Anthropogenic CO₂ monitoring satellite mission: the need for multi-angle polarimetric observations

<p>Atmospheric aerosols have been known to be a major source of uncertainties in <span class="inline-formula">CO₂</span> concentrations retrieved from space. In this study, we investigate the added value of multi-angle polarimeter (MAP) measurements in the context of the Copernicus Anthropogenic Carbon Dioxide Monitoring (CO2M) mission. To this end, we compare aerosol-induced <span class="inline-formula">XCO₂</span> errors from standard retrievals using a spectrometer only (without MAP) with those from retrievals using both MAP and a spectrometer. MAP observations are expected to provide information about aerosols that is useful for improving <span class="inline-formula">XCO₂</span> accuracy. For the purpose of this work, we generate synthetic measurements for different atmospheric and geophysical scenes over land, based on which <span class="inline-formula">XCO₂</span> retrieval errors are assessed. We show that the standard <span class="inline-formula">XCO₂</span> retrieval approach that makes no use of auxiliary aerosol observations returns <span class="inline-formula">XCO₂</span> errors with an overall bias of 1.12 <span class="inline-formula">ppm</span> and a spread (defined as half of the 15.9–84.1 percentile range) of 2.07 <span class="inline-formula">ppm</span>. The latter is far higher than the required <span class="inline-formula">XCO₂</span> accuracy (0.5 <span class="inline-formula">ppm</span>) and precision (0.7 <span class="inline-formula">ppm</span>) of the CO2M mission. Moreover, these <span class="inline-formula">XCO₂</span> errors exhibit a significantly larger bias and scatter at high aerosol optical depth, high aerosol altitude, and low solar zenith angle, which could lead to worse performance in retrieving <span class="inline-formula">XCO₂</span> from polluted areas where <span class="inline-formula">CO₂</span> and aerosols are co-emitted. We proceed to determine MAP instrument specifications in terms of wavelength range, number of viewing angles, and measurement uncertainties that are required to achieve <span class="inline-formula">XCO₂</span> accuracy and precision targets of the mission. Two different MAP instrument concepts are considered in this analysis. We find that for either concept, MAP measurement uncertainties on radiance and degree of linear polarization should be no more than 3 % and 0.003, respectively. A retrieval exercise using MAP and spectrometer measurements of the synthetic scenes is carried out for each of the two MAP concepts. The resulting <span class="inline-formula">XCO₂</span> errors have an overall bias of <span class="inline-formula">−0.004</span> <span class="inline-formula">ppm</span> and a spread of 0.54 <span class="inline-formula">ppm</span> for one concept, and a bias of 0.02 <span class="inline-formula">ppm</span> and a spread of 0.52 <span class="inline-formula">ppm</span> for the other concept. Both are compliant with the CO2M mission requirements; the very low bias is especially important for proper emission estimates. For the test ensemble, we find effectively no dependence of the <span class="inline-formula">XCO₂</span> errors on aerosol optical depth, altitude of the aerosol layer, and solar zenith angle. These results indicate a major improvement in the retrieved <span class="inline-formula">XCO₂</span> accuracy with respect to the standard retrieval approach, which could lead to a higher data yield, better global coverage, and a more comprehensive determination of <span class="inline-formula">CO₂</span> sinks and sources. As such, this outcome underlines the contribution of, and therefore the need for, a MAP instrument aboard the CO2M mission.</p>