Beijing suffers from very high concentrations of airborne pollutants, leading to adverse health and wellbeing for over twenty million people. The pollutants likely to have the greatest effects upon human health are particulate matter, nitrogen dioxide and ozone. Both particulate matter and nitrogen dioxide are emitted directly from individual sources (primary contributions, many of which are not well quantified); and are formed in the atmosphere (secondary contributions, which are highly complex). Ozone is entirely secondary in nature, formed from reactions of precursor gases, whose sources and abundance are also challenging to constrain. These uncertainties hinder understanding of the causes of air pollution in Beijing, which is needed to deliver effective and efficient strategies for pollution reduction and health improvement. AIRPOLL-Beijing project will address this challenge, through identification and quantification of the sources and emissions of air pollutants in Beijing. The project sits within the NERC/MRC-NSFC China megacity programme, which includes projects addressing the atmospheric processes affecting air pollutants, human exposure and health effects, and solutions / mitigation strategies to reduce air pollution and health impacts. The project exploits the combined experience and expertise of leading UK and Chinese scientists, applying multiple complementary approaches. The project deploys multiple atmospheric measurement and analysis strategies to characterise pollutant abundance and sources, develop novel emissions inventories, and integrate these to produce new modelling tools for use in policy development. We adopt a range of state-of-the-science approaches: -Receptor Modelling, where detailed composition measurements are used to infer pollutant sources from their chemical signatures, combining world-leading UK and Chinese capability. -Flux Measurements, where the total release of pollutants from all sources is measured, providing a key metric to refine emission inventories. We will combine near-ground measurements (using the unique Institute of Atmospheric Physics 325m tower in central Beijing), ground-based observations and fluxes derived from satellite observations. -3D spatial analysis, in which a novel sensor network will be deployed around central Beijing to measure pollutant fields. -Development of novel emissions inventories, which will predict the temporally- and spatially- resolved emissions of air pollutants from all sources, enhancing existing capability. -Development of new online modelling tools, within which to integrate emissions, atmospheric processing and meteorology to predict primary and secondary pollutant concentration fields. AIRPOLL-Beijing will integrate these approaches to provide thorough understanding of the sources and emissions of air pollutants in Beijing, at unprecedented detail and accuracy. While the project is a self-contained activity, key deliverables feed into Processes, Health and Solutions themes of the programme. This proposal seeks Newton fund support, part of the UK's Official Development Assistance (ODA) commitment. The project will directly address ODA objectives, in the categories of (i) people (through the joint development of novel scientific approaches to the understanding of megacity air pollution), (ii) programmes (as all aspects of the project are joint UK-Chinese research endeavours) and (iii) translation (through provision of detailed air pollution source assessments, in support of assessment of health impacts and development of mitigation strategies). More generally, the project will leave a legacy of improved air pollution understanding and research capacity of the Chinese teams, and, through integration with other themes of the Megacities programme, underpin improvements in the health and welfare of the population of Beijing, and across China more widely - ultimately benefitting more than a billion people.