REEVALUATION OF THE OPEN-OCEAN SOURCE OF CARBONYL SULFIDE TO THE ATMOSPHERE

Carbonyl sulfide (COS) concentrations were measured in surface seawate r samples and the overlying marine boundary layer of the Pacific Ocean by using gas chromatography (GC) and electron capture sulfur detectio n (ECD-S). A wide latitudinal range was covered (55 degrees N-70 degre es S) on two cruises 9 months apart. COS saturation ratios (SRs) in se awater were found to be less than 1 (undersaturated) across wide regio ns of the open ocean, especially in the subtropical gyres and winterti me subpolar waters. SRs were highest in coastal/shelf regions and in s pring/summertime temperate waters. Extensive undersaturation is attrib uted to a low COS photoproduction potential of the water, limited sunl ight, and/or a rapid hydrolysis rate constant. Decreasing COS concentr ations during diurnal cycles in tropical waters were fitted to first-o rder exponentials, with resulting decay times agreeing with calculated hydrolysis lifetimes to within 15%. Air-sea fluxes of COS from the op en ocean were calculated by using two different expressions for the tr ansfer velocity and averaged into six latitude bands and three seasons . On the basis of these data we report a global open ocean sea-air flu x of -0.032 (-0.010 to -0.054) Tg COS/yr, which is much lower than and of different sign from the current global estimate (0.14-0.58 Tg COS/ yr). Atmospheric COS mixing ratios averaged 470 pptv on the first crui se and 442 pptv on the second cruise, with much of the difference poss ibly a result of a seasonal decrease in the northern hemisphere COS mi xing ratio of up to 10%.