Diurnal Variations in Stratospheric Clo Measured from Mauna Kea

We present diurnal variations of ClO and O3 measurements from a new 278 GHz ground-based radiometer that has been deployed at Mauna Kea since April 2018. This new radiometer was built and tested at Mauna Loa from 2015-2018 following the removal of an old instrument from Mauna Kea in 2015 due to anti-telescope protests. Results are compared with twice daily Microwave Limb Sounder (MLS) ClO and O3 measurements and with O3 measurements from our nearby 110 GHz radiometer at Mauna Loa. Since ClO mixing ratios are much higher at daytime than nighttime, and since the diurnal variation in O3 near the ClO peak is small, previous ClO retrievals from ground-based microwave measurements at Mauna Kea (1992-2015) have used the difference in the daytime and nighttime spectra to provide a “day minus night” ClO mixing ratio. This has the advantage of minimizing the effect of instrumental artifacts, but in doing so one loses information on the O3 mixing ratio. Here we simultaneously retrieve ClO and O3 without a day minus night subtraction, and obtain good results. These O3 retrievals can be compared to O3 retrievals from the much stronger 110 GHz O3 emission line being measured by our instrument at Mauna Loa. Such O3 comparisons are extremely useful as a check on the overall calibration of the ClO measurements. In addition to measuring diurnal variations, a long-term goal is to create a ClO dataset that merges these new measurements with the 1992-2015 data, and to provide this dataset to the Network of Detection of Atmospheric Composition Change (NDACC) database.