A FIELD-STUDY ON CHEMISTRY, S(IV) OXIDATION RATES AND VERTICAL TRANSPORT DURING FOG CONDITIONS

An extensive fog study was carried out in the central plateau of Switzerland. Ninety-seven fog samples were collected along with aerosol filter and cascade impactor samples, and measurements of O3, SO2, NO, NO(x), PAN, temperature, and wind speed and direction. Maximum levels in fogwater were 4.3, 4.4, 0.033, 1.7, 0.5, 0.024 and 9.2 mmol l-1 for Cl-, NO3-, NO2-, SO4(2-), S(IV), oxalate and NH4+, respectively. pH varied between 2.9 and 7.1. Sixteen additional elements were determined in the fog samples by ICP. The sum of the concentrations of SO4(2-) and S(IV) agreed very well with the total sulfur concentration as determined by ICP. A substantial excess of S(IV) (up to 0.2 mmol l-1) compared to Henry and acid-base equilibrium calculations was found, which can probably be attributed to complex formations with aldehydes. S(IV) oxidation rates of up to 650 nmol l-1 s-1 with ozone and of up to 100 nmol l-1 s-1 with NO2 were calculated. S(IV) oxidation due to PAN, NO2- and Fe(III) was of minor importance. A substantial fraction of the major ions was present in the interstitial aerosol (aerosol particles < 4-mu-m) even during fog conditions. High correlations were found for NH4+, NO3- and SO4(2-). From their ratios in the fog water and the aerosol (< 4-mu-m) it could be concluded that at least 40% of NO3- and 20% of NH4+ in fog water was due to gas phase scavenging. Increasing concentrations in fog water were found during fog dissipation. Concentrations decreased with increasing height. A vertical transport model including turbulent diffusion and droplet sedimentation is introduced, which matches the experimental data of this vertical profile.