Modelling the atmospheric mercury cycle-chemistry in fog droplets

A model has been developed describing the mass transport and chemistry of different forms of mercury in the atmosphere (the CAM model). 48-hour simulations of an air parcel containing a fog have been used to examine the influence of a number of chemical parameters on dissolved divalent mercury, Hg(II), in fog droplets. Representation of chlorine chemistry was found to be very important for modelling of mercury species, as mercury-chloride complexes dominate the dissolved Hg(II) fraction in competition with the reactive Hg(II)---S(IV) complexes. If the pH is increased, the importance of HgCl2 will decrease in favour of Hg(II)---S(IV) complexes which, in turn, will lead to lowered concentrations of dissolved Hg(II), due to an enhanced production of volatile Hg0 via reduction of HgSO3 At low SO2 concentration (0.5 < SO2 < 10 ppb) dissolved mercury is strongly inversely dependent on the gas phase SO2 concentration. The ozone concentration is almost linearly related to the dissolved Hg(II) content. Total mercury content (dissolved plus adsorbed Hg(II)) is strongly correlated to soot concentration. At high soot concentrations all Hg(II) is expected to be found in the adsorbed form.