Identifying the main source of air pollution in Indian cities is crucial to reducing the many deaths caused by fine particulate matter (PM₂.₅) – deaths that during the harvest season can account for up to half of all air pollution-related fatalities. An international research team lead by the Paul Scherrer Institute (PSI), funded by the Swiss Agency for Development and Cooperation (SDC) has investigated in detail the sources of the organic components of fine particulate matter in the northern Indian cities of Delhi and Kanpur, located in the Indo-Gangetic Plain. Using novel high-resolution molecular measurement techniques and advanced data analysis, the researchers were able to precisely identify and quantify the sources of organic fine particulate matter.
A large share of this pollution is linked to biomass burning: it accounts for about two-thirds of organic aerosol in Kanpur and around one-third in Delhi. These include primarily local fuels such as wood, straw, and dried cow dung, which are commonly used for heating and cooking in many rural households. The research team identified a particularly relevant source - organic aerosol formed by the open burning of agricultural residues, especially during the rice harvest in autumn. These emissions originate predominantly from the northwestern state of Punjab and are transported over long distances into urban areas. During the harvest season, they account for 22% of the organic particulate matter in Delhi and as much as 43% in Kanpur. The health impacts are severe: Rural smoke during this period can contribute to up to 50% of acute PM₂.₅-related deaths in Kanpur and around 32% in Delhi during this period.
The study highlights the extent of regionally transported emissions - an often underestimated factor in many air quality models. It provides not only new scientific insights but also a solid foundation for policy measures that must extend beyond the boundaries of individual cities. In addition to controlling urban emissions, targeted regional strategies are necessary to effectively reduce the widespread practice of burning crop residues—particularly in Punjab.
The analytical framework developed in this study for quantitatively determining fine particulate matter sources can be applied to other regions facing similar air quality challenges—such as Southeast Asia or Africa—and be used there to improve air quality in a targeted matter.
The PSI scientists’ study thus confirms that agricultural fires, especially during the autumn rice harvest, are a major source of fine particulate pollution in Delhi and Kanpur. These emissions pose significant health risks and contribute substantially to PM₂.₅-related deaths. The findings emphasize the urgent need for targeted regional actions beyond individual cities to sustainably improve air quality.
- Read the publication here: https://www.sciencedirect.com/science/article/pii/S0160412025003344
- Link to Guardian Article about this study: https://www.theguardian.com/environment/2025/jun/27/pollutionwatch-fires-delhi-post-monsoon-smog-air