Carbon disulphide and carbonyl sulphide from biogenic sources and their contributions to the global sulphur cycle

Abstract

Estimates of the magnitude of biogenic sulphur emissions range from ∼70% of the total atmospheric sulphur burden^1–7, and the chemical nature of the emissions has not been clearly established. Conway¹ speculated that the principal volatile biogenic component of the sulphur cycle was hydrogen sulphide (H₂S) whereas Lovelock et al.⁸ and Rasmussen⁹ have suggested that dimethyl sulphide (DMS) contributes to the apparent source deficits. Aneja et al.¹⁰ have shown that both H²S and DMS are emitted from saltmarshes. Other volatile sulphur compounds which may contribute to the sulphur burden of the atmosphere include methyl mercaptan (CH₃SH)^11,12, dimethyl disulphide ((CH₃)₂S₂)^11,12, carbonyl sulphide (COS)¹³ and carbon disulphide (CS₂)^14,15. We report here the discovery of CS₂ and COS emanating from a saltmarsh and estimate their emission rates using emission flux reactor and bag chamber techniques. The species CS₂ and COS are relatively inert in the troposphere, so may be assumed to penetrate to the stratosphere, where they may be photolysed to form the sulphur dioxide (SO₂) and sulphates (SO₄²⁻) known to be present in the stratosphere. Based on the measured fluxes, we show that the emissions from marshes are important to the sulphate aerosol burden (≲19%) of the stratosphere, but not important for the tropospheric sulphur burden (≲0.2%).