Abstract
Abstract. The evolution of tropospheric ozone from 1850 to 2100 has been studied using data from Phase 6 of the Coupled Model Intercomparison Project (CMIP6). We evaluate long-term changes using coupled atmosphere–ocean chemistry–climate models, focusing on the CMIP Historical and ScenarioMIP ssp370 experiments, for which detailed tropospheric-ozone diagnostics were archived. The model ensemble has been evaluated against a suite of surface, sonde and satellite observations of the past several decades and found to reproduce well the salient spatial, seasonal and decadal variability and trends. The multi-model mean tropospheric-ozone burden increases from 247 ± 36 Tg in 1850 to a mean value of 356 ± 31 Tg for the period 2005–2014, an increase of 44 %. Modelled present-day values agree well with previous determinations (ACCENT: 336 ± 27 Tg; Atmospheric Chemistry and Climate Model Intercomparison Project, ACCMIP: 337 ± 23 Tg; Tropospheric Ozone Assessment Report, TOAR: 340 ± 34 Tg). In the ssp370 experiments, the ozone burden increases to 416 ± 35 Tg by 2100. The ozone budget has been examined over the same period using lumped ozone production (PO3) and loss (LO3) diagnostics. Both ozone production and chemical loss terms increase steadily over the period 1850 to 2100, with net chemical production (PO3-LO3) reaching a maximum around the year 2000. The residual term, which contains contributions from stratosphere–troposphere transport reaches a minimum around the same time before recovering in the 21st century, while dry deposition increases steadily over the period 1850–2100. Differences between the model residual terms are explained in terms of variation in tropopause height and stratospheric ozone burden.
Highlights
Tropospheric ozone (O3) is an important component of air pollution and an oxidizing species with adverse effects on human health (Jerrett et al, 2009; Turner et al, 2015; Malley et al, 2017) and vegetation (Fowler et al, 2009)
Young et al (2018) presented an updated regional evaluation of tropospheric ozone simulated by models contributing to the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) using data from ozonesonde measurements, a new compilation of long-term measurements conducted aboard commercial aircraft of internationally operating airlines (MOZAIC-IAGOS) and a comprehensive database of global surface ozone measurements that was compiled within the Tropospheric Ozone Assessment Report (TOAR) framework
CMIP6 shows nominal improvements in certain regions such as the southern tropics with respect to biases and correlations reported by the earlier ACCMIP (Young et al, 2013) and Atmospheric Composition Change: the European Network of excellence (ACCENT) (Stevenson et al, 2006) studies, it is difficult to evaluate given the smaller number of models in the CMIP6 (5) versus ACCMIP (15) and ACCENT (26) studies and given different periods of evaluation
Summary
Tropospheric ozone (O3) is an important component of air pollution and an oxidizing species with adverse effects on human health (Jerrett et al, 2009; Turner et al, 2015; Malley et al, 2017) and vegetation (Fowler et al, 2009). Young et al (2018) presented an updated regional evaluation of tropospheric ozone simulated by models contributing to the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) using data from ozonesonde measurements, a new compilation of long-term measurements conducted aboard commercial aircraft of internationally operating airlines (MOZAIC-IAGOS) and a comprehensive database of global surface ozone measurements that was compiled within the Tropospheric Ozone Assessment Report (TOAR) framework. A study using new simulations from multiple CCMs finds considerable disagreement among models regarding past and future responses to drivers of tropospheric ozone even when the same scenario is considered, with much of the model spread likely due to the uncertainty in impacts on ozone in the tropopause region driving inter-model variations in STE trends (Morgenstern et al, 2018) In addition to these stratospheric influences, further uncertainty arises from inter-model differences in tropospheric chemistry and physics (such as photolysis, convection and the boundarylayer scheme). Other papers published or under discussion at the time of writing feature an analysis of chemistry and feedbacks (Thornhill et al, 2020), stratospheric ozone (Keeble et al, 2020), ozone radiative forcing (Skeie et al, 2020; Morgenstern et al, 2020), air pollution and particulate matter (Turnock et al, 2020; Allen et al, 2020), and oxidizing capacity (Stevenson et al, 2020)
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