Fluxes of Carbonyl Sulfide at Harvard Forest EMS Tower 2010-2013

Carbonyl sulfide (OCS), the most abundant sulfur compound in the atmosphere, controls the
sulfur budget and aerosol loading of the stratosphere in times of low volcanic activity. OCS is
also closely tied to the vegetative carbon cycle and may provide an independent measure of the
photosynthetic uptake of carbon. However, the detailed nature of the biogeochemical cycling of
OCS throughout the seasons in terrestrial ecosystems has not been thoroughly explored. We
measured the seasonal response of the ecosystem flux of OCS above a deciduous temperate forest
using an infrared laser absorption spectrometer at Harvard Forest (MA, USA) in 2011. Fluxes
were calculated using two complementary approaches: gradient – flux (also known as flux –
gradient; January – July, 2011) and eddy covariance flux (August – December, 2011), which
agreed within the combined error for a period with both measurements. Strong uptake of OCS by
the forest was observed during most of the growing season, but significant uptake also occurred
when the deciduous leaves were not present. A strong diel cycle of mid-day OCS uptake was
observed in May/June and August/September, consistent with OCS uptake by processes parallel to
CO2 photosynthetic uptake, at the ecosystem scale, for these months. The results show that
while overall OCS and CO2 both show evidence of vegetative uptake, the OCS flux cannot be
explained by this process alone and the relationship between OCS and CO2 changes throughout the
year above this mixed deciduous forest. The data imply that terrestrial uptake of OCS, and
hence the stratospheric sulfur cycle, are potentially quite sensitive to extremes in
temperature and soil moisture.