Air quality impacts of crop residue burning in India and mitigation alternatives

Long-term exposure to fine particulate matter (PM2.5) poses significant health risks globally, resulting in millions of premature deaths annually. India, the world's second-largest crop producer, experiences a substantial portion of this burden, with crop residue burning being a major contributor to PM2.5 emissions. Approximately 100 million metric tons of the 500 million tons of crop residue generated annually in India are burned, primarily in northwestern India after wheat and rice harvests. This practice leads to severely elevated PM2.5 levels in densely populated areas downwind, such as New Delhi, exceeding World Health Organization guidelines by a significant margin. Exposure to this PM2.5 is directly linked to increased health risks, particularly acute respiratory infections. Despite government regulations including bans and fines, crop residue burning persists. This study aims to quantitatively assess the impact of crop residue burning on air quality and human health across India and evaluate the potential effectiveness of targeted mitigation strategies at the district level. The research questions are: 1. What is the magnitude of the health impacts resulting from crop residue burning in India? 2. How significant are the contributions to air quality impacts from specific districts and states? 3. What is the potential for mitigation through targeted interventions, such as adjusting burning times?

Existing literature extensively documents the adverse effects of PM2.5 on human health, especially in India. Studies have highlighted the significant contribution of crop residue burning to regional air pollution, impacting not only India but also neighboring countries like Pakistan, Nepal, and Bangladesh due to wind patterns. Previous research has explored the local, urban, and regional-scale impacts of PM2.5 from agricultural fires. However, there is a lack of comprehensive, nationwide quantification of the health impacts and the effectiveness of various mitigation strategies. Studies have shown the limited success of existing national and state-level regulations in reducing agricultural fires. The low nutritional value and digestibility of rice and wheat residues have hindered the development of effective alternatives, such as biofuel or animal feed. This study builds on previous research by offering a comprehensive, quantitative analysis of the nationwide impact and exploring the potential of localized interventions.

This study employed a combination of adjoint and forward modeling approaches using the GEOS-Chem atmospheric chemistry and transport model. The adjoint model, specifically GEOS-Chem adjoint (version 35), was utilized to quantify the sensitivity of annual mean population-weighted PM2.5 exposure in India to emission sources. Three sets of adjoint runs were performed, each representing a typical year with differing rainfall conditions (flood, drought, and normal). This data, combined with fire emission data (Global Fire Emissions Database v4.1s) from 2003 to 2019, allowed estimation of the impact of agricultural residue burning on population exposure. Additional adjoint runs focused on urban and highly populated areas. To assess long-term trends and impacts on neighboring countries, 23 pairs of forward simulations (GEOS-Chem Classic, version 13.0.2) were conducted, simulating the post-monsoon burning season with and without Indian agricultural emissions (1997–2019). The Integrated Exposure Response (IER) function was employed, along with India-specific population data and baseline mortality rates from GBD 2019, to estimate premature mortalities attributable to PM2.5 exposure. The Value of Statistical Life (VSL) adjusted for India was used to monetize the cost of premature mortality. The diurnal pattern of fire activity was adjusted based on MODIS satellite fire counts. Model outputs were validated against various observational data sources including satellite AOD, AERONET sites, and ground-level PM2.5 observations from India's Central Pollution Control Board and US Embassy monitors.

The study's key findings include: 1. **Significant Mortality Burden:** Crop residue burning caused an estimated 44,000–98,000 premature deaths annually in India from 2003 to 2019. 2. **Regional Disparities:** Punjab, Haryana, and Uttar Pradesh contributed disproportionately to the nationwide impact (67–90%), with six districts in Punjab alone accounting for 40% of India-wide impacts. 3. **Effectiveness of Targeted Interventions:** Shifting burning times two hours earlier in Punjab could avert approximately 9600 premature deaths annually, with a monetized value of $3.2 billion. Reducing burning emissions by 1% in Punjab and Haryana resulted in a 0.57% and 0.065% decrease in India-wide impacts, respectively. 4. **Temporal Variations:** The impact varied between seasons, with post-monsoon burning contributing more significantly. Also, inter-annual variations were observed, with drought years showing lower impacts and flood years exhibiting higher impacts. 5. **Spillover Effects:** Impacts on neighboring countries (Bangladesh, Nepal, Pakistan) were significantly lower than in India, mostly due to prevailing wind patterns and population density. 6. **Policy Implications:** The findings suggest that small-scale, localized interventions such as adjusting burning times or promoting less residue-intensive crops could yield significant improvements in air quality and public health. A subsidy introduced in 2018 to encourage mechanization did not show immediate substantial improvements due to a shift in burning from the post-monsoon to the pre-monsoon season.

The findings highlight the substantial public health and economic burden associated with crop residue burning in India. The disproportionate contribution from specific districts and states underscores the need for spatially targeted interventions. The adjoint modeling approach effectively identified areas with high potential for mitigation through relatively simple changes, such as altering burning times. The effectiveness of shifting burning times earlier in the day is linked to diurnal changes in planetary boundary layer height and other meteorological factors, although further research is needed to understand the complex interactions between aerosols and meteorology. The study's findings offer a quantitative basis for developing and optimizing mitigation strategies, emphasizing the potential for substantial improvements through targeted and potentially low-cost actions. The observed persistence of high air quality impacts despite policy interventions highlight the need for a more comprehensive approach that considers both seasonal variations and the adoption of long-term solutions, such as mechanization.

This study provides a comprehensive quantitative assessment of the air quality impacts and health burden resulting from crop residue burning in India. The findings emphasize the urgent need for targeted interventions at the district level, demonstrating the potential for significant improvements through relatively simple and low-cost strategies like adjusting burning times or promoting less residue-intensive crops. Future research should focus on more detailed cost-benefit analyses of specific interventions, further investigation of aerosol-meteorology interactions, improved observational data on burning practices, and the evaluation of downwind impacts in specific locations.

This study acknowledges several limitations. The analysis primarily focuses on premature mortality risk from PM2.5 and does not fully account for other pollutants, like ozone. The IER function employed assumes equal toxicity for all PM2.5 species and was not developed specifically for the Indian context. The assumed diurnal cycle for burning emissions might not fully reflect reality due to limited hourly burning activity data. Finally, this study provides an overall assessment across India and does not focus specifically on individual pollution hotspots like Delhi. Future research addressing these limitations would strengthen the findings and refine recommendations.