THE CHEMISTRY OF SULFUR AND NITROGEN SPECIES IN A FOG SYSTEM - A MULTIPHASE APPROACH

Concentration and phase distribution of sulfur and nitrogen species during a particular fog episode in the Po Valley are experimentally described in this paper. Chemical measurements were carried out simultaneously at different heights within the fog layer, up to 50 m. Microphysical and meteorological parameters necessary for the description of the fog multiphase system were also concurrently measured as a function of height. The fog cycle (formation, evolution, dissipation) is described in terms of the total acidity of a unit volume of air containing gas species, interstitial aerosol particles and fog droplets. The fog system was not closed and input of acidic and basic components was observed during fog evolution. The driving force which determines the acidity of the fog multiphase atmospheric system was found to be the presence of NH3 and its partitioning among the different phases. A strong decrease of fog water pH (from 5.6 down to 2.8) was observed during fog evolution and was attributed to a HNO3 input to the system. These acidic and basic inputs are described in terms of a titration/back-titration process of the fog system. The SO2 oxidation process in fog water was found to be of minor importance in determining the SO4= concentration within the fog system, due to both low SO2 concentration and limited oxidant availability during the experiment.