Nonseasalt sulfate

An overall mechanism for the DMS-OH reaction is shown in Figure 6.19. Many of the details of this mechanism are still uncertain. The principal stable products of the oxidation are DMSO, DMS02, MSA, S02, and H2S04. The ratio of MSA to SO4- is indicative of the path that is followed subsequent to formation of CH3S in the abstraction branch. This ratio is measured in the marine atmosphere as the ratio of MSA to nonseasalt sulfate (nss-S04). (Nonseasalt sulfate, the amount of sulfate present in particles in excess of that expected from seasalt particles, is a direct measure of the sulfuric acid formed.) Measurements as a function of latitude indicate that this ratio is quite temperature-dependent, with the ratio varying from about 0.1 near the equator to close to 0.4 in Antarctic waters. Thus the colder the temperature, the more favored the path to MSA formation as opposed to that to S02 and eventually to sulfuric acid. This behavior is consistent with a competition between a radical decomposition step, with a fairly large activation energy, namely... 

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