Nss SO42 Research Articles

The relationship between dimethyl sulfide and particulate sulfate in the mid-atlantic ocean atmosphere

Dimethyl sulfide (DMS) and atmospheric aerosols were sampled simultaneously over the Atlantic Ocean in the vicinity of Bermuda using the NOAA King Air research aircraft. Total and fine (50% cutoff at 2 μm diameter) aerosol fractions were sampled using two independent systems. The average nonsea-salt (nss)SO 4 2− concentrations were 1.9 and 1.0 μg m −3 (as SO 4 2−) for the total and the fine fractions in the boundary layer (BL) and 0.53 and 0.27 μg m −3 in the free troposphere (FT). Non-sea-salt SO 4 2− in the two aerosol fractions were highly correlated ( r = 0.90), however a smaller percentage (55%) was found in the fine aerosol near Bermuda relative to that (90%) near the North American continent. The BL SO 4 2− concentrations measured in this study were higher than those measured by others at remote marine locations despite the fact that the 7-day air mass back trajectories indicated little or no continental contact at altitudes of 700 mb and below; the trajectories were over subtropical oceanic areas that are expected to be rich in DMS. DMS concentrations were higher near the ocean surface and decreased with increasing altitude within the BL; the average DMS concentration was 0.13 μg m −3. Trace levels of DMS were also measured in the FT (0.01 μg m −3). Computer simultation of the oxidation and removal of DMS in the marine atmosphere suggests that <50% of the SO 4 2− observed could be related to the natural S cycle.

Local influences on the composition of precipitation on Bermuda

The understanding of atmospheric biogeochemical cycles in marine areas depends heavily on the collection and analysis of precipitation samples. However, collection of marine precipitation is difficult due to constraints on sampling locations. For example, the island of Bermuda has been used to characterize the composition of precipitation over the western north Atlantic Ocean. However, Bermuda is a small, heavily populated island and the possibility exists that local processes can influence the composition of precipitation. Using 2 years of data on the composition of precipitation at 2 locations on Bermuda, this paper documents that enrichments of sea-salt in Bermuda precipitation, relative to open ocean precipitation, can occur due to ocean-island turbulence. Smaller enrichments of non-sea-salt (nss) Ca ++ , nss SO = 4 , NH + 4 and H + are also observed. At present it is not possible to determine whether the enrichments are due to island sources or spatial variability in the precipitation scavenging process. By assuming the latter effect to be negligible, the upper limits of the effect of island sources on the concentrations of the nss components (Ca ++ , SO = 4 , NO - 3 , H + ) are ~ 5-30%, depending on the species. Precipitation associated with westerly airflow is the least affected by local sources. DOI: 10.1111/j.1600-0889.1988.tb00288.x

Particulate sulfate and nitrate in the boundary layer over the North Pacific Ocean

Concentrations of aerosol NO3− and non‐sea‐salt (nss) SO4= have been measured in samples collected weekly between January 1981 and March 1982 at seven stations in the Sea‐Air Exchange Program Asian Dust Network in the North Pacific between 4°N and 53°N. Mean NO3− concentrations were relatively uniform, ranging between 0.18 and 0.35 μg/m3 STP; mean nss SO4= ranged from 0.37 to 1.20 μg/m3 STP. At most stations there was no consistent correlation between NO3− and nss SO4=or between these species and mineral aerosol. At Midway these species did covary in a clear and consistent manner; this correlation appears to be forced by large‐scale meteorological factors. In some cases, high nss SO4= concentrations were associated with high mineral aerosol concentrations; this suggests that the SO4= in these episodes was transported from the continents. A particularly impressive event occurred in May 1981, when much of the North Pacific region was affected by extremely high nss SO4= concentrations; evidence suggests that the SO4= was not derived from the soil material itself. At Fanning Island, NO3− concentrations showed a strong seasonal variation; there is evidence that the high NO3− concentrations are attributable to transport from the continents. In contrast, nss SO4= concentrations at Fanning were relatively constant throughout the year, and they were consistent with recent estimates of the oceanic emission rates of sulfur as dimethyl sulfide. The NO3− data from the tropics suggest that there is a relatively uniform background concentration of about 0.10 μg/m3 STP; this concentration is close to that recently predicted by models that assume only lightning and the stratosphere as sources for NOx.

Methane sulfonic acid in the marine atmosphere

Methane sulfonic acid (MSA) is an oxidation product of the reaction of OH radical with dimethyl sulfide and, hence, should be an important constituent of marine air. MSA concentrations in marine aerosols ranged from 0.009 to 0.075 μg/m3 in samples from the Pacific and Indian oceans and Miami, Florida. In the samples from remote areas (Pacific and Indian oceans), MSA levels averaged 6.7% (S = 1.9) of the non‐sea‐salt (nss) SO4=. In the Miami area, ratios were occasionally lower because of the impact of local sulfur emissions (probably pollutant SO2). MSA concentrations in seven rainwater samples collected at Miami, Florida, ranged from 0.001 to 0.034 ppm. Cascade impactor samples from Miami, Florida, and the Gulf of Mexico indicate that MSA occurs primarily in the smaller particles, as does nss SO4= values. In the two samples analyzed, MSA is concentrated on slightly coarser particles than nss SO4=. The size distributions of both MSA and nss SO4= differ markedly from that of sea salt Na+, suggesting that at least the first step in the oxidation of organosulfur compounds is a gas phase reaction. The observed concentrations of MSA in marine air are the result of formation from gas phase reaction of dimethyl sulfide (and other volatile organosulfur compounds) with OH radical and destruction by aqueous phase reaction with OH radical in aerosols. Preliminary experiments indicate that MSA oxidation probably yields SO4= under atmospheric conditions. Therefore, MSA destruction is a potentially important pathway for the formation of nss SO4= in the marine atmosphere. The magnitude of the fluxes involved in the organosulfur cycle cannot be calculated from the MSA data because of the uncertainty in the free radical chemistry of aerosols.