An Integrated Study of Air Pollutant Sources in the Delhi National Capital Region (NCR) Delhi was rated the most polluted city in the world for ambient air pollution by the WHO in 2014.  Some 46 million people live in and around Delhi - 18m in the city, and the remainder in the surrounding National Capital Region (NCR).  Annual mean levels of particulate matter (PM), the most important air pollutant for health in Delhi, exceeded WHO guidelines by a factor of 15 during 2014, and are responsible for very substantial chronic and acute health impacts, with attendant economic costs.  Development of effective strategies for improvement in air quality requires  quantitative understanding of the sources, formation processes, regional budgets and chemical and physical nature of airborne particulate matter; however, such knowledge is not yet available.   ASAP-Delhi addresses this requirement:  The project will provide a quantitative assessment of the sources, characteristics, abundance and formation processes for PM in Delhi and the surrounding NCR.  The project is designed to address key science questions (below), with a philosophy of obtaining insights into pollutant sources and budgets from high quality direct observation - i.e. based upon measurement of the species actually present in the air at ground level within Delhi, without dependence upon other data.  The project represents a focussed collaboration between leading researchers from the UK (Birmingham, Surrey) and India (IITD, NPL), who have a track record of successful joint collaboration and publications from previous funded air quality projects in Delhi.  ASAP-Delhi will (i) perform the most detailed physical and chemical characterisation of PM present in Delhi, and in the surrounding National Capital Region (NCR) to date; (ii) produce source profiles (chemical signatures) for the principal PM sources in Delhi; (iii) identify and quantify the contributions of different sources to the PM burden in Delhi (and the NCR) - for both PM mass concentration and particle number [with differing implications for health], by application of multiple, independent, established and novel receptor modelling approaches, such as chemical mass balance, radiocarbon, and number size distribution based source apportionment; (iv) estimate the impact of NCR emissions upon air quality in Delhi (key to effective regional air quality policy)  and (v) evaluate a series of new hypotheses for the formation of PM during pollution episodes, essential for accurate model predictions of future PM levels.  We will address these objectives through a series of field observations, combining online measurements with state-of-the-science offline analyses, in Delhi (urban background locations - year round sampling combined with two "intensives"), the surrounding NCR (seasonal measurements at five sites) and a rural background location (150 km upwind).  Our measurement strategy combines (i) established methodologies - to deliver a high quality, systematic, coherent measurement dataset, with a novel spatial distribution - and (ii) new state-of-the-science tools and analytical approaches.  This provides a balance between international quality observations (in support of ASAP-Delhi and other programme components) and novel metrics, whilst also recognising the realities of fieldwork in such environments.  ASAP-Delhi will draw upon results of other projects in the APHH India programme, and will provide the key observational insights in support of air quality model development, evaluation of human health impacts, and development of mitigation policies in the wider programme.     In addressing public health, policy and development issues, and providing a platform to enhance air pollution research in India, ASAP-Delhi addresses the ODA support categories of (i) people and (ii) research programmes, in addition to providing a step-change in understanding of the sources of PM in Delhi (the key insight required by policymakers). The Newton Fund builds research and innovation partnerships with developing countries across the world to promote the economic development and social welfare of the partner countries.