Coupling Regional and Urban Processes: Effects on Air Quality

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Air pollution is the environmental factor with the greatest impact on human health in Europe. Despite substantial emission controls, the complexities of the processes linking emissions and air quality, means that substantial proportions - 80% and 97%, respectively, of the population in Europe lives in cities with levels of particulate matter (PM) and ozone (O3) exceeding EU limit and target values. The two pollutants are estimated to contribute 350,000 and 200,000 premature deaths across Europe. NERC's strategy document states: "In the UK, air pollution costs the economy £15 billion every year in damage to human health, not including the cost of damage to our environment and crops." Understanding the key processes driving air quality across the relevant spatial scales, especially during pollution exceedances and episodes, is essential to provide effective prediction for both policymakers and the public. It is particularly important for policy regulators to understand the drivers of local air quality that can be regulated by national policies versus the contribution from regional pollution transported from mainland Europe or elsewhere. Urban areas are of particular concern since as well as being receptors of regional pollution, they have high local emissions from heating and road transport associated with their high population densities. They are also subject to an urban heat island effect which can impact on the chemistry of air pollution. Our overall aim is to use state-of-the-art modelling and measurements to quantify and reduce uncertainties in the key regional and local processes that control poor air quality in urban areas, both for present-day and in the future.This proposal will develop a novel model framework using a nested suite of models to bridge scales from regional to urban for simulating atmospheric composition and weather including urban heat island effects across the UK and over London. The proposal will further exploit state-of-the-art NERC measurements from recent ClearfLo and REPARTEE field campaigns in London bringing together modelling and measurements experts to determine controlling factors of high O3 and PM events. A detailed box model of the chemical environment based on these field measurements will be constructed, and used to calculate in situ chemical production of O3 during both average and episodic conditions. The coupled regional to urban model will be evaluated against these box model and field campaign results as well as extensive network measurements. Multiple approaches will be used to probe the regional and local contributions to O3 during high O3 events. The key processes driving PM episodes will also be determined using speciated field measurements and coupled model results. The role of nitrous acid on O3 and PM oxidation chemistry in urban areas is a key uncertainty that will be quantified. Air pollution events in the UK are usually associated with stagnation events, which in summer may be coincident with heatwaves. During heatwaves weather conditions may alter emission and deposition processes. The relative importance of these processes, such as reduced O3 deposition, that lead to elevated pollution levels will be established. To investigate the impact of future emissions and climate change on urban air quality, high-resolution climate-chemistry simulations that consistently account for changes in chemistry and transport from the regional to city scale will be performed and future impacts on air quality extremes evaluated. Proof of concept studies with the coupled model framework and with high-resolution climate projections demonstrate the viability of the intended research. This proposal comprises a strong collaboration between modelling and measurement scientists spanning the disciplines of fundamental chemistry, atmospheric composition, and climate change, to advance our understanding of the processes driving regional to urban-scale air quality now and in the future.