Atmospheric dimethylsulfide and sulfur species in aerosol and rainwater at a coastal site in New Zealand

Abstract

Aspects of the atmospheric speciation of sulfur were investigated at a coastal site on the North Island of New Zealand between December 1990 and November 1992. Gaseous dimethylsulfide (DMS) as well as methanesulfonic acid (MSA) and non‐sea‐salt sulfate (NSSS) in aerosol and rainwater samples were determined. Atmospheric DMS concentrations exhibited seasonal variations in the range 0.25 nmole m−3 to 14.9 nmole m−3, with a median of 3.75 nmole m−3. Also, DMS levels varied diurnally and with meteorological variables such as wind speed and cloud coverage. Aerosol MSA concentrations similarly ranged seasonally from 0.02 to 1.40 nmole m−3 (median 0.36 nmole m−3). Aerosol NSSS concentrations varied from −1.04 to 15.9 nmole m−3, with a median value of 7.92 nmole m−3. MSA and NSSS concentrations in “clean” rainwater samples ranged from 0.03 to 0.31 μeq L−1 (median 0.04 μeq L−1) and 0.83 to 5.44 μeq L−1 (median 1.37 μeq L−1), respectively. The MSA:NSSS molar ratios varied seasonally from 0.44 to 21.4% (median 3.28%) in aerosols of marine character and from 2.38 to 22.3% (median 6.40%) for rainwater samples. Atmospheric DMS and MSA concentrations at this coastal location in New Zealand are comparable to those observed at other remote southern hemisphere sites. In contrast, NSSS concentrations are somewhat elevated, possibly incorporating an unknown contribution from heterogeneous SO2 oxidation. With respect to DMS flux calculations an oceanic emission rate of 83 mg S m−2 yr−1 would be required to account for oxidative losses (70 mg S m−2 yr−1) and leakage out of the marine boundary layer. The sum of wet and dry deposition for MSA and NSSS amounts to 65 mg S m−2 yr−1. The small difference between DMS oxidative losses and total MSA + NSSS deposition likely stems from an underestimate of the wet deposition rate for NSSS.