
Photo Credit: Abhisheklegit from Getty Images
Rapid industrialization and population growth has led to significant increases in emissions and air pollution to dangerous levels in India. To address this challenge, the Indian government implemented the National Clean Air Programme (NCAP) to reduce emissions of fine particulate air pollutants (particulate matter with diameters less than 2.5 microns (PM2.5) and 10 microns (PM10) and their precursors) in cities which fail to meet the Indian air quality standards (i.e., non-attainment cities). Though cities have seen noticeable decreases in fine particulate concentrations since the implementation of the program, a Princeton-led study finds that a significant portion of the improvements in air quality result from favorable meteorological conditions that are unlikely to persist as the climate changes.
In 2019, India’s Ministry of Environment, Forest, and Climate Change launched the NCAP to help reduce particulate matter (PM) air pollution by 20-30% by 2024 relative to 2017. The NCAP provided over 10,400 crores ($1.2 billion) of financial support to help the 131 non-attainment cities expand their pollution monitoring capacity, city action plans, and public awareness campaigns. However, meteorological variability can change ventilation and the dilution of pollution, thus driving large variations in surface PM2.5 concentrations that make it hard to gauge policy effectiveness.
“To disentangle the role of meteorology from emission changes, we need high-quality surface measurements and atmospheric chemistry models that can accurately reproduce observed pollutant concentrations,” explains lead author Yuanyu Xie, an associate research scholar at C-PREE. “However, very limited studies have carefully examined surface pollution data or provided comprehensive evaluations of model performance.”
In this study, Dr. Yuanyu Xie, Prof. Denise Mauzerall, and their research team used the recently expanding national surface continuous PM monitoring data (2017-2022) available from the Indian Central and State Pollution Control Boards (CPCB and SPCB) and ran regional model simulations to evaluate the effectiveness of NCAP policies in non-attainment cities. The researchers strictly controlled the quality of the collected surface measurements they used and analyzed observed annual, seasonal, and daily mean PM concentrations for comparison with the NCAP’s PM reduction target of 20-30%.
The results showed an 8.8% per-year decrease of PM2.5 pollution in the six non-attainment cities from 2017 through 2022. In 2022, four of these cities met the NCAP target and achieved over 20% reductions relative to 2017 levels of PM2.5 pollution. However, the authors note that although primary PM2.5 emissions appear to be decreasing, emission of key PM2.5 precursors (SO2, NOx, and NH3) have not been decreasing and have even increased in some locations since 2017.
“Decreasing pollution concentrations are beneficial for public health,” says Xie. “However, if these improvements are not primarily driven by substantial emission reductions, the current air quality improvements may not be sustainable. Increasing emissions of PM2.5 precursors we found could also indicate that existing policies are either ineffective or have not been fully implemented and enforced.”
Given the small change in anthropogenic emissions, the researchers then investigated the extent to which the decrease in PM2.5 pollution can be attributed to meteorological variability by conducting six years of regional atmospheric chemistry transport model simulations with fixed emissions at 2017 levels. PM2.5 reductions attributable to meteorological variability were then estimated as the difference between the simulated PM2.5 changes and the observed PM2.5 changes.
The results showed that approximately 30% of PM2.5 air quality improvements, and approximately 50% of the reductions during the heavily polluted winter months, can be attributed to favorable meteorological conditions. However, even with favorable meteorological conditions, in 2022, 44 out of the 57 non-attainment cities with continuous PM monitoring data still failed to meet the national air quality standards. The results highlight the need for significant additional mitigation measures - especially since these favorable meteorological conditions are unlikely to persist as the climate warms.
“India, along with other countries in the global South, face the dual challenge of rapid industrialization with resulting simultaneous increases in air pollutant and greenhouse gas emissions,” says Denise Mauzerall, a faculty member at Princeton’s School of Public and International Affairs and the School of Engineering and Applied Science. “Co-benefits for air quality, health and climate will result from on-going efforts in India to move away from coal and towards renewable energy. Substantial additional mitigation action beyond current air pollution control policies, especially measures that simultaneously mitigate greenhouse gas and air pollutant emissions, such as energy system decarbonization, electrification and reductions in agricultural waste burning, are essential to achieve healthier air quality and contribute to slowing the rate of global climate change.”
The paper, “Recent PM2.5 air quality improvements in India benefited from meteorological variation,” was co-authored by Yuanyu Xie (School of Public and International Affairs, Princeton University), Mi Zhou (School of Public and International Affairs, Princeton University), Kieran M. R. Hunt (Department of Meteorology and the National Centre for Atmospheric Sciences, University of Reading), and Denise Mauzerall (School of Public and International Affairs and the Department of Civil and Environmental Engineering, Princeton University). The paper appeared in Nature Sustainability on May 6th, 2024.