Measurements of Stratospheric ClO From Mauna Kea: 1992–2023

Abstract We present a reanalysis of the ground‐based microwave measurements of upper stratospheric ClO from Mauna Kea over 1992–2023 made by the Chlorine Oxide Experiment (ChlOE) instrument. In order to reduce instrumental baseline artifacts, the retrieval makes use of the difference of daytime and nighttime spectra (the nighttime ClO spectra are much smaller) to produce a day‐minus‐night ClO mixing ratio, which is the fundamental quantity analyzed throughout this study. Upper stratospheric ClO values peaked in ∼1997, and the trend in the upper stratospheric ChlOE measurements from 1997 to 2023 was found to be −0.4 ± 0.3%/yr (2 σ ). Comparisons of ChlOE measurements with a combined coincident Upper Atmosphere Research Satellite (UARS) and Aura Microwave Limb Sounder (MLS) data set showed a relative trend of +0.3% ± 0.3%/yr (2 σ ) over 1992–2023, where a positive trend indicates that the ChlOE ClO measurements are increasing relative to those from MLS. Restricting the comparisons to ChlOE and Aura MLS over 2004–2023, resulted in relative trends that varied by pressure level, from +0.15%/yr to +0.42%/yr, all agreeing to within the 2 σ uncertainty. Over the period 2004–2021, the average ChlOE trend for the three levels was −0.3 ± 0.4%/yr, but, with the addition of the unusually high ClO measured in 2022 and 2023 the trend from 2004 to 2023 became 0.0 ± 0.4%/yr. The increase in ClO during 2022 is, at least in part, caused by dynamical variations. The higher ClO in 2023 is closely correlated with the presence of increased H 2 O from the Hunga eruption, which affects the ClO chemistry.