Verification of the Atmospheric Infrared Sounder (AIRS) and the Microwave Limb Sounder (MLS) ozone algorithms based on retrieved daytime and night-time ozone

Ozone (O-3) plays a significant role in weather and climate on regional to global spatial scales. Most studies on the variability in the total column of O-3 (TCO) are typically carried out using daytime data. Based on knowledge of the chemistry and transport of O-3, significant deviations between daytime and night-time O-3 are only expected either in the planetary boundary layer (PBL) or high in the stratosphere or mesosphere, with little effect on the TCO. Hence, we expect the daytime and night-time TCO to be very similar. However, a detailed evaluation of satellite measurements of daytime and night-time TCO is still lacking, despite the existence of long-term records of both. Thus, comparing daytime and night-time TCOs provides a novel approach to verifying the retrieval algorithms of instruments such as the Atmospheric Infrared Sounder (AIRS) and the Microwave Limb Sounder (MLS). In addition, such a comparison also helps to assess the value of night-time TCO for scientific research. Applying this verification on the AIRS and the MLS data, we identified inconsistencies in observations of O-3 from both satellite instruments. For AIRS, daytime-night-time differences were found over oceans resembling cloud cover patterns and over land, mostly over dry land areas, which is likely related to infrared surface emissivity. These differences point to issues with the representation of both processes in the AIRS retrieval algorithm. For MLS, a major issue was identified with the "ascending-descending" orbit flag, used to discriminate night-time and daytime MLS measurements. Disregarding this issue, MLS day-night differences were significantly smaller than AIRS day-night differences, providing additional support for the retrieval method origin of AIRS in stratospheric column ozone (SCO) day- night differences. MLS day-night differences are dominated by the upper-stratospheric and mesospheric diurnal O-3 cycle. These results provide useful information for improving infrared O-3 products.