What Is Polluting Delhi's Air? A Review from 1990 to 2022

Delhi's annual average PM2.5 concentration in 2021-22 was 100 mu g/m(3)-20 times more than the WHO guideline of 5 mu g/m(3). This is an improvement compared to the limited information available for the pre-CNG-conversion era (similar to 30%), immediately before and after 2010 CWG (similar to 28%), and the mid-2010s (similar to 20%). These changes are a result of continuous technical and economic interventions interlaced with judicial engagement in various sectors. Still, Delhi is ranked the most polluted capital city in the world. Delhi's air quality is a major social and political concern in India, often with questions regarding its severity and primary sources, and despite several studies on the topic, there is limited consensus on source contributions. This paper offers insight by reviewing the influence of Delhi's urban growth since 1990 on pollution levels and sources and the evolution of technical, institutional, and legal measures to control emissions in the National Capital Region of Delhi.