Differences in the upper tropospheric and lower stratospheric aerosol composition

The stratosphere and troposphere are the main layers that define a significant part of the atmospheric processes of our planet. They are demarcated by the tropopause - a layer that has a stable stratification and makes it difficult to exchange air between them. As a consequence, the composition of the air differs slightly in the stratosphere and troposphere. However, the tropopause is not a fully material impermeable surface and therefore the exchange of impurities between both layers occurs. Under the conditions of a changing climate, the composition of the air in the troposphere has also noticeably changed. Therefore, it is important to study the processes of air exchange between the troposphere and stratosphere in a warming climate, especially if we take into account that one of the proposed geoengineering methods assumes to affect climate-forming factors by means of spraying sulphate particles into the stratosphere.

Here, we present the results of airborne measurements of the size distribution and chemical composition of aerosols carried out at the tropopause level and in the upper troposphere and lower stratosphere (UTLS) using the 'Optik' Tu-134 aircraft laboratory as a research platform. For the analysis, we have chosen 14 flight segments when the aircraft crossed the tropopause, which level was determined by the temperature gradient (up to 2°C/ km). All the selected profiles of atmospheric constituents were measured over the Russian Arctic seas or coastal areas, since the tropopause in the northern latitudes is much lower than in the middle ones.

Significant differences in the elemental composition of aerosol particles were revealed in the UTLS. Si was dominated in the composition of stratospheric particles, and Fe or Al in the tropospheric ones. The ionic composition of the LS aerosols was predominantly represented by sulfates (SO₄^2-), while tropospheric ones by a group of different ions.

The particle number size distributions (PNSD) in both UT and LS were dominated by the Aitken mode (20-50 nm). At the same time, there were some differences in PNSD – in the stratosphere, the distribution curve was shifted towards larger sizes that suggests the older age of particles measured there. It is also important to note that the nucleation mode particles (3–20 nm) were also detected during some flights in the lower stratosphere. This indicates that, despite the low humidity and the very low content of ammonia here, the processes of the new particle formation (NPF) in the stratosphere were taking place. Taking into account the dominance of SO₄^2- in the ionic composition, one can be assumed that sulfuric acid played a dominant role in the lower stratospheric NPF.

This work was supported by the grant of the Ministry of Science and Higher Education of the Russian Federation (Agreement No 075-15-2021-934).