Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 3 3 2003 10.5194/acp-3-581-2003 http://www.atmos-chem-phys.net/3/581/2003/ http://www.atmos-chem-phys.net/3/581/2003/acp-3-581-2003.html http://www.atmos-chem-phys.net/3/581/2003/acp-3-581-2003.pdf 581 589 2003-06-02 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 and NH₃ 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. Our results indicate that unactivated fogs may become optically very thick when the droplet concentrations are on the order of several thousand per cubic centimeter of air.