Observation of biomass burning aerosol from Canada in a warm conveyor belt outflow event over Europe during TPEx

The chemical composition, especially of aerosol particles, in the extratropical upper troposphere and lower stratosphere (exUTLS) plays a crucial role in the radiation budget of the atmosphere. This composition is affected by large-scale dynamics in the stratosphere and troposphere, and additionally by small-scale processes. The effects of aerosol particle number concentration as well as the chemical composition in the extratropical lowermost stratosphere (exLMS) are a major research topic in recent years. The measurements presented in this study were taken during the TPEx campaign (Tropopause composition gradients and mixing Experiment) in summer 2024 over the North Sea and northern Germany. In addition to aerosol and trace gas measurements taken on the main platform, a Learjet 35A, we make use of partly redundant aerosol and trace gas measurements on a fully automated sensor shuttle towed by the aircraft (TOSS; towed sensor shuttle), which yield information about the vertical distribution of the measured quantities. The aim of this study is to describe the process of warm conveyor belt (WCB) uplift as source for aerosol particles in the exLMS. Although WCBs are typically associated with cloud formation and efficient precipitation formation, we were able to observe transported aerosol originating at lower altitudes in the outflow. In more detail, we present the observation of a small-scale streamer of polluted biomass burning (BB) air masses which most probably originate from forest fires over Canada. Trajectory analyses indicate that the aerosol is transported within the lowest 2 km above the surface across the Atlantic towards Europe where it undergoes the moist uplift and consequent mixing into the exLMS. The TOSS below the aircraft allows for obtaining in-situ temperature gradients over a vertical scale of 200 m. In particular potential temperature gradients show a change, presumably caused by a potential radiative warming effect of the observed BB aerosol. We compared this observation with a 1D-radiation simulation, for which the measured chemical composition as well as the size distribution of the aerosol particles were used as input parameters.