Abstract
Abstract. We examine the effects of ozone precursor emissions from megacities on present-day air quality using the global chemistry–climate model UM-UKCA (UK Met Office Unified Model coupled to the UK Chemistry and Aerosols model). The sensitivity of megacity and regional ozone to local emissions, both from within the megacity and from surrounding regions, is important for determining air quality across many scales, which in turn is key for reducing human exposure to high levels of pollutants. We use two methods, perturbation and tagging, to quantify the impact of megacity emissions on global ozone. We also completely redistribute the anthropogenic emissions from megacities, to compare changes in local air quality going from centralised, densely populated megacities to decentralised, lower density urban areas. Focus is placed not only on how changes to megacity emissions affect regional and global NOx and O3, but also on changes to NOy deposition and to local chemical environments which are perturbed by the emission changes. The perturbation and tagging methods show broadly similar megacity impacts on total ozone, with the perturbation method underestimating the contribution partially because it perturbs the background chemical environment. The total redistribution of megacity emissions locally shifts the chemical environment towards more NOx-limited conditions in the megacities, which is more conducive to ozone production, and monthly mean surface ozone is found to increase up to 30% in megacities, depending on latitude and season. However, the displacement of emissions has little effect on the global annual ozone burden (0.12% change). Globally, megacity emissions are shown to contribute ~3% of total NOy deposition. The changes in O3, NOx and NOy deposition described here are useful for quantifying megacity impacts and for understanding the sensitivity of megacity regions to local emissions. The small global effects of the 100% redistribution carried out in this study suggest that the distribution of emissions on the local scale is unlikely to have large implications for chemistry–climate processes on the global scale.
Highlights
Over the past few decades the rise in the world’s urban population has led to an increase in the number of megacities, generally defined as cities with a population of greater than 10 million
The impact of megacity emissions has been investigated by both perturbation and tagging methods using the Unified Model (UM)-UK Chemistry and Aerosols (UKCA) global chemistry–climate model
In a comparison of the methods we show results to be broadly similar, with megacities in both cases contributing
Summary
Over the past few decades the rise in the world’s urban population has led to an increase in the number of megacities, generally defined as cities with a population of greater than 10 million. Changes to urban emissions have the potential to influence air quality on much larger scales, as investigated in the recent European collaborative project MEGAPOLI (Megacities: Emissions, urban, regional and Global Atmospheric POLlution and climate effects, and Integrated tools for assessment and mitigation) (Baklanov et al, 2010), and in a number of recent modelling studies (Mayer et al, 2000; Lawrence et al, 2007; Butler and Lawrence, 2009; Fiore et al, 2009; Butler et al, 2012) Many of these studies use pure modelling approaches to assess the impact of emission changes both in perturbation and future scenarios.
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