ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-6-4659-2006 Impact of H<sub>2</sub>SO<sub>4</sub>/H<sub>2</sub>O coating and ice crystal size on radiative properties of sub-visible cirrus Räisänen P. 1 Bogdan A. 2 Sassen K. 3 Kulmala M. 2 Molina M. J. 4 Finnish Meteorological Institute, P.O.&nbsp;Box 503, Helsinki, Finland Department of Physical Sciences, P.O.&nbsp;Box 64, 00014 University of Helsinki, Finland Geophysical Institute, University of Alaska, P.O.&nbsp;Box 757320, Fairbanks, AK 99775-7320, USA Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093-0356, USA 18 10 2006 6 12 4659 4667 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/6/4659/2006/acp-6-4659-2006.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/6/4659/2006/acp-6-4659-2006.pdf

Recent laboratory experiments showed that at conditions resembling those near the tropopause region, small ice particles can be coated by a liquid H<sub>2</sub>SO<sub>4</sub>/H<sub>2</sub>O over-layer formed after the freezing of diluted sulfuric acid/water aerosol drops. Here, idealized radiative transfer tests are conducted to evaluate the impact that such an over-layer would have on the radiative effects produced by sub-visible cirrus clouds (SVCs). Spherical particle shape is assumed to keep the problem tractable. The calculations show that the over-layer increases both the shortwave (SW) and longwave (LW) cloud radiative effects (CRE), but the impact is small: ~0.02 W m<sup>&minus;2</sup>, or even less, for the total (LW+SW) CRE at the top of the atmosphere. For the smallest ice particles, for which the over-layer is thickest, the fractional change in CRE can, however, reach ~20% for the SW CRE and over 50% for the LW CRE. The dependence of LW and SW CRE on particle size is also studied in the paper. Calculations for spherical and spheroidal uncoated ice particles show that even for high, optically thin cirrus, the total CRE can be negative, if the diameter of the particles is smaller than about 3&ndash;4 μm. Apart from the SVCs, this result could be relevant for contrail cirrus clouds, which are believed to consist of large numbers of very small ice particles.

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