| Keywords: ice supersaturation, upper tropospheric cirrus clouds, freezing of aqueous
aerosol.
Observations often reveal enhanced and persistent upper tropospheric (UT) ice
supersaturation, Si up to 100%, independently of whether cirrus ice clouds are
present or not (Krämer et al., 2009; Lawson et al., 2008). However, a water activity
criterion (WAC) (Koop et al., 2000) does not allow the formation of Si > ~67%
by the homogeneous freezing of aqueous droplets even at the lowest atmospheric
temperature of ~185 K. For aqueous aerosol the WAC predicts the existence of a so called
homogeneous ice nucleation threshold which, being expressed as Si, is between ~52 and
67% in the temperature range of ~220 – 185 K. The nature of the formation of
large Si remains unclear. Since water vapor is the dominant greenhouse gas it is
important to know the nature of the accumulation and persistence of water vapor in the
UT.
We studied the freezing behavior of micrometer-scaled 3-, 4-, and 5-component droplets,
which contain different weight fractions of H2O, H2SO4, HNO3, (NH4)2SO4, (NH4)HSO4,
NH4NO3, and (NH4)3H(SO4)2. The study was performed between 133 and 278 K at cooling
rates of 3, 0.1, and 0.05 K/min using differential scanning calorimetry (DSC) (Bogdan and
Molina, 2010). The cooling rates of 0.1 and 0.05 K/min (6 and 3 K/h) are similar to the
smallest reported synoptic temperature change of ~2 K/h (Carslaw et al., 1998). Using the
measured freezing temperature of ice, Ti, and the thermodynamic E-AIM model of
the system of H+ - NH4+ - SO42-- NO3-- H2O (Clegg et al., 1998), we
calculated the corresponding clear-sky Si which would be built up immediately
prior to the formation of ice cirrus clouds by the homogeneous freezing of aqueous
aerosol of similar composition. We found that our calculated values of Si are both
larger and smaller than the homogeneous ice nucleation threshold. For example, for
the droplets of compositions of 15/10 and 20/10 wt % (NH4)3H(SO4)2/H2SO4,
which freeze at 194 and 186 K, respectively, the calculated clear-sky Si can exceed
80%.
Although our Si values are smaller than the largest observed value of Si - 100%, they are
nevertheless larger than the Si - 67% predicted by the WAC at  185 K. Our results can
give an impetus for the study of whether multi-component aqueous aerosol, which
besides inorganic components also contains organics, may produce the observed Si -
100%.
Krämer, M., Schiller, C., Afchine, A., Bauer, R., Gensch, I., Mangold, A..,
Schlicht, S., Spelten, N., Sitnikov, N., Borrmann, S., de Reus, M., Spichtinger, P.
(2009), Atmos. Chem. Phys. 9, 3505.
Lawson, R. P., Pilson, B., Baker, B., Mo, Q., Jensen, E., Pfister, L., Bui, P. (2008),
Atmos. Chem. Phys. 8, 1609.
Koop, T., Luo, B., Tsias, A., Peter, T. (2000), Nature, 406, 611.
Bogdan, A. and Moilna, M. J. (2010), J. Phys. Chem. A (Published online: 5
February).
Carslaw, K. S., Wirth, M., Tsias, A., Luo, B. P., Dörnbrack, A., Leutbecher, M.,
Volkert, H., Renger, W., Bacmeister, J. T., Peter, T. (1998), J. Geophys. Res. 103,
5785.
Clegg, S. L., Brimblecombe, P., Wexler, A. S. (1998), J. Phys. Chem. A 102,
2137. |