Reevaluation of the role of dimethyl sulphide in the sulphur budget

Abstract

THE possible existence of an oceanic source of atmospheric SO₂ has recently been suggested for the following reasons. First, to account for the observed¹ SO₂ background concentration of 0.1 × 10⁻⁶g m⁻³ of air, even very far from any continental source and in areas as remote as the Antarctic ocean—despite a residence time for SO₂ in the marine atmosphere of ∼0.7 d (refs 2, 3). Second, because another major source such as the ocean is necessary to complete the global atmospheric sulphur budget^4–7. Because of the basic pH of seawater, SO₂ cannot be emanated directly from the oceans. According to Lovelock et al.⁸, the dimethyl sulphide (DMS) produced in the sea by many living systems, could be evolved in a first step into the atmosphere and subsequently oxidised to SO₂. These authors observed a mean DMS concentration of the order of 12 × 10⁻⁹ gl⁻¹ in Atlantic ocean water. We have developed an improved method for the measurement of DMS in seawater and have used this technique on several recent oceanographic research cruises. Our results suggest that the oceans could contribute over 30% of the amount of sulphur required to balance the sulphur budget.