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Abstract:
In this paper we analyze aerosol particle composition measurements from five research missions conducted between 2014 and 2018 sampling the upper troposphere and lower stratosphere (UTLS), to assess the meridional extent of particles containing meteoric material. Additional data sets from a ground based study and from a low altitude aircraft mission are used to confirm the existence of meteoric material in lower tropospheric particles. Single particle laser ablation techniques with bipolar ion detection were used to measure the chemical composition of particles in a size range of approximately 150 nm to 3 μm. The five UTLS aircraft missions cover a latitude range from 15 to 68° N, altitudes up to 21 km, and a potential temperature range from 280 to 480 K. In total, 338 363 single particles were analyzed, of which 147 338 particles were measured in the stratosphere. Of these particles, 50 688 were characterized by high abundances of magnesium, iron, and rare iron oxide compounds, together with sulfuric acid. This particle type was found almost exclusively in the stratosphere (48 610 particles) and is interpreted as meteoric material immersed or dissolved within stratospheric sulfuric acid particles. Below the tropopause, the observed fraction of this particle type decreases sharply. However, small fractional abundances were observed below 3000 m a.s.l. in the Canadian Arctic and also at the Jungfraujoch high altitude station (3600 m a.s.l.). Thus, the removal pathway by sedimentation and/or mixing into the troposphere is confirmed. In the tropical lower stratosphere, only a small fraction (< 10 %) of of the analyzed particles contain meteoric material. In contrast, in the extratropics the observed fraction of meteoric particles reaches 20–40 % directly above the tropopause. At potential temperature levels of more than 40 K above the thermal tropopause, particles containing meteoric material were found in higher relative abundances than near the tropopause and, at these altitudes, occurring at similar abundance-fraction across all latitudes and seasons measured. Above 440 K, the observed fraction of meteoric particle ranges between 60 and 80 % at latitudes between 20 and 42° N. This finding suggests that the meteoric material is transported from the mesosphere into the stratosphere within the winter polar vortex, and is efficiently distributed towards low latitudes by isentropic mixing above 440 K potential temperature. This process can explain that meteoric material is found in particles of the stratospheric aerosol layer at all latitudes.
