Transport of Biomass Burning Aerosol into the Extratropical Tropopause Region over Europe via Warm Conveyor Belt Uplift

Abstract. Aerosol particles in the extratropical upper troposphere and lower stratosphere (exUTLS) play a crucial role for the Earth‘s radiative budget. High temporal and spatial resolution measurements in the exUTLS are important to study mixing processes and their climate impact. Here, we present measurements from the TPEx mission (Tropopause composition gradients and mixing Experiment) an aircraft mission in June 2024 over Europe. The measurement platform, a Learjet 35A, was equipped with in-situ trace gas and aerosol measurements and filter samplers for offline analysis. For vertical gradient measurements of trace species and aerosol, we conducted redundant measurements on a fully automated towed sensor shuttle (TOSS) 200 m below the aircraft.

On 17 June 2024, we observed a streamer with elevated aerosol number concentration of up to 800 particles per cm³ between 100 nm and 1 μm. This is higher by a factor of two than the local background. Carbon monoxide (CO) mixing ratios were larger than 100 ppbv. Backward trajectories indicate that this pollution is transported from Canadian wildfires in the lower troposphere towards Europe, where it was uplifted on the edge of a warm conveyor belt into the tropopause region. There mixing with chemically stratospheric air occurred. The TOSS measurements also allow the calculation of the potential temperature gradient (∆θ · ∆z⁻¹). We observed a change towards smaller gradients within the region of the polluted air masses, which is presumably due to an increase of θ at lower altitudes by radiative heating as a consequence of the transported refractory black carbon.