India’s air quality management has historically focused on PM10, with limited attention to PM2.5 and almost none to the secondary particulate fraction that forms through atmospheric chemical processes rather than being emitted directly. Yet, understanding how PM2.5 is chemically constituted, especially the share arising from secondary formation, is essential for designing targeted, sector-specific interventions.
While ground monitoring networks provide high-resolution data on total PM2.5 concentrations, they rarely offer consistent information on particle composition or precursor-driven secondary aerosols. Satellite-based reanalysis products such as NASA’s MERRA-2 (Modern-Era Retrospective Analysis for Research and Applications, Version 2) help address this gap by providing spatially and temporally consistent estimates of both total PM2.5 and its modeled chemical components.
This brief presents an assessment of PM2.5 composition across India’s states and airsheds, with a specific focus on secondary particulate matter, particularly ammonium sulfate [(NH₄)₂SO₄], using MERRA-2 reanalysis data.
Ammonium sulfate represents secondary PM2.5 formed from interactions between sulfur dioxide and ammonia emissions, linking coal-based power generation, industrial activity, and agricultural ammonia to PM2.5 pollution. The government had already diluted SO₂ emission norms for coal-fired power plants, and this approach delayed action, weakened compliance efforts, and underestimated the role of sulfur dioxide emissions in forming secondary PM2.5.
The findings highlight the limitations of blanket PM control strategies and demonstrate the need for airshed and region-specific approaches that address precursor gases and secondary aerosol formation.
This evidence is directly relevant for the upcoming revision of the National Clean Air Programme (NCAP), supporting a stronger and more explicit focus on sulfur dioxide, ammonia, and combustion-related emissions to achieve sustained and structurally durable reductions in PM2.5.
Figure 1. Annual ammonium sulfate contribution (%) to PM2.5 mass across Indian states
