Estimates of 4.7 μm surface emissivity and their impact on the retrieval of tropospheric carbon monoxide by Measurements of Pollution in the Troposphere (MOPITT)

Abstract

Carbon monoxide (CO) is an important tropospheric trace species. The Measurements of Pollution in the Troposphere (MOPITT) instrument uses the 4.7 μm CO band to measure the global CO profile and total column amount in the troposphere from space. In the operational MOPITT CO retrieval algorithm, surface skin temperature (Ts) and emissivity (E) are retrieved simultaneously with the CO profile. However, because both Ts and E are retrieved from the same piece of information from the MOPITT measurements, the accuracy of both variables may be limited, which leads to an increase of uncertainty in the CO retrievals. An accurate specification of the surface skin temperature is required to determine surface emissivity and vice versa. In this study, a method is developed which uses Ts from the Moderate Resolution Imaging Spectroradiometer (MODIS) and MOPITT radiances to derive an improved 4.7 μm surface emissivity estimate (E) for use in retrievals by the MOPITT instrument. Monthly mean 4.7 μm surface emissivity maps for 1 year are generated and used as the a priori E in the MOPITT Ts and CO retrieval algorithm. We show that the geographical distribution of the 4.7 μm emissivity is very consistent with MODIS normalized difference vegetation index distribution, which is strongly tied to the surface emissivity. This a priori E has a much smaller standard deviation than values currently used in the MOPITT retrieval. As a result, more radiance information tends to be used in the MOPITT Ts and CO retrievals. By using the improved a priori E over the land, the information content of MOPITT radiances increases 15% at night and 5% during the day relative to the current version MOPITT data products. The difference between day and night information content (or diurnal difference) decreases from 0.3 (current version) to 0.21, showing that nighttime retrievals are improved. Over the global ocean the diurnal difference of the MOPITT information content decreases from 0.15 (current version) to 0.06. The differences between the new profile retrievals and those of current profile retrievals are very consistent with their corresponding diurnal and geographical information content distributions. Over the global ocean the new MOPITT CO profile is lower by 3–11% during the night in the lower troposphere. Over the global land the new CO profile is higher by 3.2% in the lower troposphere during the night. The differences between the new profile retrieval and those of current retrieval are small during the day.