropopause Aerosol Layer inferred from airborne aerosol mass spectrometry

The Asian tropopause aerosol layer (ATAL) is a feature occurring within the anticyclone of the Asian summer monsoon in the UTLS region between 13 and 18 km. This aerosol layer can have significant implications for the Earth’s radiative budget (Vernier et al., 2015) and the chemistry of the stratosphere depending on its chemical composition. So far, ammonium nitrate, organics and sulfate have been identified as the main particle compounds found in the ATAL (Appel et al., 2022). High emissions of ammonia in northern India play a crucial role for the formation of ammonium nitrate in the ATAL (Höpfner et al., 2019). However, the effect of different origin regions on the chemical composition of the ATAL remains unclear.

Here, we present a comparison between aircraft-based measurements above India and Nepal during the StratoClim campaign in summer 2017 and above the Western Pacific during the ACCLIP campaign in summer 2022. For both airborne missions, the chemical composition of aerosol particles was measured using the hybrid aerosol mass spectrometer ERICA (ERC instrument for chemical composition of aerosols; Hünig et al., 2022; Dragoneas et al., 2022). In addition, the air mass origin was determined based on kinematic backward trajectories combined with satellite-derived convective cloud top altitudes.

Our results from the non-refractory particle composition measurements reveal a larger contribution of organics and sulfate and less ammonium nitrate mass fractions during the ACCLIP mission compared to the StratoClim measurements. Combining the ERICA results and the trajectory-based product of air mass history, the differences could be explained by a large contribution from east Asian sources. In 2022, the monsoon anticyclone extended further to the northeast compared to the climatological mean. Thus, our results suggest the convection above eastern China with high emissions of SO₂ and volatile organic compounds as driver of the observed changes in the ATAL composition.

 

References:

Appel, O., Köllner, F., Dragoneas, A., et al.: Chemical analysis of the Asian tropopause aerosol layer (ATAL) with emphasis on secondary aerosol particles using aircraft-based in situ aerosol mass spectrometry, Atmos. Chem. Phys., 22, 13607–13630, https://doi.org/10.5194/acp-22-13607-2022, 2022.

Dragoneas, A., Molleker, S., Appel, O., et al.: The realization of autonomous, aircraft-based, real-time aerosol mass spectrometry in the upper troposphere and lower stratosphere, Atmos. Meas. Tech., 15, 5719–5742, https://doi.org/10.5194/amt-15-5719-2022, 2022.

Höpfner, M., Ungermann, J., Borrmann, S. et al.: Ammonium nitrate particles formed in upper troposphere from ground ammonia sources during Asian monsoons. Nat. Geosci., 12, 608–612, https://doi.org/10.1038/s41561-019-0385-8, 2019.

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