On the global distribution, seasonality, and budget of atmospheric carbonyl sulfide (COS) and some similarities to CO2

Abstract

Measurements of carbonyl sulfide (COS) from a global air‐monitoring network over multiple years suggest that atmospheric mixing ratios of COS are strongly influenced by terrestrial vegetation in the Northern Hemisphere (NH) and by the oceans in the Southern Hemisphere (SH). The annual mean NH mixing ratio estimated from results at seven surface sites during 2000.2–2005.2 was 476 ± 4 ppt, or slightly less than the mean of 491 ± 2 ppt derived from results at three surface sites in the SH. The lowest annual mean mixing ratios were measured at low‐altitude continental sites in the midlatitude and high‐latitude NH. Mixing ratios undergo substantial seasonal variations at nearly all sites across the globe; the largest seasonal variations are observed at the NH sites having the lowest annual means. There is little coherence in the seasonality in the NH and SH, suggesting that the COS seasonality is driven by different processes in each hemisphere. These seasonal changes cause the NH/SH ratio, as estimated from the available surface data, to vary regularly from 0.91 ± 0.01 to 1.04 ± 0.02 across a year; the annual mean NH/SH ratio was 0.97 ± 0.01. Results from over 160 aircraft profiles regularly collected at eight sites over the continental United States throughout an entire year reveal substantial vertical gradients for COS mixing ratios that vary with season. While similar mixing ratios are observed throughout the NH troposphere during January–April (up to 8 km above sea level (asl)), during the growing season substantially reduced mixing ratios are observed in the boundary layer above the continental United States (defined here as <2 km asl). The surface and aircraft results for COS show strong similarities to atmospheric CO 2, though both the amplitude of seasonal variations measured at Earth's surface and the observed vertical gradients during the growing season are 5–6 times larger for COS than for CO 2 on a relative basis. A qualitative analysis of the results in light of known sources and sinks suggests (1) that terrestrial uptake, most likely due to photosynthetically active vegetation, dominates the seasonality observed throughout the Northern Hemisphere at a rate that is about 5 times greater than estimates based upon scaling net primary production by mean, ambient air mixing ratios of COS to CO 2, (2) the oceans dominate the seasonality observed in the Southern Hemisphere, and (3) biomass burning has a small influence on the seasonality observed for COS in the extratropics of both hemispheres.