Did the scheme reduce air pollution?

There was definite, if not marked, improvement.

Fewer cars on the road and shorter idling times would have certainly reduced emissions, say experts. In bumper-to-bumper traffic, vehicles pollute roughly twice as much, said Anumita Roy Chowdhury, head of Centre for Science and Environment's (CSE) Clean Air programme.

But assessing the difference this made to air quality is less straightforward. The benefits of reduced emissions were offset by adverse weather, said Sumit Sharma, fellow at The Energy and Resources Institute (Teri). "For a few days there was no vertical or horizontal dispersal at all," he said, referring to the western disturbance during January 4-6.

Observations of the air quality monitoring agencies confirm this. Delhi Pollution Control Committee claimed significant reduction in air pollution at various places only in the second week of the scheme. Scientists at Central Pollution Control Board said on condition of anonymity that any reduction in emissions would have reduced pollution but the impact was felt when the weather conditions were normal.

Environment Pollution Control Authority (EPCA) told Delhi high court that DPCC data showed a dip in peak pollution levels from days before the scheme started. CSE, which is continuing to monitor air quality, said the scheme helped slow down the peaking of pollution levels.

Naresh Kumar, associate professor of environmental health in epidemiology and public health at the University of Miami, also pointed to significant gains from the scheme. His data "suggests a slight decline of about 2 micrograms per cubic metres in levels of PM2.5 (fine, respirable particles) inside Delhi" during the two-week trial, compared with the preceding fortnight. But in the adjoining towns, which did not enforce odd-even, PM2.5 levels rose by 14% due to the weather conditions. Kumar therefore infers that odd-even actually reduced PM2.5 in Delhi by 25 micrograms per cubic metres.

Did people's exposure to air pollution decrease?

At least 55% of Delhi's population breathed cleaner air.

Reduced congestion would have lowered exposure to vehicular emissions for people living near roads and traffic points. Citing two American studies, EPCA had told the high court that 55% of Delhi's population lives within 500 metres of roads where the pollution level is 1.5 times higher than the citywide average.

Did the weather play spoiler?

It certainly dulled the scheme's impact on some days.

On New Year's Day PM2.5 levels fell sharply between 7am and noon, compared with December 31. DPCC reported a dip in pollution at AnandVihar and Mandir Marg-two heavily polluted areas. January 2 was similar, but pollution levels started increasing from January 3 onwards. The met office told TOI a western disturbance at the time increased humidity and slowed down wind speed drastically, trapping the pollutants in a smog. During January 4-7, the weather was "unpredictable" and strongly affected air quality. Thereafter, data from System of Air Quality and Weather Forecasting Research (Safar), a central government agency, showed a decline in pollution with an uptick on some days.

Could the method of assessment have been better?

That would involve simulations on supercomputers.

Sharma said simulation modelling using supercomputers would have been better. "It can tell us more accurately what the impact is going to be like," he said. Safar is likely to release air quality assessment data based on simulation models on Saturday.

Did the exemptions make odd-even less effective?

Absolutely. Two-wheelers pollute more than cars in Delhi.

Scientists at most monitoring agencies want the experiment repeated without exemptions for two-wheelers and women drivers, because two-wheelers account for a third of the particulate matter emissions from traffic in Delhi. They are second only to diesel trucks, according to a recent draft IIT Kanpur report. Overall, the scheme kept only about 1 lakh cars off the roads on most days, traffic police said.

Shouldn't dust, trucks and generators have been tackled as well?

They are among the chief polluters in the NCR.

The IIT report says dust is the biggest component of particulate matter (38%) in Delhi's air. The government needs to crack down on thermal plants, industries and heavy diesel vehicles too. A recent Supreme Court order has helped by restricting the entry of pre-Euro III trucks, and EPCA has ordered measures to detect overloaded trucks that pollute more. Teri has also pointed out that diesel generators used in neighbouring towns due to frequent outages are built around truck engines and emit significant amounts of particulates and oxides of nitrogen. It has recommended fitting them with tailpipe control devices and 24x7 electricity supply in the NCR.