Abstract
We present a reanalysis of the ground-based microwave measurements of
upper stratospheric ClO from Mauna Kea from 1992-2023. In order to
reduce instrumental baseline artifacts from the Chlorine Oxide
Experiment (ChlOE) measurements, the retrieval makes use of the
difference of daytime and nighttime spectra to produce a day minus night
ClO mixing ratio. The trend in the upper stratospheric ChlOE
measurements from 1997-2023 was found to be -0.4+/-0.3%/yr (2σ).
Comparisons of ChlOE measurements with a combined coincident UARS and
Aura Microwave Limb Sounder (MLS) dataset shows a relative trend of
+0.3%+/-0.3%/yr (2σ) from 1992-2023. Restricting the comparisons to
ChlOE and Aura MLS from 2004-2023 results in relative trends that varied
by pressure level (3.2hPa, 4.2 hPa, and 5.6 hPa), from +0.15%/yr to
+0.42%/yr, all agreeing to within the 2σ uncertainty. From 2004-2021
the average ChlOE trend for the three levels was -0.3+/-0.4%/yr, but,
with the addition of the 2022 and 2023 measurements the trend from
2004-2023 becomes 0.0+/-0.4%/yr. The increase in ClO during 2022 is, at
least in part, caused by dynamical variations that affect CH4 mixing
ratios, which in turn affect the ClO chemistry. The higher ClO in 2023
is closely correlated with the presence of increased H2O from the Hunga
eruption which also affects the ClO chemistry.