Upper stratospheric ClO and HOCl trends (2005–2020): Aura Microwave Limb Sounder and model results

Abstract

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.

Tropical upper tropospheric trends in ozone and carbon monoxide (2005–2020): observational and model results

“…We have also included a fitted component that follows variations in solar radio flux (at 10.7 cm), based on Canadian solar measurements (Tapping, 2013); this component typically plays a negligible role in our results. For trend uncertainty estimates, as discussed also by Froidevaux et al (2019Froidevaux et al ( , 2022, we use the block bootstrap resampling method (Efron and Tibshirani, 1993), as done by Bourassa et al (2014) and others in such atmospheric composition analyses. For every fitted time series, we analyze thousands of re-samplings of the fit residuals, with year-long blocks of residual values replaced by residual series from randomly chosen years; twice the standard deviations in these random distributions' trends provide the (2) trend uncertainty values that we use as trend error bars throughout this work.…”

“…Our tropical analyses will focus on results between 215 and 147 hPa, in order to largely obtain upper tropospheric results, as more influence from the stratosphere occurs as one gets closer to 100 hPa in the tropics. In the upper troposphere, the vertical resolution of the O3 and CO products is about 3 km and 5 km respectively (Livesey et al, 2022). In this region, the single-profile precision (1 random uncertainty) is 20-30 ppbv for O3 and 15-20 ppbv for CO.…”

“…The MLS retrievals (Livesey et al, 2006) use the optimal estimation approach (Rodgers, 2000); there is no assumption of atmospheric homogeneity along the line of sight (see Livesey and Read, 2000), and the retrievals make use of the MLS antenna's views along overlapping tangent rays during consecutive scans of the Earth's limb. The specifics of MLS data characterization and data quality, along with estimated errors and related information can be found in the documentation by Livesey et al (2022).…”

“…In this work, we analyze both monthly zonal mean series (based on monthly averages of binned 24-hr averaged fields) and a longitude-gridded subset (see below) of these monthly mean series; the typical number of MLS profiles in a monthly zonal mean 4° bin is or order 2400, and about 200 for each of the 12 mapped (monthly) longitude/latitude bins. Prior to averaging the MLS data, the standard MLS data quality screening criteria (Livesey et al, 2022) have been applied to all the O3 and CO Level 2 profiles; this screening removes only a very small fraction (typically of order a few percent or less) of the retrieved profiles. In the troposphere and stratosphere, the MLS O3 retrieval grid is defined by a subset of the pressure levels given by p(n) =1000 x 10 -n/12 hPa, where n is the pressure level index number; for CO, the grid is twice as coarse, meaning that n/6 is used as an exponent in the above equation, rather than n/12.…”

“…In addition, the methodology used by the MLS team to assess the aggregate effects of estimated errors in various input parameters, coupled with validation results (see Livesey et al, 2022), leads to systematic uncertainty estimates (1) of 5-12 ppbv and 15-25 ppbv for tropical upper tropospheric O3 and CO, respectively.…”