Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 4 6 2004 10.5194/acpd-4-7859-2004 http://www.atmos-chem-phys-discuss.net/4/7859/2004/ http://www.atmos-chem-phys-discuss.net/4/7859/2004/acpd-4-7859-2004.html http://www.atmos-chem-phys-discuss.net/4/7859/2004/acpd-4-7859-2004.pdf 7859 7879 2004-12-02 Parameterization of the nitric acid effect on CCN activation S. Romakkaniemi sami.romakkaniemi@uku.fi H. Kokkola A. Laaksonen Department of applied physics, University of Kuopio, Kuopio, Finland In this paper we present a parameterization of the nitric acid effect on cloud droplet formation. The new parameterization is intended to be used in large scale models in order to obtain regional and global estimates of the effect of nitric acid on cloud drop concentrations and the radiative balance. The parameterization is based on numerical air parcel model simulations and can be applied for unimodal and bimodal lognormal aerosol particle size distributions in a large variety of different conditions. In addition to the aerosol particle distribution and gas-phase HNO₃ concentration, the parameterization requires temperature, total pressure, updraft velocity, and the number concentration of cloud droplets formed at zero nitric acid concentration, as input parameters. The parametrization is also suitable for describing the effect of hydrochloric acid on the cloud drop concentrations, and in practice, the HNO₃ and HCl concentrations can be summed up to yield the total effect. The comparison between the parameterization and the results from numerical air parcel model simulations show good consistency.