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On the role of heterogeneous chemistry in ozone depletion and recovery

Conclusive verification that stratospheric ozone destruction is lessening as expected in response to international controls on anthropogenic ozone-depleting substances (ODSs) enacted under the Montreal Protocol is one of today’s atmospheric science imperatives, but robust detection of such ozone “recovery” is complicated by large natural variability. One confounding factor is volcanic enhancement of stratospheric aerosol, which facilitates heterogeneous chlorine-catalyzed ozone destruction.

 On the role of heterogeneous chemistry in ozone depletion and recovery

Modeled and measured (SWOOSH) changes generally agree in the upper stratosphere, confirming that gas-phase chemistry is well represented in models. Differences between model runs with (Chem-Dyn-Vol) and without (Vol-Clean) volcanic aerosol show that in the lower stratosphere, volcanoes enhanced ozone loss during the depletion era (top panels) and have delayed ozone recovery (bottom panels) in both polar and midlatitude regions.

Simulations from a state-of-the-art chemistry-climate model driven by meteorological reanalyses are analyzed to quantify the contributions from gas-phase and heterogeneous chemistry to ozone trends in both the depletion and the recovery eras.

Model results are compared to the long-term “SWOOSH” record of vertically resolved ozone profile observations from the Aura Microwave Limb Sounder and other sensors, as well as total column ozone. As expected, gas-phase chemistry is found to largely account for upper stratospheric changes, whereas heterogeneous chemistry dominates lower stratospheric ozone depletion.

Several large volcanic eruptions exacerbated ozone loss during the depletion era (1984–1998). The start of the post-peak ODS period was characterized by very clean (low-aerosol) conditions, but a series of moderate eruptions after 2004 has impeded the rate of ozone recovery, flattening trends over the 1999–2014 period.


This work demonstrates the need to accurately represent heterogeneous chemistry and volcanically enhanced stratospheric aerosol loading in quantifying ozone trends.


Technical description of figure:

Top set of panels: Figure 2 of above reference. Contour plots of annual-average linear trends during the SWOOSH depletion era (1984–1998) for the Chem-Dyn-Vol (the best representation of both background and volcanic aerosols since 1979), Vol-Clean (background aerosols under volcanically clean conditions), and gas-phase only (no heterogeneous chemistry) model runs, as well as the SWOOSH data set. A 3-year running mean is applied to smooth the quasi-biennial oscillation (QBO) and other high-frequency variability. Trends are fitted separately at each latitude-pressure point and then expressed as percent per decade relative to the mean of the ozone mixing ratio over the time period. The white line marks the tropopause location in the model. Bottom set of panels: Figure 3 of above reference. Same as above but for the recovery era (1999–2014).

Scientific significance, societal relevance, and relationships to future missions:

Observations of volcanic aerosol loading and ozone (vertical profiles and total column) are used together with simulations from a state-of-the-art chemistry-climate model to investigate how heterogeneous chemistry has affected ozone trends in both the depletion and the recovery eras. Gas-phase ozone depletion is also evaluated; the contribution of gas-phase chemistry alone has rarely been revisited since the discovery of heterogeneous chemistry in the 1980s. Comparisons of the different model runs show where and by how much volcanoes have exacerbated ozone destruction. These results indicate that heterogeneous chemical reactions on volcanic sulfate aerosol increased the severity of ozone depletion and delayed its recovery, underscoring the necessity of accurately accounting for changes in stratospheric aerosol content in quantifying ozone trends. Careful observations and analysis will be needed to discern and attribute significant trends against the backdrop of natural variability as the ozone layer evolves over the coming decades. In addition to the continuing record from Aura MLS and other current sensors, vertically resolved limb measurements of O3 will be available from the planned Ozone Mapper and Profiler Suite Limb Profiler (OMPS-LP) scheduled to be launched on the Joint Polar Satellite System (JPSS)-2 spacecraft in 2022.

Data sources:

The SWOOSH (Stratospheric Water and OzOne Homogenized) merged data set, to which Aura Microwave Limb Sounder measurements are central, is available from https://www.esrl.noaa.gov/csd/groups/csd8/swoosh/. The complete time history of calculated aerosol properties is available on the Earth System Grid (https://doi.org/10.5065/D6S180JM). Model results presented in this paper are available upon request to the WACCM liaison, Michael Mills (mmills@ucar.edu). MERRA data can be accessed freely online at http://disc.sci.gsfc.nasa.gov/.


References: Wilka, C., K. Shah, K. Stone, S. Solomon, D. Kinnison, M. Mills, A. Schmidt, R.R. Neely III, On the role of heterogeneous chemistry in ozone depletion and recovery, Geophys. Res. Lett., 45, 7835–7842, doi:10.1029/2018GL078596, 2018.


6.2019


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