A vehicle rationing briefly introduced last year by the Delhi government to cut down toxic pollutants in the city air may have had a negligible impact on the levels of ultrafine particulate matter, PM2.5, which is linked to premature mortality, a scientific analysis has shown.

Minor reduction

The scientists who studied the first odd-even experiment, which lasted 15 days from January 1, 2016, found that there was only a marginal drop in the PM2.5 levels during the period.

While the levels of the toxic particles came down by 8 to 10 per cent in some pockets, the rest of the capital recorded a drop of only 2 to 3 per cent.

The study was published online in the journal Environmental Science and Policy early this week.

“The reduction was abysmally small,” said Sachida Nand Tripathi, who heads the Centre for Environmental Science & Engineering at the Indian Institute of Technology (IIT) Kanpur, who led the study.

The PM2.5 levels in Delhi, particularly in winter months, are on average three times higher than 60 micrograms per cubic metre which the World Health Organisation has declared as permissible limit.

A recent study by two of the authors of the present paper – IIT Delhi researchers Sourangsu Chowdhury and Sagnik Dey – exposure to alarmingly high levels of PM2.5 are responsible for approximately 12,000 premature deaths in Delhi every year.

Apex court fiats

The situation in the capital has been so severe that the Supreme Court has been demanding stricter action from both the state and Central governments.

“This is the first-ever study that scientifically analysed the vehicle rationing scheme in the capital,” said Dey, who is with the Centre for Atmospheric Sciences at IIT Delhi.

The scientists used data gathered from satellite and ground stations to simulate two conditions – with odd-even rule and without the rule. They were surprised to see that the difference between the two was extremely small.

Multiple sources

“Restricting traffic volume alone cannot control the PM2.5 concentration over Delhi, where there are multiple other sources contributing towards making the city’s air dirty,” the scientists said.

“We are not talking about an enclosed entity, but open atmosphere,” said Tripathi.

There have been studies in the past that showed the contribution of vehicles to the PM2.5 levels is very small. The share of private cars could be even smaller, he said.