Tropical sources and sinks of COS

What is it about?

The seasonal variation of carbonyl sulfide (COS) is governed by oceanic release and vegetation uptake. Its assimilation by plants is assumed to be similar to the uptake of CO2 but, contrary to the latter process, to be irreversible. Therefore COS has been suggested as co-tracer of the carbon cycle. However, observations of COS are sparse, especially in tropical regions. We derive its global distribution from the comprehensive spaceborne data set of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS). Two major features are observed in the tropical upper troposphere at about 11 km altitude: enhanced amounts over the western Pacific and the Maritime Continent, peaking around 550 parts per trillion by volume (pptv) in boreal summer, and a seasonally varying depletion of COS extending from tropical South America to Africa. The large-scale COS depletion has not been observed before and reveals the seasonality of COS uptake through tropical vegetation. The observations can only be reproduced by global models, when a large vegetation uptake and a corresponding increase in oceanic emissions as proposed in several recent publications are assumed.

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