Model Prediction of Sulfuric Acid in the Tropical/Subtropical Cirrus Clouds as a Part of CRYSTAL-FACE Mission

Recent studies show that new particle formation takes place in the cirrus clouds, however, it has not been examined how the formation of new particle occur. Recent experiments, in cirrus clouds, provide in-situ data for particles of diameter 4-6 nm; unfortunately, simultaneous measurement of precursor gases e.g. SO2 and/or H2SO4 concentration is not available. This hinders model prediction of the newly formed particles. In-situ measurements of ultrafine particles, diameter 4 to 6 nm, carried out during 23 July 2002 (WB-57 mission), as a part of CRYSTAL-FACE mission, over Florida, were used for prediction of sulfuric acid and consequently SO2 concentration in the tropical/subtropical cirrus clouds. Using aerosol microphysical model, driven by parameterized binary homogenous nucleation [Vehkamaki et al., 2002] and ion induced nucleation [Modgil et al., 2005] (one at a time), and in-situ measured environmental conditions i.e. temperature, relative humidity and ultrafine particle number concentration, we have calculated likely source strength of sulfuric acid, which is used to infer the dominating nucleation mechanism for ultrafine particles. Model results, for particles diameter 4 to 6 nm, show that the binary homogenous nucleation rates are ~6 fold higher than the ion induced nucleation rates, suggesting that the binary homogenous nucleation mechanism is likely to dominate over ion induced nucleation mechanism in the tropical/subtropical cirrus clouds. The binary homogenous nucleation model and ion induced nucleation model, respectively, predicts sulfuric acid concentrations in the range 2-8 105 molecules cm-3 (SO2=0.3-0.75 ppbv) and 7.5 105 106 molecules cm-3 (SO2=1-3 ppbv),. About 75% of the model simulations show reasonable agreement between predicted and observed particles of diameter from 6 to 9 nm and 4 to 9 nm. Refrences Modgil, M. S. et al., A parameterization of ion-induced nucleation of sulfuric acid and water for atmospheric conditions, J. Geophys. Res., In press, 2005. Vehkamaki H. et al., An improved parameterization for sulfuric acid water nucleation rates for tropospheric and stratospheric conditions, J. Geophys. Res., 107 (D22), 4622-4632, 2002.