Abstract
We analyze Aura Microwave Limb Sounder (MLS) monthly zonal mean time series of ClO and HOCl between 50° S and 50° N to estimate upper stratospheric trends in these chlorine species from 2005 through 2020. We compare these observations to those from the Whole Atmosphere Community Climate Model version 6 (WACCM6), run under the specified dynamics configuration. The model sampling follows the MLS coverage in space and local time. We use version 5 MLS ClO zonal mean daytime profiles and similarly binned daytime ClO model profiles from 32 to 1.5 hPa. For MLS HOCl, we use the version 5 offline product derived from daily zonal mean radiances rather than averaged Level 2 profiles; MLS HOCl is scientifically useful between 10 and 2 hPa, and the HOCl monthly zonal means are separated into day and night for comparison to WACCM6. We find good agreement (mostly within ~10 %) between the climatological MLS ClO daytime distributions and the model ClO climatology for 2005–2020. The model HOCl climatology, however, underestimates the MLS HOCl climatology by about 30 %. This could well be caused by a combination of fairly large systematic uncertainties in both the model-assumed rate constant for the formation of HOCl and the MLS HOCl retrievals themselves. The model daytime ClO trends versus latitude and pressure agree quite well with those from MLS. MLS-derived near-global upper stratospheric daytime trends between 7 and 2 hPa are −0.73 ± 0.40 % yr−1 for ClO and –0.39 ± 0.35 % yr−1 for HOCl, with 2σ uncertainty estimates used here. The corresponding model decreases are somewhat faster than observed (although the difference is not statistically significant), with trend values of –0.85 ± 0.45 % yr−1 for ClO and –0.64 ± 0.37 % yr−1 for HOCl. Both data and model results point to a faster trend in ClO than in HOCl. The MLS ClO trends are consistent with past estimates of upper stratospheric ClO trends from satellite and ground-based microwave data. As discussed in the past, trends in other species (in particular, positive trends in CH4 and H2O) can lead to a ClO decrease that is faster than the decrease in total inorganic chlorine. Regarding trends in HOCl, positive trends in HO2 can lead to a faster rate of formation for HOCl as a function of time, which partially offsets the decreasing trend in active chlorine. The decreasing trends in upper stratospheric ClO and HOCl provide additional confirmation of the effectiveness of the Montreal Protocol and its amendments, which have led to the early stages of an expected long-term ozone recovery from the effects of ozone-depleting substances.
Highlights
Changes in the gaseous chlorine content of the atmosphere have been scrutinized since the late 1970s, when prescient warnings (Molina and Rowland, 1974) were made regarding likely threats to the Earth’s stratospheric ozone (O3) layer from the decomposition of various chlorofluorocarbons (CFCs) emitted at the surface by human industrial activities
As a consequence of the photochemical balance between Cl and chlorine monoxide (ClO) radicals in the upper stratosphere, the largest ClO abundances occur at pressure levels near 2 to 3 hPa; in the mid- to lower stratosphere, the availability of reactive chlorine is limited by the conversion of ClO and NO2 to ClONO2
Near 20–30 hPa, the model ClO values in the winter hemisphere mid- to high latitudes are lower than observed by ~30%, there is not much available ClO in this region, and the systematic uncertainty estimates for Microwave Limb Sounder (MLS) ClO are of order 0.1 ppbv, which can be as much as 50–100%
Summary
Changes in the gaseous chlorine content of the atmosphere have been scrutinized since the late 1970s, when prescient warnings (Molina and Rowland, 1974) were made regarding likely threats to the Earth’s stratospheric ozone (O3) layer from the decomposition of various chlorofluorocarbons (CFCs) emitted at the surface by human industrial activities. Since the 1987 Montreal Protocol and its subsequent amendments, established to strongly reduce worldwide surface emissions of halogenated compounds harmful to the ozone layer, both the tropospheric and stratospheric chlorine budgets have been carefully studied and monitored by the atmospheric science community. This was motivated by enhanced concerns regarding ozone decreases in the lower stratosphere, after the discovery of the seasonal appearance of an ozone hole over Antarctica (Farman et al, 1985)
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