Secondary pollutants constitute up to third of PM2.5 pollution
Secondary pollutants, particularly ammonium sulphate, that results from the reaction between sulphur dioxide (SO2) and ammonia (NH3) in the atmosphere, are responsible for nearly one-third of India's fine particulate matter (PM2.5) pollution, said a study from the Centre for Research on Energy and Clean Air (CREA).
Primary pollutants are those that result from the burning of fossil fuels from automobiles, coal plants and organic matter. These pollutants, once they climb into the atmosphere, react with each other or other gases, water vapour in the atmosphere to form more complex particles — some of which are harmful. These are called secondary pollutants.
The CREA findings emerge from a combination of satellite-data imagery and modelling analysis.
'This widespread presence of ammonium sulphate has been observed in several cities, irrespective of their proximity to major emission sources, due to the trans-boundary nature of pollution,' an accompanying press release noted.
CREA's study says that the nation-wide average concentration of ammonium sulphate is 11.9 μg/m3, accounting for approximately 34% of the PM2.5 mass. The main driver of ammonium sulphate formation and more than 60% of SO2 emissions in India originate from coal-fired thermal power plants.
'This makes them a critical target for reducing secondary PM2.5 pollution through the implementation of flue gas desulphurisation (FGD) systems,' the report notes. Though it is mandatory for Indian coal thermal plants to install such units, only about 8% have installed them, and the government is also considering doing away with such a requirement altogether.
Ammonium sulphate concentrations constitute a greater proportion of particulate matter nearer to coal plants and fall with distance. It is 2.5 times higher within 10 km of coal-fired power plants (15 μg/m3) compared to areas beyond 10 km (6 μg/m3). While the contribution to PM2.5 is 36% near coal-fired thermal power plants, it remains significant at 23% in other areas, indicating both local and trans-boundary impacts of SO2 emissions.
Across the 130 National Clean Air Programme (NCAP) cities, concentrations ranged from 3.9 to 22.5 μg/m3, with the share of PM2.5 from ammonium sulphate ranging between 20% and 43%. Notably, ammonium sulphate made up more than 30% of the total PM2.5 levels in 114 out of 130 NCAP cities. These are cities marked out as having a considerable level of air pollution and required to reduce pollution by a certain percentage by 2025-26.
In addition to ammonium sulphate, other secondary pollutants like ammonium nitrate also contribute significantly, up to 50% of PM2.5 mass. 'Alongside targeted strategies such as deploying flue-gas desulphurisation in coal-fired thermal power plants to reduce sulphur dioxide and promoting efficient fertiliser management to reduce ammonia, it is essential that all polluting sectors fully comply with existing emission norms. Ensuring both source-specific actions and regulatory compliance is critical for reducing precursor gas emissions and tackling the growing burden of secondary PM2.5 pollution in India,' said Manoj Kumar, Analyst at CREA.
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