Suppression of air pollution via aerosol mediated removal of peroxy radicals

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Air quality is a pressing societal concern with around 7 million people dying per year from the combined effect of outdoor and indoor air pollution. Two of the key pollutants are gaseous ozone (which can also influence crop yields and damage ecosystems) and particular matter (PM), both of which also influence the climate system through the absorption and scattering of solar radiation. Policies to reduce the concentrations of these compounds are often implemented independently of each other and focus on reducing the emissions of different compounds with different sources. Ozone is a secondary pollutant, and so is not directly emitted into the atmosphere but produced by chemical chain reactions where volatile organic compounds (VOCs) are oxidized in the presence of oxides of nitrogen (NOx).For ozone, the scientific basis for emission reduction policy depends on the photochemical regime of a given region. Two well established regimes for ozone production are the so-called "VOC limited" regime where reductions in the VOC emissions are required in order to reduce ozone, and the "NOx limited" regime where reduction in NOx emissions is required. Which regime operates depends on which chemical reactions limit ozone production. This separation between NOx or VOC limited policies has underpinned ozone control strategies over the last three decades where policymakers have either reduced NOx or VOC emissions to reduce ozone. Controls of PM pollution, on the other hand, have focussed on the reduction of a different set of compounds (sulfur, black carbon, dust etc.). Thus, for decades ozone and PM pollution have been considered as effectively separate problems with unrelated policies for controlling PM and ozone.This proposal builds on the discovery by one of the investigators of a new third "aerosol inhibited" regime where the dominant process limiting ozone production is the reactive removal of peroxy radicals onto aerosol surfaces. In these locations there is a strong interaction between policies to reduce particulate matter and ozone. Efforts to reduce aerosol pollution would lead to the unintended consequence of increasing ozone concentration.Given the uncertainties on the rate of uptake of HO2 onto aerosols, a better understanding of this regime is necessary if policy is to be updated. This proposal will address a number of uncertainties that remain in these calculations. We will develop a new instrument for the direct field measurement of the rate of removal of peroxy radicals onto aerosols, and will measure for the first time in the laboratory how quickly organic peroxy radicals react on the surface of aerosols. The new field capability will be deployed in the UK and the measurements (together with those from a Japanese collaborator) and the results from the laboratory will be used to formulate new parameterizations which will allow heterogeneous removal to be more accurately represented in atmospheric models. We will use simple zero-dimensional box models and more complex three dimensional chemical transport models to quantify the impact on surface and global ozone concentrations, and atmospheric oxidation rates in the past, present and future. In this way we will explore the importance of the new ozone regime for air quality and climate.This proposal brings together leading complementary expertise from groups in Leeds, York and Manchester who have considerable experience in field measurements, laboratory measurements of gas-phase and heterogeneous aerosol processes, and numerical modelling on a range of spatial scales.