Limiting Factors for Satellite-Based Retrievals of Surface-Level Carbon Monoxide

Abstract:

CO is mostly produced in the lower troposphere by incomplete combustion of biomass and fuels. CO oxidation consumes ~75% of the tropospheric OH, which then is not available to remove CH₄ and other greenhouse gases. CO oxidation also leads to the production of tropospheric O₃. These critical impacts of CO on air quality and climate require accurate determination of the abundance and evolution of CO near the surface.

Satellite retrievals would be well-suited to monitor surface CO globally. However, how do they compare to actual surface abundances? Some aspects to be considered include: the vertical sensitivity of retrievals (given by the averaging kernels), or how thick are the atmospheric layers that can be resolved; the vertical correlation length of CO with respect to the thickness of those layers; and the horizontal variability of CO with respect to the instrument's footprint.

To investigate these questions we analyze MOPITT retrievals, DISCOVER-AQ and NOAA profiles, as well as WDCGG surface measurements. MOPITT, on board NASA's Terra satellite, has been measuring tropospheric CO since 2000, providing the longest global CO record to date. Its unique multispectral CO product offers enhanced sensitivity to CO near the surface. Vertical profiles of the lower troposphere were acquired during the DISCOVER-AQ airborne campaigns over selected regions of the USA. NOAA's airborne flask sampling program results in a multi-year, multi-seasonal record of vertical profiles from near the surface up to the mid troposphere, acquired over a number of stations, mostly in North America. Long-term, cross-calibrated surface CO data from ground stations worldwide are available through the WDCGG.

Statistical analyses of the DISCOVER-AQ and NOAA profiles indicate that surface vertical correlation length varies greatly depending on geographic location. This may explain contrasting results obtained for different ground stations when comparing MOPITT and WDCGG co-located data and timeseries.