Delhi air particles hampering pollution fight by fettering sunlight, weakening winds: Study

New Delhi: Particles present in Delhi's air could be making the fight against pollution difficult, as they obstruct sunlight reaching the ground and weaken winds that help disperse pollutants, according to a new study.

The study, titled "Improving aerosol-radiation interactions in the operational forecasting system - AIRWISE", was published in Atmospheric Chemistry and Physics on September 21.

The study found that aerosols, tiny particles suspended in the air, can absorb and scatter sunlight. With less sunlight reaching the ground, the surface heats less, weakening the natural movement of air that helps mix and disperse pollutants.


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This can make the atmosphere more stable and reduce the height of the layer of air close to the ground where pollutants normally mix. Near-surface winds also weaken, meaning there is less movement to carry pollutants away. In turn, more PM2.5 can remain concentrated close to the ground.

In simple terms, the study suggests that once pollution becomes heavy, the particles themselves can alter local weather conditions in a way that makes it harder for that pollution to disperse.

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Researchers from the Indian Institute of Tropical Meteorology, Savitribai Phule Pune University, the National Centre for Medium Range Weather Forecasting, the University of California, Los Angeles, and the National Center for Atmospheric Research studied the national capital's pollution and weather conditions from September 2023 to January 2024.

During the November 1 to 10, 2023 pollution episode, heavy pollution reduced the sunlight reaching the ground by up to about 80 watts per square metre. This slightly cooled the surface, weakened winds and reduced the air movement that helps disperse pollution.

The atmosphere's ability to disperse pollution fell by up to 20 per cent in November and 10-12 per cent in January, the study said.

In November, PM2.5 levels reached about 500 micrograms per cubic metre, with daytime levels generally between 200 and 300 micrograms per cubic metre.

The layer of air that helps spread pollution became about 100-200 metres thinner during the afternoon, while winds weakened by up to 0.4 metres per second. The model showed that near-ground PM2.5 increased by up to 37-40 micrograms per cubic metre.

In January, PM2.5 levels peaked at about 550 micrograms per cubic metre amid low temperatures, high humidity, weak winds, fog and low clouds. These conditions already favour the build-up of pollution near the ground, further limiting its dispersal during the episode. The impact was smaller because there was less sunlight.

Winds weakened by about 0.2 metres per second and the air layer that helps disperse pollution became 40-50 metres thinner. Near-ground PM2.5 increased by about 25 micrograms per cubic metre in the model.

The researchers found that their calculated sunlight levels were closer to the measurements recorded at IGI Airport.

"The Air Quality Warning and Integrated Decision Support System for Emissions (AIRWISE), developed through a collaborative effort between the Indian Institute of Tropical Meteorology, the India Meteorological Department and the National Center for Atmospheric Research, has been operational since 2018 and provides real-time air quality forecasts across India," the study said.

Despite its operational success, the system has limitations in reproducing extreme pollution loading events over northern India, particularly over Delhi during the crop-residue burning season in the surrounding states, the researchers said.

Such high-impact episodes are often underestimated partly due to inaccuracies in representing aerosol-radiation-meteorology interactions, they said, adding that improving the representation of meteorological feedback processes is crucial for enhancing forecast skill.

The findings highlight how closely pollution and weather conditions can interact during Delhi's severe post-monsoon and winter pollution episodes, and the researchers said representing these interactions more accurately could help improve air-quality forecasts.

The study reinforces the need to reduce air pollution at its sources rather than relying mainly on temporary measures after pollution has built up, said Manoj Kumar, an analyst at the Centre for Research on Energy and Clean Air (CREA).

"When pollution levels are high, aerosols can block sunlight from reaching the surface, reducing heating and slowing the movement of air near the ground. This can make it harder for pollution to disperse, allowing it to build up further," he said.

"This makes consistent, targeted and time-bound emission reductions across major sources, including industry, power plants and other combustion sources, essential for reducing the overall pollution burden," Kumar said.