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Monitoring ambient air quality in Indian city airsheds

An understanding of sources contributing to a city’s air pollution problem is crucial for building an effective clean air action plan. All the non-attainment cities under the National Clean Air Programme (NCAP) are planning or conducting a source apportionment study. Irrespective of the approaches and equipment selected to conduct these studies, the initiation process requires an understanding of the pollution loads, mix of sources, and geography of the city to decide how much to monitor for better spatial representation and how many times to monitor for better temporal representation. In this report, we defined (a) The size of NCAP city airsheds (b) The recommended number of ambient air quality monitoring sites in an airshed (c) The operational sampling frequency to support receptor-based source apportionment studies. These resources are necessary for strengthening the monitoring needs of an airshed to track pollution levels, to conduct receptor-model-based source apportionment studies, and to support long-term air quality management plans. 
 
Together, 122 NCAP cities were clubbed into 94 airsheds representing a total of 154 cities and a total population of 290.6 million - 66 airsheds contain only one city; 16 airsheds contain two cities, nine airsheds contain three cities and four airsheds (Chandigarh, Delhi, Mumbai, and Kolkata) contain 11, 7, 7, and 4 cities respectively. For the 94 airsheds, a total of 1700 sampling sites are recommended to measure and analyse particulate matter, at an average of 18 per airshed. The Tier-1 cities on the NCAP list - Ahmedabad, Bengaluru, Delhi, Hyderabad, Kolkata, Mumbai, and Pune require at least 33, 41, 78, 36, 70, 68, and 32 sampling sites, respectively, which is more than the current operational monitoring capacity in the cities.