Processes Affecting Oxygen Isotope Ratios of Atmospheric and Ecosystem Sulfate in Two Contrasting Forest Catchments in Central Europe

Sulfate aerosols are harmful as respirable particles. They also play a role as cloud condensation nuclei and have radiative effects on global climate. A combination of δ18O−SO4 data with catchment sulfur mass balances was used to constrain processes affecting S cycling in the atmosphere and spruce forests of the Czech Republic. Extremely high S fluxes via spruce throughfall and runoff were measured at Jezeri (49 and 80 kg S ha-1 yr-1, respectively). The second catchment, Na Lizu, was 10 times less polluted. In both catchments, δ18O−SO4 decreased in the following order: open-area precipitation > throughfall > runoff. The δ18O−SO4 values of throughfall exhibited a seasonal pattern at both sites, with maxima in summer and minima in winter. This seasonal pattern paralleled δ18O−H2O values, which were offset by −18‰. Sulfate in throughfall was predominantly formed by heterogeneous (aqueous) oxidation of SO2. Wet-deposited sulfate in an open area did not show systematic δ18O−SO4 trends, suggesting formation by homogeneous (gaseous) oxidation and/or transport from large distances. The percentage of incoming S that is organically cycled in soil was similar under the high and the low pollution. High-temperature 18O-rich sulfate was not detected, which contrasts with North American industrial sites.