Overlooking PM2.5 precursors hurting India's clean air programme: Study

Bengaluru: Nearly six years since it was launched, the National Clean Air Programme (NCAP) overlooks precursors to PM 2.5, the smaller pollutant that have serious implications for human health, a new study has pointed out.

The study by Manojkumar N and Monish Raj from the Centre for Research on Energy and Clean Air (CREA) said that a large amount of PM 2.5, linked to lung serious diseases, was not generated directly. Instead, the particles form through atmospheric chemical processes.

"Understanding the share arising from this secondary formation is essential for designing targeted, sector specific interventions," it noted.

India emits 11.2 million tonne sulfur dioxide (SO2), highest in the globe, and is next only to China when it comes to the emission of nitrogen oxides (NOx) at 9.4 million tonnes and ammonia (NH3) at 10.4 million tonnes.

CREA said that the secondary pollutants like ammonium sulfate, formed from reaction between SO2 and NH3 in the atmosphere, were responsible for nearly one-third of PM 2.5 pollution. Ammonium sulfate thereby links coal-based power generation, industrial activity and agricultural ammonia to PM2.5 pollution.

Ammonium sulfate contribution to PM 2.5 goes up to 40 per cent during the winter and post-monsoon season in almost all of the country save for the northern and western airsheds.

While this reduces in Summer in other states, the northeast states continue to see higher (31-40 per cent) contribution.

Shift in strategy

The study called for a shift in strategy to decisively target beyond particulate matter. "Given that ammonium sulfate alone accounts for roughly one-quarter to two-fifths of PM2.5 mass across most states, effective PM2.5 mitigation is not possible without explicitly targeting the precursor gases sulfur dioxide and ammonia," it said.

It also called for a check on SO2 emissions, particularly from coal-fired power plants. The finding makes a strong case for reinstating mandatory flue gas desulfurisation requirements, reversing the dilutions of emission norms by the Centre.

The study also called for explicit regulation on ammonia emissions, multi-state coordination based on air-shed models and strengthening the monitoring of PM 2.5.

"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 per cent 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.