ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-11-6229-2011 Observations of ice nuclei and heterogeneous freezing in a Western Pacific extratropical storm Stith J. L. 1 Twohy C. H. 2 DeMott P. J. 3 Baumgardner D. 4 Campos T. 1 Gao R. 5 Anderson J. 6 National Center for Atmospheric Research, Boulder, Colorado, USA College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon, USA Department of Atmospheric Sciences, Colorado State University, Fort Collins, Colorado, USA Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, Mexico City, Mexico, USA Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, Colorado, USA School for Engineering of Matter, Transport and Energy, Tempe, Arizona, USA 01 07 2011 11 13 6229 6243 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/11/6229/2011/acp-11-6229-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/6229/2011/acp-11-6229-2011.pdf

In situ airborne sampling of refractory black carbon (rBC) particles and Ice Nuclei (IN) was conducted in and near an extratropical cyclonic storm in the western Pacific Ocean during the Pacific Dust Experiment, PACDEX, in the spring of 2007. Airmass origins were from Eastern Asia. Clouds associated primarily with the warm sector of the storm were sampled at various locations and altitudes. Cloud hydrometeors were evaporated by a counterflow virtual impactor (CVI) and the residuals were sampled by a single particle soot photometer (SP2) instrument, a continuous flow diffusion chamber ice nucleus detector (CFDC) and collected for electron microscope analysis. In clouds containing large ice particles, multiple residual particles were observed downstream of the CVI for each ice particle sampled on average. The fraction of rBC compared to total particles in the residual particles increased with decreasing condensed water content, while the fraction of IN compared to total particles did not, suggesting that the scavenging process for rBC is different than for IN. In the warm sector storm midlevels at temperatures where heterogeneous freezing is expected to be significant (here −24 to −29 °C), IN concentrations from ice particle residuals generally agreed with simultaneous measurements of total ice concentrations or were higher in regions where aggregates of crystals were found, suggesting heterogeneous freezing as the dominant ice formation process in the mid levels of these warm sector clouds. Lower in the storm, at warmer temperatures, ice concentrations were affected by aggregation and were somewhat less than measured IN concentrations at colder temperatures. The results are consistent with ice particles forming at storm mid-levels by heterogeneous freezing on IN, followed by aggregation and sedimentation to lower altitudes. Compositional analysis of the aerosol and back trajectories of the air in the warm sector suggested a possible biomass burning source for much of the aerosol. Comparison of the particles from the CFDC with the other aerosol in the residuals of ice particles suggested that the largest portion of IN had similar inferred origins (from biomass burning with minor amounts of rBC) as the other aerosol, but contained slightly elevated amounts of calcium and less influence from sea salt.

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