New Delhi: Delhi has 39 continuous air quality monitoring stations (CAAQMS), the most for an Indian city. However, their distribution across an area of 1,483 square kilometres is uneven, with very few stations in southwest, northwest and peripheral areas of Delhi. A report released by Centre for Science and Environment (CSE) has found that the monitoring network covers only 26% of the city’s area within a 2-km radius, leaving a 74% data shadow. If we consider a 5-km radius, 25% of the city’s area is beyond the network.Due to the same skewed distribution pattern in 29 other NCR cities, 21 have one station each, despite experts emphasising the need for multi-dimensional monitoring systems that integrate air quality sensors and satellite-based monitoring alongside such stations. Though there is no rule specifying how much the coverage radius of a station should be, official guidelines tie the number to the size of population. Towns with fewer than 1 lakh inhabitants require a minimum of four stations to monitor particulate matter (PM), three to keep a tab on sulphur dioxide (SO2), four to monitor nitrogen dioxide (NO2) and one for carbon monoxide (CO) and oxidants like surface ozone.A city with a population between 1 lakh and 1 million ideally needs 2,417 PM monitors, 1,863 SO2 monitors, 2,417 NO2 monitors, and 525 for CO and surface ozone each, according to CSE, which analysed the guidelines of ‘Indian Standard 5182′, used to measure air pollution. Out of 39 stations in Delhi, most of them are in its south and central parts. A total of six stations have been proposed at JNU and IGNOU in south Delhi, Netaji Subhash University (west campus) in west Delhi, ISRO earth station in Central Ridge, Delhi cantonment and the Commonwealth Games Sports Complex in east Delhi. “Delhi is one of India’s few regions meeting or exceeding its target number of stations, but the widening gap between demand and actual monitoring infrastructure underlines the urgent need to expand or redistribute them more equitably. The city’s outer zones are home to rapidly expanding populations and carry high-exposure risks but remain largely invisible in the monitoring network,” stated a CSE report titled ‘Air quality monitoring: addressing data shadow regions.‘ Among NCR cities, Noida, Gurgaon and Faridabad have the highest number of stations at four each, followed by Meerut at three. Greater Noida, Bhiwadi and Baghpat have two each. Anumita Roychowdhury, executive director (research and advocacy) of Centre for Science and Environment, said the regulatory CAAQM network also needs to be representative of land-use patterns and background air quality to provide reliable data for risk assessment, policy making and compliance reporting. “But a high-density network can be very expensive. So, it’s necessary to support regulatory monitoring with a multilayered system, including satellite-based pollution mapping and sensor-based local assessment of exposure,” she said. Sunil Dahiya, founder and lead analyst of the think tank EnviroCatalysts, said the uneven distribution of the stations leaves most cities in NCR as blind spots. “Having only one station also makes it susceptible to practices like sprinkling of water to alter air quality reading. We need a robust network of regulator-grade stations coupled with low-cost sensors to give granular data for cities and rural areas,” he added.
