Impact of H2SO4/H2O coating and ice crystal size on radiative properties of sub-visible cirrus P. Räisänen1, A. Bogdan2, K. Sassen3, M. Kulmala2, and M. J. Molina4 1Finnish Meteorological Institute, P.O. Box 503, Helsinki, Finland 2Department of Physical Sciences, P.O. Box 64, 00014 University of Helsinki, Finland 3Geophysical Institute, University of Alaska, P.O. Box 757320, Fairbanks, AK 99775-7320, USA 4Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093-0356, USA
Abstract. Recent laboratory experiments showed that at conditions resembling those
near the tropopause region, small ice particles can
be coated by a liquid H2SO4/H2O 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−2, 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 3–4 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.
Citation: Räisänen, P., Bogdan, A., Sassen, K., Kulmala, M., and Molina, M. J.: Impact of H2SO4/H2O coating and ice crystal size on radiative properties of sub-visible cirrus, Atmos. Chem. Phys., 6, 4659-4667, doi:10.5194/acp-6-4659-2006, 2006.