Atmospheric aerosols have been known to be a major source of uncertainties in CO₂ 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 XCO₂ 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 XCO₂ accuracy. For the purpose of this work, we generate synthetic measurements for different atmospheric and geophysical scenes over land, based on which XCO₂ retrieval errors are assessed. We show that the standard XCO₂ retrieval approach that makes no use of auxiliary aerosol observations returns XCO₂ errors with an overall bias of 1.12 ppm and a spread (defined as half of the 15.9–84.1 percentile range) of 2.07 ppm. The latter is far higher than the required XCO₂ accuracy (0.5 ppm) and precision (0.7 ppm) of the CO2M mission. Moreover, these XCO₂ 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 XCO₂ from polluted areas where CO₂ 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 XCO₂ 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 XCO₂ errors have an overall bias of −0.004 ppm and a spread of 0.54 ppm for one concept, and a bias of 0.02 ppm and a spread of 0.52 ppm 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 XCO₂ errors on aerosol optical depth, altitude of the aerosol layer, and solar zenith angle. These results indicate a major improvement in the retrieved XCO₂ 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 CO₂ sinks and sources. As such, this outcome underlines the contribution of, and therefore the need for, a MAP instrument aboard the CO2M mission.

中文翻译：

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人为CO ₂监测卫星任务：多角度极化观测的需要

众所周知，大气气溶胶是从太空中获取的CO ₂浓度不确定性的主要来源。在这项研究中，我们研究了哥白尼人为二氧化碳监测（CO2M）任务中多角度偏振计（MAP）测量的附加值。为此，我们将仅使用光谱仪（不带MAP）的标准检索与使用MAP和光谱仪检索的气溶胶引起的XCO ₂误差进行了比较。预计MAP观测将提供有关可改善XCO _2的气溶胶信息。准确性。出于这项工作的目的，我们针对陆地上不同的大气和地球物理场景生成了综合测量值，并以此为依据对XCO ₂检索误差进行了评估。我们表明，不使用辅助气溶胶观测的标准XCO ₂检索方法会返回XCO ₂误差，其总偏差为1.12  ppm，并且散布度（定义为15.9–84.1百分位数范围的一半）为2.07  ppm。后者远远高于CO2M任务要求的XCO ₂精度（0.5  ppm）和精度（0.7  ppm）。而且，这些XCO ₂在高气溶胶光学深度，高气溶胶高度和低太阳天顶角时，这些误差会表现出明显更大的偏差和散射，这可能会导致在从CO ₂和气溶胶共排放的污染区域检索XCO _2的性能降低。我们将根据实现XCO ₂所需的波长范围，视角数量和测量不确定度来确定MAP仪器规格任务的精确度和精确度目标。在此分析中考虑了两种不同的MAP仪器概念。我们发现，对于这两个概念，MAP的辐射度和线性极化度的不确定度均应分别不超过3％和0.003。针对这两个MAP概念中的每个概念，使用MAP和光谱仪对合成场景的测量结果进行了检索练习。对于一个概念，所得的XCO ₂误差的总偏差为-0.004  ppm，扩展为0.54  ppm，偏差为0.02  ppm，扩展为0.52  ppm对于另一个概念。两者都符合CO2M任务要求；对于适当的排放估算，非常低的偏差尤其重要。对于测试合奏，我们实际上没有发现XCO ₂误差与气溶胶光学深度，气溶胶层高度和太阳天顶角无关。这些结果表明，相对于标准检索方法，检索到的XCO ₂准确性有了显着提高，这可能会导致更高的数据产出，更好的全球覆盖范围以及更全面的CO ₂汇和源确定。因此，这一结果强调了CO2M任务中MAP仪器的贡献，因此也需要它。