The 2018 UNEP/WMO Scientific Assessment of Ozone Depletion

. The influence of stratospheric ozone on the interannual variability and trends in tropospheric ozone is evaluated between 30 and 90° N from 1990–2009 using ozone measurements and a global chemical transport model, the Community Atmospheric Model with chemistry (CAM-chem). Long-term measurements from ozonesondes, at 150 and 500 hPa, and the Measurements of OZone and water vapour by in-service Airbus aircraft programme (MOZAIC), at 500 hPa, are analyzed over Japan, Canada, the Eastern US and Northern and Central Europe. The measurements generally emphasize northern latitudes, although the simulation suggests that measurements over the Canadian, Northern and Central European regions are representative of the large-scale interannual ozone variability from 30 to 90° N at 500 hPa. CAM-chem is run with input meteorology from the National Center for Environmental Prediction; a tagging methodology is used to identify the stratospheric contribution to tropospheric ozone concentrations. A variant of the synthetic ozone tracer (synoz) is used to represent stratospheric ozone. Both the model and measurements indicate that on large spatial scales stratospheric interannual ozone variability drives significant tropospheric variability at 500 hPa and the surface. In particular, the simulation and the measurements suggest large stratospheric influence at the surface sites of Mace Head (Ireland) and Jungfraujoch (Switzerland) as well as many 500 hPa measurement locations. Both the measurements and simulation suggest the stratosphere has contributed to tropospheric ozone trends. In many locations between 30–90° N 500 hPa ozone significantly increased from 1990–2000, but has leveled off since (from 2000–2009). The simulated global ozone budget suggests global stratosphere-troposphere exchange increased in 1998–1999 in association with a global ozone anomaly. Discrepancies between the simulated and measured ozone budget include a large underestimation of measured ozone variability and discrepancies in long-term stratospheric ozone trends. This suggests the need for more sophisticated simulations including better representations of stratospheric chemistry and circulation.

Stratospheric Impact on Tropospheric Ozone Variability and Trends: 1990-2009

Ozone profile measurements collected at L'Aquila (Italy, 42.4°N) during seventeen years of radio-sounding 30 (2000-2016) are presented here, with an analysis of derived trends. Model results from the SPARC-CCMI exercise are used in parallel to highlight the physical and chemical mechanisms regulating mid-latitude ozone trends. The statistically significant trends highlighted in time series at L'Aquila are those in the mid-upper stratosphere (+5.9±4.2), mid troposphere https://doi.org/10.5194/acp-2020-525 Preprint. Discussion started: 1 July 2020 c Author(s) 2020. CC BY 4.0 License. 2 (+5.9±2.4) and upper troposphere (+2.5±0.9), all in percent/decade. The upper stratospheric positive trend was already well documented in recent WMO assessments and attributed to the starting decline of stratospheric Cly and Bry and to the 35 stratospheric cooling induced by increasing well mixed greenhouse gases, thus slowing down gas-phase reactions that destroy ozone in the upper stratosphere. The ozone increase in the mid-upper troposphere is largely regulated by the increasing strength of the Brewer-Dobson circulation, which moves more ozone from the tropics to the extratropics and enhances the tropospheric influx from the lowermost stratosphere. This climate feedback mechanism on tropospheric ozone is only partially compensated by the increasing chemical ozone loss associated to higher H2O values in response to the 40 tropospheric warming. We also note that ozone trends obtained in the lower stratosphere are negative (-2.2 percent/decade), but do not result to be statistically significant in our analyses.

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Seventeen years of ozone sounding at L'Aquila, Italy: evidence of mid-latitude stratospheric ozone recovery and tropospheric profile changes

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Seventeen years of ozone sounding at L'Aquila, Italy: evidence of mid-latitude stratospheric ozone recovery and tropospheric profile changes