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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 30 Oct 2018

Research article | 30 Oct 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Characterising the Seasonal and Geographical Variability of Tropospheric Ozone, Stratospheric Influence and Recent Changes

Ryan S. Williams1, Michaela I. Hegglin1, Brian J. Kerridge2, Patrick Jöckel3, Barry G. Latter2, and David A. Plummer4 Ryan S. Williams et al.
  • 1University of Reading, Reading, UK
  • 2Rutherford Appleton Laboratory (RAL), Harwell Oxford, Didcot, UK
  • 3German Aerospace Center (DLR), Institute of Atmospheric Physics, Oberpfaffenhofen-Wessling, Germany
  • 4Canadian Centre for Climate Modelling and Analysis, Environment and Climate Change Canada, Montréal, QC, Canada

Abstract. The stratospheric contribution to tropospheric ozone (O3) has been a subject of much debate in recent decades, but is known to have an important influence. Recent improvements in diagnostic and modelling tools provide new evidence that the stratosphere has a much larger influence than previously thought. This study aims to characterise the seasonal and geographical distribution of tropospheric ozone, its variability and changes, and provide quantification of the stratospheric influence on these measures. To this end, we evaluate hindcast specified dynamics chemistry-climate model (CCM) simulations from the ECHAM/MESSy Atmospheric Chemistry (EMAC) model and the Canadian Middle Atmosphere Model (CMAM), as contributed to the IGAC/SPARC Chemistry Climate Model Initiative (CCMI) activity, together with satellite observations from the Ozone Monitoring Instrument (OMI) and ozonesonde profile measurements from the World Ozone and Ultraviolet Radiation Data Centre (WOUDC) over a period of concurrent data availability (2005–2010). An overall positive, seasonally dependent bias in 1000–450hPa (~0–5.5km) subcolumn ozone is found for EMAC, ranging from 2–8 Dobson Units (DU), whereas CMAM is found to be in closer agreement with the observations, although with substantial seasonal and regional variation in the sign and magnitude of the bias (~−4 to +4DU). Although the application of OMI averaging kernels (AKs) improves agreement with model estimates from both EMAC and CMAM as expected, comparisons with ozonesondes indicate a positive ozone bias in the lower stratosphere in CMAM, together with an underestimation of photochemical ozone production (negative bias) in the troposphere. Model variability is found to be more similar in magnitude to that implied from ozonesondes, in comparison with OMI which has significantly larger variability. Noting the overall consistency of the CCMs, the influence of the model chemistry schemes and internal dynamics is discussed in relation to the inter-model differences found. In particular, it is shown that CMAM simulates a faster and shallower Brewer-Dobson Circulation (BDC) relative to both EMAC and observational estimates, which has implications for the distribution and magnitude of the downward flux of stratospheric ozone, over the most recent climatological period (1980–2010). Nonetheless, it is shown that the stratospheric influence on tropospheric ozone is larger than previously thought and is estimated to exceed 50% in the wintertime extratropics, even in the lower troposphere. Finally, long term changes in the CCM ozone tracers are calculated for different seasons between 1980–89 and 2001–10. An overall statistically significant increase in tropospheric ozone is found across much of the world, but particularly in the Northern Hemisphere and in the middle to upper troposphere, where the increase is on the order of 4–6ppbv (5–10%). Our model study implies that attribution from stratosphere-troposphere exchange (STE) to such ozone changes ranges from 25–30% at the surface to as much as 50–80% in the upper troposphere-lower stratosphere (UTLS) across many regions of the world. These findings highlight the importance of a well-resolved stratosphere in simulations of tropospheric ozone and its implications for the radiative forcing, air quality and oxidation capacity of the troposphere.

Ryan S. Williams et al.
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Status: final response (author comments only)
Status: final response (author comments only)
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Ryan S. Williams et al.
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Short summary
Tropospheric ozone has important implications for air quality and climate change, but is poorly understood at a regional and seasonal level. Analysis of model simulations indicate that downward transport of ozone from the stratosphere has a larger influence than previously thought (as much as ~ 50 % even near the surface). Recent estimated changes in tropospheric ozone (1980–89 to 2001–10) are generally positive, with substantial attribution from the stratosphere identified over some regions.
Tropospheric ozone has important implications for air quality and climate change, but is poorly...