Measurements of Greenhouse Gases from Near-infrared Solar Absorption Spectra

This thesis presents improvements to retrievals of greenhouse gas concentrations, with a focus on CO2 and the Arctic. Near-infrared solar absorption spectra were collected at the Polar Environment Research Laboratory (PEARL) in Eureka, Nunavut as part of the Total Carbon Column Observing Network (TCCON), extending the data record to July 2020. Data processing was improved by the application of solar zenith angle corrections to account for pointing offsets of the solar tracker. Issues related to surface pressure records were resolved. TCCON measurements were used to validate simulations of CO2 and CH4 by GEM-MACH-GHG, a model in development at Environment and Climate Change Canada. Vertical profile retrievals of CO2 from TCCON spectra were evaluated, using improved spectroscopy and line shapes. CO2 profiles were obtained from sequential retrievals in five spectral windows using synthetic and real spectra. A sensitivity study showed that the leading source of uncertainty in the retrieved CO2 profiles is errors in the a priori temperature profile as small as 2°C between 600-850 hPa. To distinguish the effect of errors in the instrument alignment and spectroscopic parameters from other error sources, CO2 profiles were retrieved using an a prioriprofile built from coincident in-situ measurements. With real spectra, the deviations in retrieved CO2 profiles were larger than typical vertical variations of CO2. Remaining errors in the forward iiimodel limit the accuracy of the retrieved profiles. Implementing a temperature retrieval or correction is critical to improve CO2 profile retrievals. A study was conducted in support of the proposed Canadian satellite mission AIM-North. The ReFRACtor algorithm was adapted to generate synthetic spectra for a Fourier transform spectrometer and a grating spectrometer. Retrievals were performed on these synthetic spectra to estimate the precision and accuracy of retrieved XCH4, XCO, and XCO2, in different conditions.Over a standard scene corresponding to a boreal forest, the retrieval precision for the given instrument characteristics was ~0.6% for XCH4, ~8% for XCO, and ~0.4% for XCO2. These results can be used by the AIM-North team to decide whether the instrument design should be adapted to meet the mission’s precision and accuracy goals and thresholds over specific scenes.Ph.D