Seasonal variations of dimethylsulfide emissions and atmospheric sulfur and nitrogen species over the western north Atlantic Ocean

Abstract

Seasonal measurements of atmospheric dimethylsulfide (DMS), sulfur dioxide (SO₂), aerosol methanesulfonate (MSA), non-sea salt sulfate (nss-SO₄^2-), ammonium (NH₄⁺), nitrate (NO₃^-), and other major ions were made between September 1985-September 1986 in the Western North Atlantic Ocean (WNAO). DMS was also measured in surface seawater. In addition, rainwater samples were collected and analyzed for various trace compounds, including dimethylsulfoxide (DMSO) and carboxylic acids. The DMS flux from the open ocean ranged from 1.3 μmol/m²/d in winter to 5—10 μmol/m₂/d in late summer and fall. The annual DMS flux from the WNAO area is estimated to be 0.1 Tg S/yr. This is a negligible contribution (3%) compared to the advection of sulfur from North America during westerly wind conditions. However, under easterly wind conditions oceanic DMS emissions may dominate the sulfur budget in the WNAO boundary layer. SO₂ and nss-SO₄^2- concentrations varied strongly with wind direction. SO₂ levels were as low as 10 pptv in marine air and reached several ppbv in continentally derived air. No significant differences between MSA levels in polluted and relatively unpolluted air were found. The yield of MSA from DMS oxidation may be similar in both air masses. Relatively weak and statistically not significant seasonal variations were observed for MSA, NO₃^-, and MSA/nss-SO₄^2- and NO₃^-nss-SO₄^2- ratios. In contrast, NH₄⁺ concentrations showed a clear seasonal variation with a maximum in late summer/early fall and a minimum in winter. Oceanic ammonia emissions may be important for marine background levels of ammonia and NH₄⁺ in the WNAO atmosphere. Aerosol particles were predominantly acidic with NH₄⁺/nss-SO₄^2- ratios of 1—2 mol/mol.