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We examine the effects of ozone precursor emissions from megacities onpresent-day air quality using the global chemistry–climate model UM-UKCA(UK Met Office Unified Model coupled to the UK Chemistry and Aerosols model). Thesensitivity of megacity and regional ozone to local emissions, both fromwithin the megacity and from surrounding regions, is important fordetermining air quality across many scales, which in turn is key for reducinghuman exposure to high levels of pollutants. We use two methods, perturbationand tagging, to quantify the impact of megacity emissions on global ozone. Wealso completely redistribute the anthropogenic emissions from megacities, tocompare changes in local air quality going from centralised, denselypopulated megacities to decentralised, lower density urban areas. Focus isplaced not only on how changes to megacity emissions affect regional andglobal NOx and O3, but also on changes to NOydeposition and to local chemical environments which are perturbed by theemission changes. The perturbation and tagging methods show broadly similar megacity impacts ontotal ozone, with the perturbation method underestimating the contributionpartially because it perturbs the background chemical environment. The totalredistribution of megacity emissions locally shifts the chemical environmenttowards more NOx-limited conditions in the megacities, which is moreconducive to ozone production, and monthly mean surface ozone is found toincrease up to 30% in megacities, depending on latitude and season. However,the displacement of emissions has little effect on the global annual ozoneburden (0.12% change). Globally, megacity emissions are shown to contribute~3% of total NOy deposition.The changes in O3, NOx and NOy deposition describedhere are useful for quantifying megacity impacts and for understanding thesensitivity of megacity regions to local emissions. The small global effectsof the 100% redistribution carried out in this study suggest that thedistribution of emissions on the local scale is unlikely to have largeimplications for chemistry–climate processes on the global scale

Modelling the impact of megacities on local, regional and global tropospheric ozone and the deposition of nitrogen species