What is polluting Delhi’s air? A quantitative review from 1990 to 2022

“How bad is Delhi’s air quality?” and “What are the main sources of Delhi’s air pollution problem?” are perennial questions in India, despite Delhi being the most studied city. Delhi’s air pollution peaks during the winter months starting with Diwali and post-harvest agricultural waste burning in late October and early November and deteriorates further with lower surface temperatures resulting in an increase in demand for space heating. The pollution levels are the lowest during the monsoon months of July and August, but not negligible. This cyclical nature also overlaps with the overall interest in the topic of air pollution and efforts to address the issue, peaking at the start of the winter pollution episodes, with the most the media coverage (based on the number of articles published), public interest (based on social media activity and google search trends), and political will (based on the number of political statements made). This review of Delhi’s air quality from 1990 to 2022 from data, sectoral, judicial, and institutional perspectives is published as an open access data resource and covers databases and story lines on (a) geography and meteorology (b) changes in ambient air quality (as PM2.5 concentrations) using information from ground measurements, reanalysis fields, and satellite retrievals (c) source apportionment studies (d) sectoral history of road transport, agricultural waste burning, residential (cooking and heating) emissions, open waste burning, construction sector (including brick kilns), road dust, power generation and demand, and diwali fireworks and (e) judicial and institutional engagement. All the data resources collated for this review are accessible @ https://www.urbanemissions.info