Comparison of modeled versus measured MSA:nss SO=4 ratios: A global analysis

Abstract

The MSA:nss SO=4 ratio, which is a measure of the relative marine biogenic contribution to the total atmospheric sulphur burden, has long been measured in various parts of the globe. Transect studies and observations from a network of stations have provided some idea of the spatial and temporal behavior of the ratio in various regions, but gaps in knowledge still exist in other parts of the globe. Here we present results of a global 3-D chemical transport modeling study which complement these measurements and provide a globe-wide picture of the spatial variation and distribution of this ratio. Comparison of modeled versus measured data on the MSA:nss SO=4 ratio resulting from all sulphur sources considered shows fair model performance (i.e., a general overestimation of 23%; degrees of freedom = 90) in all areas of the globe where actual measurements of the ratio have been made. On the other hand, the model-observation comparisons for the MSA:nss SO=4 ratio derived solely from the oceanic DMS source are not as satisfactory (an overall overestimation of a factor of 3; degrees of freedom = 50). The MSA:nss SO=4 ratio that is derived from the oceanic DMS source alone provides information on the relative yields of MSA and SO=4 from atmospheric DMS oxidation. Our model results are consistent with measurements, showing that the ratio is highest around the polar regions and lowest within the tropics. This spatial trend is attributed to the fact that MSA production occurs best under low temperatures (maximum ambient temperature of 27°C). Despite MSA being preferably produced under low temperatures, observations at high latitudes have consistently shown summer maxima and winter minima in the MSA:nss SO=4 ratio. This has raised many questions on the robustness of the theory of the MSA production mechanism. Diminished marine biological activity and low seawater DMS conditions in winter have widely been cited as the cause of this observed trend. In this study, we further propose that since photochemical hydroxyl radical (OH) production during the dark winter months at polar latitudes is non-existent, reduced wintertime oxidation of DMS by OH to form MSA results in summer maxima and winter minima in MSA concentrations at these latitudes. Temperature and marine biological activity are, therefore, not the only major determining factors for MSA production at high latitudes on a seasonal scale. Light conditions are also important. Throughout the year, the highest ratios occur in the Southern Hemisphere, where the atmospheric DMS burden is highest. This is in agreement with both short- and long-term measurements in literature. Copyright 2004 by the American Geophysical Union.