Comparison of modeled versus measured MSA:nss SO₄⁼ ratios: A global analysis

Abstract

[1] 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.

Sulfate source apportionment in the Ny-Ålesund (Svalbard Islands) Arctic aerosol

“…In East Antarctic Plateau (Concordia Station), by using the same approach, Udisti et al (2012) found the value of 2.6 for a multi-year aerosol database. The ratio = 3.0 evaluated in the 2014 GVB data set is very close to that obtained by the AOE-96 box-model (mean nss-SO 4 2-/MSA = 3.1, Karl et al 2007), and slightly higher than those calculated by a chemical transport model (1.5-2.6; Gondwe et al 2006). By using the SO 4 2-/MSA ratio = 3 in biogenic aerosol originated in the Arctic sea areas, the bio-SO 4 2-fraction can be evaluated by multiplying the MSA concentrations, measured in the Ny-Å lesund aerosol samples, by this value.…”

Year-round record of bulk and size-segregated aerosol composition in central Antarctica (Concordia site) – Part 2: Biogenic sulfur (sulfate and methanesulfonate) aerosol

“…These values may be overestimated since an influence of the city of La Paz (3600 m a.s.l., located only at 25 km away from the Chacaltaya site) on the sulfate concentration remains here possible. Note also that other continents, such as Australia, certainly contribute to the long-range transport of continental 210 Pb and sulfate towards Antarctica, in particular in the case of East Antarctica (Heimann et al, 1990). For the first time, 10 Be concentrations are documented in the atmosphere of the high East Antarctic plateau (Fig.…”

Observational evidence for the formation of DMS-derived aerosols during Arctic phytoplankton blooms

“…Therefore, this asymptotic value adequately represents the nss-SO 2− 4 / MSA ratio derived exclusively from DMS . The biogenic SO 2− 4 / MSA ratio has been reported to vary considerably in space and time (Gondwe et al, 2004), because the ratio is sensitive to temperature, and to a lesser extent photochemical species or reactions with halogen radicals (Bates et al, 1992). When this method was applied to data for aerosol samples (PM 10 ) collected in 2014 at the Gruvebadet observatory, in a vicinity of our DMS measurement site, the biogenic SO 2− 4 / MSA ratio was estimated to be 3.0 (Fig.…”