Crop-residue burning, a major contributor to black carbon pollution: Study

A new study finds that crop-residue burning contributes substantially to black carbon levels in North India, especially during the post-monsoon season.

Emissions deteriorate air quality across the Indo-Gangetic Plain and in downwind metropolitan cities such as Delhi.

The study provides quantitative evidence for targeted crop-residue burning policy interventions in the region. See All Key Ideas

Stubble burning, or crop-residue burning, has been known to contribute to the seasonal smog across northern India, especially in the Indo-Gangetic Plain region. Despite directions issued by the National Green Tribunal in 2015 to prevent agricultural residue burning, biomass burning remains a seasonal activity in states such as Punjab, Haryana, Uttar Pradesh, Delhi, and Rajasthan, contributing to the region’s air pollution.

A new study by researchers from IIT, New Delhi, has quantified the impact of crop-residue burning on particulate air pollution, particularly black carbon, commonly known as soot, that is produced when carbon-containing fuels are not burned completely.

The study found that emissions from crop-residue burning in rural Punjab deteriorate air quality across the Indo-Gangetic Plain (IGP) and in downwind metropolitan cities such as Delhi, due to the transport of pollutants by prevailing winds.

Results from real-time measurements at an agricultural-field site in Kheral village in rural Punjab showed that crop-residue burning contributed, on average, 48% to the total black carbon during the post-monsoon period, and periodically exceeded 90% during active burning hours.

The study was conducted during October and November, the peak season for crop-residue burning in North India after rice harvest.

A dominant source of black carbon

“For the study, we mainly focused on black carbon levels and their dependence on stubble burning,” said Ajit Kumar, a Ph.D. researcher at IIT Delhi and one of the paper’s authors.

Black carbon is a short-lived, carbon-rich component of PM2.5 (particulate matter smaller than 2.5 micrometres), Kumar said. A mobile monitoring lab was deployed to measure detailed PM2.5 composition data from the site equipped with an Aethalometer, a device to measure black carbon, as well as other devices to measure elemental composition, wind speed and direction.

The average black carbon concentration at the study site was 12.6 micrograms per cubic meter (µg/m3), slightly lower than what has been previously reported from Delhi (17.23 µg/m3) post monsoon in 2022 and Agra (approximately 13 µg/m3 in 2014).

The scientists used two models, Aethalometer and Positive Matrix Factorisation, to measure black carbon.

The Aethalometer model showed that the black carbon at the site came from two nearly equal sources. Biomass burning and fossil fuel burning contributed 48.5% and 51.5% of black carbon respectively. The Positive Matrix Factorisation (PMF) model, meanwhile, provided a more detailed breakdown. It identified biomass burning (48%) as the largest contributor, followed by power plant emissions (24%) and traffic emissions (20.2%). Smaller contributions came from a Pb-rich industrial source (6.1%) and waste incineration (2%).

Interestingly, both PMF and the Aethalometer models showed “good agreement” in estimating the contribution of stubble burning, Kumar said. “The study provides strong evidence that stubble burning can temporarily become a dominant source of local black carbon pollution,” said Pratima Gupta, a postdoctoral researcher at the University of Lille who was not involved with the study.

But Gupta highlighted that even in an agricultural setting, the study shows how black carbon was apportioned to multiple other sources alongside stubble burning.

There is no single percentage that can represent the contribution of stubble burning or any other source to black carbon across India, Gupta added, citing two other studies. A 2024 study analysed three years (2020-2022) of black carbon observations in Delhi and estimated stubble burning contributed between 15% in May and 37% in November. Meanwhile, a 2025 study looked at the broader IGP region and identified household biofuel combustion as the dominant contributor at 58%, while stubble burning contributed just 3.5% during the post-monsoon period.

“While stubble burning can produce extremely intense, short-term and seasonal black carbon episodes, household energy, transport, industry and other combustion sources can contribute more continuously,” Gupta said.

The researchers were also able to confirm the transport of stubble-burning emissions from the sampling site in Punjab to all across the IGP via a HYSPLIT model developed by the National Oceanic and Atmospheric Administration (NOAA), highlighting that pollutants travel from the source region and affect nearby and downwind areas.

