Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 3 1 2003 10.5194/acpd-3-389-2003 http://www.atmos-chem-phys-discuss.net/3/389/2003/ http://www.atmos-chem-phys-discuss.net/3/389/2003/acpd-3-389-2003.html http://www.atmos-chem-phys-discuss.net/3/389/2003/acpd-3-389-2003.pdf 389 411 2003-02-03 On the formation of radiation fogs under heavily polluted conditions H. Kokkola S. Romakkaniemi A Laaksonen Department of Applied Physics, University of Kuopio, Finland We have studied the effect of gaseous pollutants on fog droplet growth in heavily polluted air using a model that describes time-dependent sulfate production in the liquid phase and thermodynamical equilibrium between the droplets and the gas phase. Our research indicates that the oxidation of  SO₂ to sulfate has a significant effect on fog droplet growth especially when hygroscopic trace gases, for example HNO₃ andNH₃ are present. The increased sulfate production by dissolution of hygroscopic gases results from increased pH (caused by absorption of ammonia) and from the increased size of the fog/smog droplets. As a result of the enhancement of the droplet growth the optical thickness of the fog will increase.