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In situ observations of aerosol particles remaining from evaporated cirrus crystals: Comparing clean and polluted air masses

by M. Seifert, J. Ström, R. Krejci, A. Minikin, A. Petzold, J.-F. Gayet, U. Schumann, J. Ovarlez
Atmospheric Chemistry and Physics Discussions ()

Abstract

In-situ observations of aerosol particles contained in cirrus crystals\nare presented and compared to interstitial aerosol size distributions\n(non-activated particles in between the cirrus crystals). The\nobservations were conducted in cirrus clouds in the Southern and\nNorthern Hemisphere midlatitudes during the INCA project. The first\ncampaign in March and April 2000 was performed from Punta Arenas, Chile\n(54degreesS) in pristine air. The second campaign in September and\nOctober 2000 was performed from Prestwick, Scotland (53degreesN) in the\nvicinity of the North Atlantic flight corridor. Size distribution\nmeasurements of crystal residuals (particles remaining after evaporation\nof the crystals) show that small aerosol particles (D-p < 0.1 μm)\ndominate the number density of residuals. The crystal residual size\ndistributions were significantly different in the two campaigns. On\naverage the residual size distributions were shifted towards larger\nsizes in the Southern Hemisphere. For a given integral residual number\ndensity, the calculated particle volume was on average three times\nlarger in the Southern Hemisphere. This may be of significance to the\nvertical redistribution of aerosol mass by clouds in the tropopause\nregion. In both campaigns the mean residual size increased slightly with\nincreasing crystal number density. The form of the residual size\ndistribution did not depend on temperature as one might have expected\nconsidering different modes of nucleation. The observations of ambient\naerosol particles were consistent with the expected higher pollution\nlevel in the Northern Hemisphere. The fraction of residual particles\nonly contributes to approximately a percent or less of the total number\nof particles, which is the sum of the residual and interstitial\nparticles. Excellent agreement between the CVI and FSSP-300 probes was\nfound supporting the assumption that each crystal is associated with\nonly one residual particle.

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