Continuous and high-precision atmospheric concentration measurements of COS, CO2, CO and H2O using a quantum cascade laser spectrometer (QCLS)
Other literature type
Kooijmans, Linda M. J.
Uitslag, Nelly A. M.
Zahniser, Mark S.
Nelson, David D.
Montzka, Stephen A.
(issn: 1867-8548, eissn: 1867-8548)
Carbonyl sulfide (COS) has been suggested as a useful tracer for gross
primary production as it is taken up by plants in a similar way as CO<sub>2</sub>. To
explore and verify the application of this novel tracer, it is highly desired
to develop the ability to perform continuous and high-precision in situ
atmospheric measurements of COS and CO<sub>2</sub>. In this study we have tested a
quantum cascade laser spectrometer (QCLS) for its suitability to obtain
accurate and high-precision measurements of COS and CO<sub>2</sub>. The instrument is
capable of simultaneously measuring COS, CO<sub>2</sub>, CO and H<sub>2</sub>O after
including a weak CO absorption line in the extended wavelength range. An
optimal background and calibration strategy was developed based on laboratory
tests to ensure accurate field measurements. We have derived water vapor
correction factors based on a set of laboratory experiments and found that
for COS the interference associated with a water absorption line can dominate
over the effect of dilution. This interference can be solved mathematically
by fitting the COS spectral line separately from the H<sub>2</sub>O spectral line.
Furthermore, we improved the temperature stability of the QCLS by isolating
it in an enclosed box and actively cooling its electronics with the same
thermoelectric chiller used to cool the laser. The QCLS was deployed at the
Lutjewad atmospheric monitoring station (60 m; 6°21′ E,
53°24′ N; 1 m a.s.l.) in the Netherlands from July 2014 to April
2015. The QCLS measurements of independent working standards while deployed
in the field showed a mean difference with the assigned cylinder value within
3.3 ppt COS, 0.05 ppm for CO<sub>2</sub> and 1.7 ppb for CO over a period of 35 days. The different contributions to uncertainty in measurements of COS,
CO<sub>2</sub> and CO were summarized and the overall uncertainty was determined to
be 7.5 ppt for COS, 0.23 ppm for CO<sub>2</sub> and 3.3 ppb for CO for 1-minute
data. A comparison of in situ QCLS measurements with those from concurrently
filled flasks that were subsequently measured by the QCLS showed a difference
of −9.7 ± 4.6 ppt for COS. Comparison of the QCLS with a cavity ring-down
spectrometer showed a difference of 0.12 ± 0.77 ppm for CO<sub>2</sub> and −0.9 ± 3.8 ppb for CO.