Aircraft-based observation of meteoric material in lower-stratospheric aerosol particles between 15 and 68°\u2009N

Abstract

Abstract. We analyse aerosol particle composition measurements from five research missions between 2014 and 2018 to assess the meridional extent of particles\ncontaining meteoric material in the upper troposphere and lower stratosphere (UTLS). Measurements from the Jungfraujoch mountaintop site and a\nlow-altitude aircraft mission show that meteoric material is also present within middle- and lower-tropospheric aerosol but within only a very small\nproportion of particles. For both the UTLS campaigns and the lower- and mid-troposphere observations, the measurements were conducted with single-particle laser ablation mass spectrometers with bipolar-ion detection, which enabled us to measure the chemical composition of particles in a diameter\nrange of approximately 150\u2009 nm to 3\u2009 µm . The five UTLS aircraft missions cover a latitude range from 15 to 68 ∘ \u2009N,\naltitudes up to 21\u2009 km , and a potential temperature range from 280 to 480\u2009 K . In total, 338\u2009363 single particles were analysed, of\nwhich 147\u2009338 were measured in the stratosphere. Of these total particles, 50\u2009688 were characterized by high abundances of magnesium and iron,\ntogether with sulfuric ions, the vast majority (48\u2009610) in the stratosphere, and are interpreted as meteoric material immersed or dissolved within\nsulfuric acid. It must be noted that the relative abundance of such meteoric particles may be overestimated by about 10\u2009% to 30\u2009% due to the\npresence of pure sulfuric acid particles in the stratosphere which are not detected by the instruments used here. Below the tropopause, the observed\nfraction of the meteoric particle type decreased sharply with 0.2\u2009%–1\u2009% abundance at Jungfraujoch, and smaller abundances\n(0.025\u2009%–0.05\u2009%) were observed during the lower-altitude Canadian Arctic aircraft measurements. The size distribution of the meteoric sulfuric\nparticles measured in the UTLS campaigns is consistent with earlier aircraft-based mass-spectrometric measurements, with only 5\u2009%–10\u2009%\nfractions in the smallest particles detected (200–300\u2009 nm diameter) but with substantial ( > \u200940\u2009%) abundance fractions for particles\nfrom 300–350 up to 900\u2009 nm in diameter, suggesting sedimentation is the primary loss mechanism. In the tropical lower stratosphere, only a\nsmall fraction ( \u200910\u2009%) of the analysed particles contained meteoric material. In contrast, in the extratropics the observed fraction of\nmeteoric particles reached 20\u2009%–40\u2009% directly above the tropopause. At potential temperature levels of more than 40\u2009 K above the\nthermal tropopause, particles containing meteoric material were observed in much higher relative abundances than near the tropopause, and, at these\naltitudes, they occurred at a similar abundance fraction across all latitudes and seasons measured. Above 440\u2009 K , the observed fraction of meteoric\nparticles is above 60\u2009% at latitudes between 20 and 42 ∘ \u2009N. Meteoric smoke particles are transported from the mesosphere into the\nstratosphere within the winter polar vortex and are subsequently distributed towards low latitudes by isentropic mixing, typically below a potential temperature of 440\u2009 K . By contrast, the findings from the UTLS measurements show that meteoric material is found in stratospheric\naerosol particles at all latitudes and seasons, which suggests that either isentropic mixing is effective also above 440\u2009 K or that meteoric\nfragments may be the source of a substantial proportion of the observed meteoric material.