Verification of the AIRS and MLS ozone algorithms based on retrieved daytime and nighttime ozone

Ozone (O3) plays a significant role in weather and climate on regional to global spatial scales. Most studies on the variability in the total column of O3 (TCO) are typically analysed using daytime data. Based on knowledge of the chemistry and transport of O3, significant deviations between daytime and nighttime O3 are only expected either in the planetary boundary layer (PBL) or high in the stratosphere or mesosphere, having little effect on the TCO. Hence, we expect the daytime and nighttime TCO to be very similar. Comparing daytime and nighttime TCOs thus provides an approach to verify the retrieval algorithms of infrared instruments like the Atmospheric InfraRed Sounder (AIRS) and the Microwave Limb Sounder (MLS). Applying this verification on the AIRS and the MLS data we identified inconsistencies in observations of O3 from both satellite instruments. For AIRS, daytime-nighttime differences were found over oceans resembling cloud cover patterns, and over land, mostly over dry land areas, 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 nighttime and daytime MLS measurements. Disregarding this issue, MLS day-night differences were significantly smaller than AIRS day-night differences, providing additional support for retrieval method origin of AIRS day-night TCO differences. MLS day-night differences are dominated by the upper stratospheric and mesospheric diurnal O3 cycle. These results provide useful information for improving infrared O3 products and at the same time will allow study the day-night differences of stratospheric and mesospheric O3.