Feasibility study of atmospheric carbon dioxide satellite retrievals over snow in the context of the Copernicus Anthropogenic CO₂ Monitoring Mission

Abstract. Northern high latitudes pose significant challenges to reliable space-based observations of total column carbon dioxide (XCO₂). In addition to large solar zenith angles and frequent cloud coverage over the Arctic and boreal regions, snow-covered surfaces absorb strongly in the shortwave-infrared wavelengths coinciding with the CO₂ absorption channels used by several current missions, e.g., Japanese Greenhouse Gases Observing Satellite and Nasa Orbiting Carbon Observatory-2, and the upcoming Copernicus Anthropogenic CO₂ Monitoring Mission (CO2M). Because of the resulting low radiances of the reflection measured by the satellite instruments, retrievals over snow may be less reliable and, for current missions, are typically filtered or flagged for potentially poor quality. In this work, we present the first feasibility study dedicated to XCO₂ retrievals over snow. We introduce a measurement-based snow reflectance model, develop this into a kernel format following the Ross-Thick-Snow (RTS) kernel formulation, and use this to simulate CO2M radiances. Furthermore, we also present a sample of simulated XCO₂ retrievals for the CO2M with the University of Leicester Full Physics (UoL-FP) retrieval framework, considering a variety of solar zenith angles as well as nadir and specular reflection (glint) observing geometries, the two planned observation modes of CO2M. The results indicate the advantages of using a snow kernel model in the retrievals of XCO₂, in terms of improved convergence. Glint-mode observations are shown to lead to higher signal-to-noise ratios and improved retrievals in terms of reduced errors compared to the nadir geometry in the Northern high latitudes over snow. The signal-to-noise ratio increases towards forward-scattering direction, which indicates that the exact specular reflection (glint) geometry is not required but forward scattering directions in general have advantages at large solar zenith angles. However, we also find that constraining the snow reflectance model is essential for the retrieval improvements. The results show the potential to significantly increase the number of observations at high latitudes and widen the seasonal coverage by approximately 1-3 months. The relevance of a potentially increased coverage does not limit to CO2M but extends to other current and future CO₂ missions as well as other atmospheric constituents. Ultimately, the increased coverage may contribute to the quantification and an improved understanding of high-latitude CO₂ sources and sinks, especially in the late winter and spring seasons where a reliable quantification of the XCO₂ seasonal cycle is essential.