Trends in Upper Stratospheric Chlorine

What are the trends in stratospheric chlorine species over the past decade and a half, and do they follow expectations, based on decreases in total surface chlorine since the turn of the century?

This work examines trends in upper stratospheric chlorine species for 2005–2020, with an emphasis on chlorine monoxide (ClO) and hypochlorous acid (HOCl), measured by the Aura Microwave Limb Sounder (MLS) . Results are compared to those from a state-of-the-art atmospheric model (WACCM, the Whole Atmosphere Chemistry Climate Model), constrained by the chlorine source gases (and other surface gases) and driven by meteorological fields.

Multiple regression fits are applied to the monthly zonal mean time series, and decreasing linear trends are extracted from both MLS and model time series (see example in Fig. 1). Averaged (50ᴼS–50ᴼN) results (Fig. 2) show MLS and model ClO trends of about -0.8%/year for ClO and -0.5%/year for HOCl. Trends in related upper stratospheric species (e.g., increases in methane) are also analyzed in this paper, to help explain differences between the ClO and HOCl trends.

The 1987 Montreal Protocol and its amendments have led to global chlorine decreases in the stratosphere, in accord with expectations. These international agreements have strongly limited the deleterious effects of chlorine (and bromine) source gases in terms of potential stratospheric ozone depletion and related solar UV radiation increases at the surface. ClO, in particular, is of critical importance regarding stratospheric ozone depletion and recovery.

As for the continuation of stratospheric data records of chlorine species from Aura MLS or any other planned sensors, there are currently no concrete plans for future satellite measurements of such species.

Data Sources:

The Aura MLS data files used here are available from the NASA GSFC DISC

The model results are available online

Reference:

Froidevaux, L., Douglas E. Kinnison, D. E., Michelle L. Santee, M. L., Millán, L. F., Nathaniel J. Livesey, N. J., Read, W. G., Bardeen, C. G., Orlando, J. J., and Fuller, R. A., Upper stratospheric ClO and HOCl trends (2005–2020): Aura Microwave Limb Sounder and model results , Atmos. Chem. Phys., 22, 4779–4799, :10.5194/acp-22-4779-2022, 2022.

10.2022