Carbonyl Sulfide (OCS) and Ecosystems

For the past decade, observations of carbonyl sulfide (OCS or COS) have been investigated as a proxy for carbon uptake by plants. Completed Project: Mercury in forests


OCS is destroyed by enzymes that interact with CO2 during photosynthesis, namely carbonic anhydrase (CA) and RuBisCO (CA is the dominant sink). The majority of sources of OCS to the atmosphere are geographically separated from this large plant sink, whereas the sources and sinks of CO2 are co-located in ecosystems. The drawdown of OCS can therefore be related to the uptake of CO2 without the added complication of co-located emissions comparable in magnitude. We are working on using OCS to link the carbon, water and mercury cycles in forest ecosystems. 



Our first paper from our NSF AGS project, led by Daniel Obrist, is out in PNAS. Róisín Commane, Jamie Harrison, and Charlotte Kwong are all coauthors on the paper “Previously unaccounted atmospheric mercury deposition in a midlatitude deciduous forest.” We found that at Harvard Forest, over a period of 16 months, three-quarters of the mercury deposition in the forest was in a gaseous form. That is five times greater than the amount of mercury stored in rain and snow, and three times that of litterfall.

This work is being continued at  Howland Forest, a spruce dominated forest in Maine, through Summer 2021. 


Previous news

May 2021: QCLS Analyzer deployed at Howland Forest to measure CO2 and OCS gradients above and below the canopy. We also added a soil chamber to our measurements at this site to examine soil fluxes. This work made possible in collaboration with Mary Whelan at Rutgers University. 

December 2020: Jamie Harrison and Charlotte Kwong present at AGU virtual meeting. Jamie gave an eLightening talk on OCS fluxes and Charlotte presented a poster on ozone, both at Harvard Forest.

January 2020: Luke gave an oral presentation on his global OCS work at the American Meteorological Society meeting in Boston. 

November 2019: Roisin attended the 2nd International OCS workshop in Innsbruck, Austria

August 2019: Charlotte presented her summer work

Charlotte has continued her research as part of her undergraduate senior thesis at Columbia College, where she is using ozone fluxes to learn about the uptake mechanisms of mercury.

June 2019: Group outing to Harvard Forest

Chandler MorrisCharlotte Kwong and Emily Follansbee joined a recent visit to Harvard Forest.


We were funded by NSF Atmospheric Chemistry to measure mercury fluxes from midlatitude and tropical forests with Daniel Obrist (UMass Lowell) and measure OCS and ozone fluxes to learn about the uptake mechanisms. We began the project at Harvard Forest in summer 2019 and moved to Howland Forest in Maine in spring 2021. See the Obrist Lab website for a wonderful overview of mercury fluxes in ecosystems. The completed research project is described here.


As part of a recent overview of carbonyl sulfide and its role in terrestrial ecosystems (Whelan et al., 2018), reviewed the state of our understanding of the global OCS cycle and its applications to ecosystem carbon cycle science. They found that OCS uptake is correlated well to plant carbon uptake, especially at the regional scale and that OCS can be used in conjunction with other independent measures of ecosystem function, like solar-induced fluorescence and carbon and water isotope studies. This review highlighted a number of important gaps in our understanding of the total OCS budget and more work needs to be done to generate global coverage for OCS observations and to link this powerful atmospheric tracer to systems where fundamental questions concerning the carbon and water cycle remain.

Measurements of OCS fluxes above Harvard Forest (a mixed forest east of Boston, MA) for three years allow us to compile an extensive dataset to investigate the role of stomatal conductance in the ecosystem carbon cycle in a mixed mid-latitude forest. Recent publications: Commane et al., PNAS, 2015, Wehr et al., Biogeosciences, 2017.

We measured OCS mixing ratios along the coast of California, through San Francisco Bay into the Sacramento as part of the CalNex project. We sampled outflow from the Redwood forests of California and measured high OCS emissions from some unexpected anthropogenic sources: Commane et al., 2013. Campbell et al., JGR-Biogeosciences, 2018 also used this data to estimate the impact of fog on stomatal conductance on the regional scale. 


From years of measurements of OCS fluxes above Harvard Forest (a mixed forest east of Boston, MA) to the measurement of the OCS flux on a cruise to the North Atlantic (WACS) and OCS mixing ratios into the San Francisco Bay area (CalNex), we have learned a great deal about carbonyl sulfide in the past few years.