Measurement of OCS, CO2, CO and H2O aboard NASA’s WB-57 High Altitude Platform Using Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS)

Abstract:

Carbonyl sulfide (OCS) is the most abundant sulfur gas in the atmosphere and has been implicated in controlling the sulfur budget and aerosol loading of the stratosphere. In the troposphere, OCS is irreversibly consumed during photosynthesis and may serve as a tracer for gross primary production (GPP). Its primary sources are ocean outgassing, industrial processes, and biomass burning. Its primary sinks are vegetation and soils. Despite the importance of OCS in atmospheric processes, the OCS atmospheric budget is poorly determined and has high uncertainty.

OCS is typically monitored using either canisters analyzed by gas chromatography or integrated atmospheric column measurements. Improved in-situ terrestrial flux and airborne measurements are required to constrain the OCS budget and further elucidate its role in stratospheric aerosol formation and as a tracer for biogenic volatile organics and photosynthesis.

Los Gatos Research has developed a flight capable mid-infrared Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS) analyzer to simultaneously quantify OCS, CO₂, CO, and H₂O in ambient air at up to 2 Hz. The prototype was tested on diluted, certified samples and found to be precise (OCS, CO₂, CO, and H₂O to better than ±4 ppt, ±0.2 ppm, ±0.31 ppb, and ±3.7 ppm respectively, 1s in 1 sec) and linear (R² > 0.9997 for all gases) over a wide dynamic range (OCS, CO₂, CO, and H₂O ranging from 0.2 – 70 ppb, 500 – 3000 ppm, 150 – 480 ppb, and 7000 – 21000 ppm respectively). Cross-interference measurements showed no appreciable change in measured OCS concentration with variations in CO₂ (500 – 3500 ppm) or CO.

We report on high altitude measurements made aboard NASA’s WB-57 research aircraft. Two research flights were conducted from Houston, TX. The concentration of OCS, CO₂, CO, and H₂O were continuously recorded from sea level to approximately 60,000 feet. The concentration of OCS was observed to increase with altitude through the troposphere due to the consumption in photosynthesis at ground level and was well correlated with CO₂.

These results demonstrate that the OA-ICOS instrument is capable of high altitude airborne operation that will advance our understanding of OCS’s role in Earth’s atmosphere by providing precise and accurate measurements throughout the troposphere and into the stratosphere.