Chemical analysis of the 2017 ATAL measured during StratoClim – New insights into refractory aerosol components

The chemical nature of the Asian aerosol tropopause layer (ATAL) was controversially discussed in the past decade. Modeling studies show the importance of black carbon and mineral dust aerosol for the formation of the ATAL (e.g., Bossolasco et al., 2021).  However, in-situ measurements at these high altitudes are sparse. We present the first in-situ measurements of the ATAL chemical composition conducted during the aircraft-based campaign StratoClim in July/August 2017 out of Kathmandu. Our ERICA instrument combines the laser desorption ionization mass spectrometry and the thermal desorption with subsequent electron impact ionization techniques, allowing measurements of refractory and non-refractory aerosol components. The ERICA is able to detect particles in the size range from 120 nm to 3500 nm (d_va, d₅₀ cutoff; Hünig et al., 2022). In parallel, particle samples were also collected in-situ and examined a-posteriori using scanning electron microscopy (SEM) and X-ray microanalysis (EDX). Results of both methods will be shown and discussed.

In our recent publication, we demonstrated that a large fraction (up to 70 %) of the ATAL particles is of purely secondary origin (Appel et al., 2022). Nitrate and organics are the dominant non-refractory components. In contrast to the secondary particle type, we found that a non-negligible fraction (up to 50 % in the lower ATAL region) of the particles include refractory components. In regions above 400 K potential temperature, the aerosol can be attributed to meteoric material (Schneider et al., 2021). Below 400 K, we found that refractory components are mainly linked to the presence of potassium, internally mixed with nitrate, sulfate, and organics. However, the vertical profile of elemental carbon (EC) shows its presence within the ATAL, albeit with an abundance in the lower percentage range. Likewise, the abundance of iron, sodium, and calcium indicative for the transport from ground sources is in the lower percentage range. Nonetheless, we observed these refractory particles in the boundary layer above Kathmandu with a higher abundance as compared to that within the ATAL. We thus assume that the transport efficiency of refractory particles from ground sources to the UTLS is strongly limited by wet deposition.  

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.

Bossolasco, A., Jegou, F., Sellitto, P., et al.: Global modeling  studies of composition and decadal trends of the Asian Tropopause Aerosol Layer, Atmos. Chem. Phys., 21, 2745–2764, https://doi.org/10.5194/acp-21-2745-2021, 2021

Hünig, A., Appel, O., Dragoneas, A., et al.: Design, characterization, and first field deployment of a novel aircraft-based aerosol mass spectrometer combining the laser ablation and flash vaporization techniques, Atmos. Meas. Tech., 15, 2889–2921, https://doi.org/10.5194/amt-15-2889-2022, 2022.

Schneider, J., Weigel, R., Klimach, T., et al.: Aircraft-based observation of meteoric material in lower-stratospheric aerosol particles between 15 and 68° N, Atmos. Chem. Phys., 21, 989–1013, https://doi.org/10.5194/acp-21-989-2021, 2021.