Measurements made by the MOPITT (“Measurements of Pollution in the Troposphere”) instrument on the NASA Terra polar-orbiting platform enable the retrieval of tropospheric carbon monoxide (CO) concentrations. As determined by the Terra orbit and MOPITT swath width, the frequency of MOPITT observations at a specific location, or measurement sampling frequency, is typically about once every three to four days. However, because the MOPITT retrieval algorithm is only applicable to clear-sky scenes, MOPITT retrieval sampling frequency strongly depends on regional cloudiness and can be much smaller than the measurement sampling frequency. Moreover, highly polluted scenes, characterized by high aerosol optical depths, can be confused with cloudy scenes and thus be discarded unnecessarily by the MOPITT cloud detection algorithm. Herein are described revisions to this algorithm which substantially increase retrieval sampling over land in varying pollution conditions. The performance of the revised cloud detection algorithm is evaluated through validation, case studies, and continental-scale maps of retrieval sampling frequency. Presented case studies illustrate (1) why the current operational MOPITT cloud detection algorithm excludes extended areas of potentially valuable cloud-free MOPITT observations and (2) how, for the same scenes, improved retrieval coverage benefits analyses of regional CO variability. Maps of retrieval sampling frequency for South America and Asia exhibit well-defined improvements, especially in regions with poor sampling frequency in the current product.

利用MOPITT（“对流层中的污染测量”）仪器在NASA Terra极地轨道平台上进行的测量能够检索对流层一氧化碳（CO）的浓度。由Terra轨道和MOPITT测绘带宽度确定，特定位置的MOPITT观测频率或测量采样频率通常大约每三到四天一次。但是，由于MOPITT检索算法仅适用于晴朗的天空，因此MOPITT检索采样频率在很大程度上取决于区域云量，并且可能比测量采样频率小得多。此外，以高气溶胶光学深度为特征的高污染场景可以与多云场景混淆，因此可以通过MOPITT云检测算法将其不必要地丢弃。本文介绍了此算法的修订版本，该版本可在变化的污染条件下大幅增加土地上的取回采样。修改后的云检测算法的性能通过验证，案例研究和检索采样频率的大陆比例图进行评估。提出的案例研究说明了（1）为什么当前可操作的MOPITT云检测算法排除了可能有价值的无云MOPITT观测值的扩展区域，以及（2）对于相同的场景，改进的取回覆盖范围如何有益于区域CO变异性分析。南美和亚洲的取回采样频率图显示出明确的改进，尤其是在当前产品的采样频率不佳的地区。