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There is presently renewed interest in diurnal variations of stratosphericand mesospheric ozone for the purpose of supporting homogenization ofrecords of various ozone measurements that are limited by the techniqueemployed to being made at certain times of day. We have made suchmeasurements for 19 years using a passive microwave remote sensing techniqueat the Mauna Loa Observatory (MLO) in Hawaii, which is a primary station in theNetwork for Detection of Atmospheric Composition Change (NDACC). We haverecently reprocessed these data with hourly time resolution to study diurnalvariations. We inspected differences between pairs of the ozone spectra(e.g., day and night) from which the ozone profiles are derived to determinethe extent to which they may be contaminated by diurnally varying systematicinstrumental or measurement effects. These are small, and we have reducedthem further by selecting data that meet certain criteria that weestablished. We have calculated differences between profiles measured atdifferent times: morning–night, afternoon–night, and morning–afternoon andhave intercompared these with like profiles derived from the Aura Microwave Limb Sounder (Aura-MLS), the Upper Atmosphere Research Satellite Microwave Limb Sounder (UARS-MLS),the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES), and Solar Backscatter Ultraviolet version 2 (SBUV/2) measurements. Differences between averages of coincidentprofiles are typically < 1.5% of typical nighttime values overmost of the covered altitude range with some exceptions. We calculatedaverages of ozone values for each hour from the Mauna Loa microwave data,and normalized these to the average for the first hour after midnight forcomparison with corresponding values calculated with the Goddard EarthObserving System Chemistry Climate Model (GEOSCCM). We found that themeasurements and model output mostly agree to better than 1.5% of themidnight value, with one noteworthy exception: The measured morning–nightvalues are significantly (2–3 %) higher than the modeled ones from 3.2 to1.8 hPa (~39–43 km), and there is evidence that the measuredvalues are increasing compared to the modeled values before sunrise in thisregion

atmospheric chemistry and physics

Diurnal variations of stratospheric ozone measured by ground-based microwave remote sensing at the Mauna Loa NDACC site: measurement validation and GEOSCCM model comparison