ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-3-581-2003 On the formation of radiation fogs under heavily polluted conditions Kokkola H. 1 Romakkaniemi S. 1 Laaksonen A. 1 Department of Applied Physics, University of Kuopio, Finland 02 06 2003 3 3 581 589 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/3/581/2003/acp-3-581-2003.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/3/581/2003/acp-3-581-2003.pdf

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 SO<sub>2</sub> to sulfate has a significant effect on fog droplet growth especially when hygroscopic trace gases, for example HNO<sub>3</sub> and NH<sub>3</sub> 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.