As India prepares to revise its National Clean Air Programme (NCAP), experts warn that the country’s air pollution strategy risks missing its most dangerous target. While current policies remain heavily focused on PM10, evidence shows that PM2.5 poses far greater risks to public health and demands urgent, central attention in the next phase of NCAP.
A new analysis by the Centre for Research on Energy and Clean Air (CREA) highlights a critical gap in India’s clean air approach. The study finds that a substantial share of PM2.5 pollution is not emitted directly, but forms in the atmosphere through chemical reactions involving precursor gases. According to CREA, up to 42 percent of India’s PM2.5 burden consists of secondary particulate matter, dominated by ammonium sulfate formed from sulfur dioxide.
India’s SO2 challenge and the coal power link
India is now the world’s largest emitter of sulfur dioxide. Coal-fired power plants account for at least 60 percent of national SO2 emissions, placing them at the center of India’s PM2.5 problem. Despite this, regulatory exemptions have allowed around 78 percent of coal-based power plants to avoid installing flue gas desulphurisation systems, significantly weakening SO2 controls at the source.
CREA’s assessment shows that states with a high concentration of coal-fired power plants face the greatest burden of secondary ammonium sulfate. Chhattisgarh records the highest annual contribution at 42 percent of PM2.5, followed closely by Odisha at 41 percent. These findings underline the importance of reinstating mandatory FGD requirements across all coal-fired thermal power plants to effectively reduce secondary PM2.5 formation under NCAP.
Delhi’s pollution episodes driven by secondary PM2.5
The analysis also sheds new light on Delhi’s air quality crisis. Around one-third of the capital’s annual PM2.5 pollution comes from secondary ammonium sulfate. During the most polluted periods, post-monsoon and winter, ammonium sulfate dominates PM2.5 levels, contributing 49 percent and 41 percent respectively. This compares with 21 percent during summer and the monsoon season.
These patterns suggest that Delhi’s worst pollution episodes are driven not only by local sources, but by region-wide SO2 emissions and atmospheric chemistry. This challenges the prevailing focus on visible and local pollution sources alone.
Secondary particulate matter is a national issue
Using NASA’s MERRA-2 reanalysis data for 2024, CREA finds that ammonium sulfate contributes between 17 percent and 42 percent of PM2.5 mass across Indian states, with most states clustering between 30 percent and 40 percent annually. Seasonal analysis shows that secondary particulate matter remains significant throughout the year, peaking in winter and post-monsoon months and remaining substantial even in summer and the monsoon.
The widespread nature of secondary sulfate formation confirms that PM2.5 pollution in India is a national, year-round challenge rather than a localized or seasonal problem.
Gaps in current air quality strategy
The findings expose major shortcomings in existing air quality policies, which continue to prioritise PM10, road dust, and other visible sources of pollution. The role of precursor gases such as sulfur dioxide, nitrogen dioxide, and ammonia remains largely overlooked. Without addressing secondary particulate matter, improvements in air quality are likely to be limited and short-lived.
“As the NCAP is revised, India must focus not only on PM2.5 concentrations but also on what the pollution is made of. With secondary ammonium sulfate accounting for up to 42 percent of PM2.5, largely driven by SO2 from coal-based power plants, precursor controls and composition monitoring are essential for air quality improvement,” said Manoj Kumar, India Analyst at CREA.
Power plant emissions outweigh stubble burning
CREA’s broader research shows that capturing emissions from coal-fired power plants could reduce PM2.5 pollution across India by about 8 percent, with benefits rising to as much as 30 percent in areas close to power plants. This is particularly significant given that most of the world’s 100 most polluted places are in India.
The analysis also challenges the dominant narrative around stubble burning. Coal-fired power plants produce 10 times more particulate matter pollution than stubble burning in Punjab and Haryana, emitting an estimated 277 kilotonnes compared with 26.7 kilotonnes from farm fires. The contrast is even sharper for sulfur dioxide, with coal plants emitting about 4,327 kilotonnes, roughly 240 times more than stubble burning.
Policy reversal raises concerns
CREA’s latest assessment was prompted by the July 2025 reversal of a decade-old policy that mandated the installation of FGDs in 537 power plant units. The study finds that nearly 90 percent of the 416 coal-fired power plants assessed exceed national SO2 emission limits.
While the scale of the challenge is large, CREA notes that installing SO2 control technology is feasible. India has previously demonstrated the ability to implement ambitious policies at scale, such as the Pradhan Mantri Ujjwala Yojana, which significantly reduced indoor air pollution in millions of households.
BABURAJAN KIZHAKEDATH