More than just a pollutant

“What makes black carbon particularly important is that it sits at the intersection of air pollution, public health and climate change,” Gupta said, adding: “Aerosol components, such as sulphates, predominantly scatter incoming sunlight and can have a cooling influence on the atmosphere. Black carbon behaves differently because it strongly absorbs solar radiation and converts it into heat, warming the surrounding atmosphere.”

Black carbon deposited on snow and ice can accelerate melting by reducing their reflectivity and increasing the absorption of solar energy, according to her. It is also a fine particulate pollutant that can penetrate deep into the respiratory system and damage health. A 2023 study showed that high exposure has been associated with cardiovascular and respiratory health effects, as well as premature mortality.

“Unlike major pollutants such as PM2.5, PM10, NO2 and ozone, there is currently no specific ambient air-quality standard for black carbon,” Gupta said. There is also a lack of long-term studies on black carbon and its impacts on public health, Gupta added.

To address this gap, Gupta and team developed high-resolution estimates of black carbon exposure across India at approximately 1 km x 1 km resolution. They then combined these estimates with demographic and health information to examine how black-carbon-associated mortality varied geographically and across different sections of the population.

The most important finding was the strong geographical variation in black carbon exposure. Across India, population-weighted black carbon exposure ranged from 0.4 to 13.7 µg/m³. High exposure (above 8 µg/m³) was observed in three regions: the state of Assam, the industrial region around Mumbai, and the Indo-Gangetic Plain.

This was somewhat mirrored in the mortality burden data, with Uttar Pradesh having an estimated annual black carbon-attributable mortality of 293 deaths per 100,000 population, followed by Delhi at 235 and Bihar at 230.

“The patterns of mortality, however, are more complex,” Gupta said. “Mortality burden is determined not only by pollution exposure but also by underlying characteristics such as age structure, baseline disease rates, socioeconomic conditions and other vulnerabilities.”

The study observed a clear decreasing gradient from north to south, said Gupta, as states with lower socio-demographic indices accounted for nearly 54% of black carbon-attributable mortality. Females, other backward classes and economically deprived populations had higher mortality when compared to their respective demographic counterparts, she added.

“The central message of our study is that black carbon is not only an air-pollution and climate issue,” Gupta said. “It is also an environmental-health-equity issue.”

Mitigation can deliver on multiple fronts

The black carbon study from rural Punjab provides policymakers with the evidence needed to design targeted interventions such as concentrated enforcement during the afternoon burning window across the October-November paddy harvest period, said Shefali Khanna, a research fellow in energy economics and policy at the London School of Economics and Political Science.

Khanna, who has a working paper on the effectiveness of the ban on crop residue burning, said that she believed their paper complemented the findings on black carbon by highlighting the importance of ban enforcement.

The working paper found that the National Green Tribunal ban reduced crop-residue burning fires by around 30% relative to pre-ban levels, but only where enforcement was credible. “Farmers initially reduced burning out of uncertainty about how strictly the ban would be enforced,” Khanna said. “But once it became clear that enforcement would remain weak and fines negligible, burning returned to pre-ban levels within two to three years.”

According to their data, fines collected under the ban were extremely low, averaging less than ₹38 per estimated burnt landholding, even in Haryana, the best-performing state. “At most, 4% of burning landholdings were ever fined (in Haryana),” Khanna said.

Evidence also suggests a carrot-rather-than-stick approach, Khanna said. “In the absence of credible enforcement, policies that directly reduce the cost of proper residue disposal are likely to be more effective.”

According to Gupta, black carbon mitigation represents an important opportunity for India. “The short atmospheric lifetime [of black carbon] means reducing emissions can provide relatively rapid improvements in air quality and public health, while also contributing to near-term climate mitigation,” she said. But it must complement, and not replace, the mitigation of CO2 and other long-lived greenhouse gases, Gupta stressed.

Banner image: A farmer sets fire to crop residue on the out skirts of Jalandhar, Punjab. (AP Photo)