Simultaneous, in situ detection of gas-phase carbon disulfide (CS₂) and carbonyl sulfide (OCS) via O₂⁺ chemical ionization mass spectrometry

Abstract. Carbonyl sulfide (OCS) is the largest gas-phase sulfur reservoir in the troposphere. OCS is both directly emitted to the atmosphere and formed chemically in the atmosphere following the oxidation of reduced sulfur species such as carbon disulfide (CS₂) and dimethyl sulfide (C₂H₆S, DMS). OCS is the largest continuous contributor to stratospheric sulfate aerosol and a key diagnostic for the photosynthetic uptake of carbon dioxide (CO₂). Despite its importance to the OCS budget, in situ measurements of CS₂ are extremely limited. Here, we describe a new technique for the simultaneous measurement of CS₂, OCS, and DMS utilizing O₂⁺ chemical ionization time-of-flight mass spectrometry (O₂⁺ CI-ToFMS). We highlight the utility of the measurement through the reanalysis of three distinct data sets taken in remote marine, remote continental, and urban environments. We report mean CS₂ mixing ratios for the Eastern North Atlantic (2.2 ppt) and Downtown Chicago, IL (9.5 ppt), while CS₂ was not detected in Northern Wisconsin. In the Eastern North Atlantic, CS₂ showed limited temporal variability during the summer. In Chicago, CS₂ was highly variable, with episodic large enhancements (>85 ppt) suggesting local, anthropogenic sources. Results of laboratory measurements and success in the quantification of field measurements suggest that primary and secondary O₂⁺ CI-ToFMS is suitable for simultaneous, in situ detection of CS₂ and OCS.