The first three working days after the end of the odd-even scheme saw rapid worsening of air quality with levels of fine particulate matter (PM2.5) registering a jump of more than 57 per cent on the first working day itself, according to an analysis by a green body.

Centre for Science and Environment (CSE) said that Delhi has lost the air quality gains of odd and even scheme and the delayed winter will make pollution worse if quicker steps are not taken to sustain the gains.

It urged the Delhi government to immediately roll out its plan for the remaining part of the winter season and reduce traffic volume on a more sustained basis and allow public transport to perform more efficiently as was possible during the odd and even week.

CSE also urged the government to implement the slew of orders which the Supreme Court gave yesterday including quick augmentation of bus and metro, stronger enforcement on diversion of truck traffic, closure notice on coal-based Badarpur Power Station power plant and serving notice to further advance implementation of Euro VI emissions standards for vehicles.

"The gains of odd and even scheme were immediately lost after full traffic was back on the first Monday after the scheme came to a close. This was further aggravated by lower wind speed. On January 18, the PM2.5 levels shot up by 57 per cent within the day and once again rose to 'severe' category.

"With an average city wide concentration of 277 microgram per cubic metre (cum), Delhi's air quality was back in 'severe' category. The PM2.5 levels continued to remain in the 'severe' category on Tuesday with the levels further increasing to 281 micro gram per cum.

"On Monday, pollution build up within the 24 hours was massive – by as much as 101 micro gram per cum. Despite being a windier day this was 35 per cent greater build up on January 18 than the highest observed during odd-even period – a jump of 75 micro gram per cum on January 11," the CSE analysis said.

CSE further said that this winter, out of all the severe smog episodes so far, the peak pollution during odd and even programme has been the lowest and this shows that despite hostile weather conditions - no wind, temperature dip and western disturbance – peak pollution during odd-even scheme was much lower.

"This validates the importance of emergency action. The fortnight, when the programme was implemented has clearly demonstrated that the peak pollution levels are lower than the normal smog peaks of the season – despite adverse weather conditions," said Anumita Roy Chowdhury, Executive Director, CSE.

The green body said January 15, which was the last day of the odd and even scheme was the "cleanest" day of this winter when PM2.5 levels dropped to 155 micro gram per cum. The only day when lower levels were recorded before that was on November 5, 2015 when it had rained in the city, it said.

CSE came out with the data after it analysed air quality data from real-time monitoring of Delhi Pollution Control Board for the entire winter – November 2015 to January 2016 – to assess the benefits of the odd and even scheme and the loss of air quality gains after the completion of the scheme.

CSE said that it is clearly evident from the air pollution data that despite the lower wind speed on some days during of the odd-even scheme, pollution fell during those hours and in fact, during days before the programme was started, pollution levels had increased when wind speed was low.

CSE said that Delhi government should consider more such action sooner keeping in view Delhi is witnessing delayed winter and also greatly augment public transport, walking and cycling facility so that the city can go car-free more often and as a general practice.

CSE said that scientists from the Department of Public Health Sciences, University of Miami, CSE said have also studied how pollution in the air blocks sunlight by absorbing or by scattering light and prevents direct sunlight from reaching the ground.

"A value of 0.01 corresponds to an extremely clean atmosphere and a value of 0.4 would correspond to a very hazy condition. An average aerosol optical depth for the US is 0.1 to 0.15.

"They have found that this improved in Delhi after the odd-even scheme was implemented. But in areas bordering the capital and the surrounding NCR, there was no effect, and saw 35 per cent worsening," it said.

While referring to another assessment by a team of researchers from University of Chicago and University of Harvard, CSE said that study found that the odd-even programme has helped improve air quality in Delhi – by reducing particulate air pollution concentrations by 10-13 per cent in the period.