Abstract
AbstractWe study the evolution of tropospheric ozone over the period 1979–2010 using a chemistry‐climate model employing a stratosphere‐troposphere chemistry scheme. By running with specified dynamics, the key feedback of composition on meteorology is suppressed, isolating the chemical response. By using historical forcings and emissions, interactions between processes are realistically represented. We use the model to assess how the ozone responds over time and to investigate model responses and trends. We find that the chlorofluorocarbon (CFC)‐driven decrease in stratospheric ozone plays a significant role in the tropospheric ozone burden. Over the period 1979–1994, the decline in transport of ozone from the stratosphere, partially offsets an emissions‐driven increase in tropospheric ozone production. From 1994–2010, despite a leveling off in emissions, increased stratosphere‐to‐troposphere transport of ozone drives a small increase in the tropospheric ozone burden. These results have implications for the impact of future stratospheric ozone recovery on air quality and radiative forcing.
Highlights
The changes in tropospheric ozone since the pre-industrial era are estimated to have resulted in an increase in radiative forcing of 0.4 W m-2 (Stevenson et al, 2013, Myhre et al, 2013), making tropospheric ozone the third most important anthropogenic greenhouse gas
We show that stratospheric ozone depletion over the period 1979-2010 has a critical effect on tropospheric composition – with less ozone in the lower stratosphere, there is less transport to the troposphere, and this offsets an emissions-driven increase in ozone production in the troposphere
The mean ozone value is significantly closer to the OMI/MLS mean than the configuration of the models employing UKCA used in the CCMI refC1 integrations, analysed in (Revell et al, 2018), which, in contrast to the scheme used here, used a reduced complexity tropospheric chemistry scheme that does not treat NMVOC
Summary
The changes in tropospheric ozone since the pre-industrial era are estimated to have resulted in an increase in radiative forcing of 0.4 W m-2 (Stevenson et al, 2013, Myhre et al, 2013), making tropospheric ozone the third most important anthropogenic greenhouse gas. Unlike the major greenhouse gases, carbon dioxide (CO2) and methane (CH4), ozone is not emitted directly, but is the result of the oxidation of VOCs in the presence of NOx (Monks et al, 2015). Ozone is important since it indirectly affects the lifetime of other greenhouse gases, methane, through its role in the formation of the hydroxyl radical (OH) (Voulgarakis et al, 2013). OH and ozone have an impact on aerosol radiative forcing, a major source of uncertainty in the climate system, as secondary aerosols such as sulfate, nitrate and secondary organic aerosol are mediated by tropospheric oxidants and play a major role in the aerosol budget and burden (Karset et al, 2018). Ozone is linked throughout the Earth system, as changes in ozone can have knock on impacts on emissions of ozone precursors through feedbacks induced by changes in temperature and the hydrological cycle (driven by the changes in aerosols and clouds and radiative forcing), which themselves will modify ozone
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