Odd-even: Fewer cars on roads, but air in Delhi remains polluted

Any gains in air quality made because of the low traffic volume on Delhi roads on Friday morning were offset by a drop in morning temperatures and heavy traffic in the evening.

At most air quality monitoring stations in the city, Particulate Matter 10 and Particulate Matter 2.5 was higher on Friday afternoon than that calculated on Thursday at the same time. According to scientist Gufran Beig, who heads Ministry of Earth Sciences’ SAFAR air quality index, the reason for higher pollutants was low temperature.

“Last night was bad for air quality and the reason probably was a lot of vehicle movement and firecrackers combined with a fall in temperature by at least 1.5 degrees Celsius from the previous night. This elevated the PM 2.5 level significantly. However as the day began, it took some time for the previous night’s emission to settle. PM 2.5 level started to fall in the morning hours from 8am onwards sharply and later slowly until 2pm. However, again after 2 PM, there was an increase in PM2.5 with respect to previous days. This will have to be examined more loosely,” Beig said.

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The Delhi government set up six live pollution meters to monitoring real-time air quality on Friday. The metres displayed real-time values of the particulate matter (PM2.5 and PM 10 in city air. Portable machines were also sent around to monitor air quality at 200 locations.

Several Delhi government officials cited the drop in pollutant levels between early morning (at around 6 am) and the levels at 9 am to highlight how well the odd-even restriction was working. This, however, is not a correct parameter as pollutants are always higher in the early morning as compared to later in the day. This is because of low temperatures that trap pollutants close to the surface of earth. The Delhi Pollution Control Committee was more cautious and said it would take a few days to understand the trends that the data was showing.

Scientists, however, are confused by the sudden unusual spike in pollutants (PM 2.5) after 2pm and many said it was probably because of a combination of weather conditions and traffic.

Friday being the first day of the New Year saw very few cars on the roads. Most of those that were on the road were odd numbered in keeping with the government’s order. The low traffic volume can be attributed to it being a holiday for a number of professionals as well as all Delhi schools. Towards the afternoon, however, as people stirred out of their homes and traffic on the roads increased manifold, the pollution levels also started to shoot up almost at the same time.

“The quick dip in pollution between 8am and 2pm shows that there is a clear connection between air quality and vehicular emissions, which many have been denying. The spike after that could be attributed to low wind speed, accumulation effect and the traffic towards the afternoon and evening,” said Anumita Roy Chowdhury, executive director, Centre for Science and Environment.

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