ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-10-209-2010 Aerosols that form subvisible cirrus at the tropical tropopause Froyd K. D. 1 2 Murphy D. M. 1 Lawson P. 3 Baumgardner D. 4 Herman R. L. 5 NOAA Earth System Research Laboratory, Chemical Sciences Division, Boulder, CO, USA Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, CO, USA SPEC Incorporated, Boulder, CO, USA Universidad Nacional Autonoma de Mexico, Ciudad Universitaria, Mexico City, Mexico Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA 12 01 2010 10 1 209 218 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/10/209/2010/acp-10-209-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/209/2010/acp-10-209-2010.pdf

The composition of residual particles from evaporated cirrus ice crystals near the tropical tropopause as well as unfrozen aerosols were measured with a single particle mass spectrometer. Subvisible cirrus residuals were predominantly composed of internal mixtures of neutralized sulfate with organic material and were chemically indistinguishable from unfrozen sulfate-organic aerosols. Ice residuals were also similar in size to unfrozen aerosol. Heterogeneous ice nuclei such as mineral dust were not enhanced in these subvisible cirrus residuals. Biomass burning particles were depleted in the residuals. Cloud probe measurements showing low cirrus ice crystal number concentrations were inconsistent with conventional homogeneous freezing. Recent laboratory studies provide heterogeneous nucleation scenarios that may explain tropopause level subvisible cirrus formation.

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