Small-scale gradients in aerosol size distribution and aerosol chemical composition across the extratropical tropopause
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Abstract
The chemical composition of the tropopause region, and more precisely the upper troposphere and lower stratosphere (UTLS), plays an important role on surface temperature changes. Changes in the chemical composition may have large impacts on the radiation budget of the Earth’s atmosphere. The UTLS and its chemical composition are influenced by horizontal transport of air masses, vertical transport associated with (deep) convective systems and warm conveyor belts (WCBs), rapid turbulent mixing as well as photochemical reactions. The presented studies are based on two different airborne measurement campaigns over Europe in 2020 and 2024 which were analyzed with the focus on cross-tropopause mixing of aerosol particles. Therefore, the chemical composition of non-refractory submicron aerosol particles as well as the total aerosol number concentration and size distribution measurements were investigated in combination with in-situ trace gas data to identify mixing processes. It is shown that the sulfate aerosol in the lowermost stratosphere ( LMS) is influenced by upper tropospheric volcanic eruptions and subsequent cross-tropopause mixing and gas-to-particle conversion. Additionally, a filament with high aerosol number concentration (> 800 cm−1) in combination with high carbon monoxide ( CO) mixing ratios (> 100 ppbv) was observed in chemically stratospheric air (nitrous oxide ( N2O) < 338 ppbv) within a WCB outflow region. Size distribution measurements as well as offline analysis of impactor samples show indications for biomass burning ( BB). Trajectory data support this by showing the air mass origin over Canada close to active wildfires with subsequent low-level transport towards Europe and uplift associated with a WCB to the UTLS. Summarizing, this thesis yields valuable information on the influence of small-scale mixing processes on aerosol particles in the extratropical UTLS. Furthermore, it shows the importance of different transport processes that need to be considered for aerosol transport. It builds a strong basis for future
studies and possible model development in order to represent the observed processes in current models correctly.