A field study on chemistry, S(IV) oxidation rates and vertical transport during fog conditions

Abstract

An extensive fog study was carried out in the central plateu of Switzerland. Ninety-seven fog samples were collected along with aerosol filter and cascade impactor samples, and measurements of O 3, SO 2, 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 −, NO 3 −, NO 2 −, SO 4 2−, S(IV), oxalate and NH 4 +, 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 SO 4 2− and S(IV) agreed very 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 NO 2 were calculated. S(IV) oxidation due to PAN, NO 2 − and Fe(III) was of minor importance. A substantial fraction of the major ions was present in the intersitial aerosol (aerosol particles < 4 μm) even during fog conditions. High correlations were found for NH 4 +, NO 3 2−. From their ratios in the fog water and the aerosol (< 4 μm) it could be concluded that at least 40% of NO 3 − and 20% of NH 4 + 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.