Odd-even scheme back in Delhi: Has it helped bring down air pollution in the past?

On CPCB’s scale, an AQI between 201 and 300 is “poor”, 301-400 is “very poor”, 401-500 is “severe”, and anything higher is in the “severe plus” category.

November 13-20 will be the fourth time the vehicle-rationing scheme will be implemented in Delhi in the last seven years. Odd-even is among the most drastic responses to the annual spike in air pollution that the capital and its neighbourhood sees every year around this time.

What is the odd-even scheme?

When odd-even is in force, cars can ply only on alternate days, based on their registration number. So, cars with numbers ending in an odd digit will be allowed on the street on odd dates (November 13, 15, 17, and 19), and those with numbers ending in an even digit will be allowed on even dates (November 14, 16, 18, and 20). The idea is to cut the number of cars on the road by roughly half which, the Delhi government hopes, will bring down AQI levels somewhat.

The scheme was tried out twice in 2016, and then in 2019. Several categories of vehicles were exempt, including taxis (which are CNG-run), cars driven by women (for considerations of safety), electric and hybrid vehicles, and all two-wheelers. The details of the new scheme have not been announced yet.

Sources in the Delhi Transport Department said there are an estimated 75 lakh active Delhi-registered vehicles currently, about a third of which are cars. The odd-even scheme would, therefore, mean that about 12.5 lakh cars would go off the roads each day, not considering electrics and hybrids.

About 20 lakh vehicles that are registered in the NCR cities or elsewhere also ply in Delhi. Broadly speaking though, they do not necessarily add to the traffic load in Delhi at any given time, because roughly the same number of Delhi-registered vehicles can be expected to be in the NCR at the same time.

According to the Delhi Statistical Handbook 2022 published by the Directorate of Economics & Statistics of the Delhi government, as on March 31, 2022, the number of registered “cars and jeeps” in Delhi was 20,57,657, and the number of “motor cycles and scooters” was 51,35,821.

The rest of the total 77,39,369 vehicles were made up by autorickshaws, taxis, buses, goods vehicles and tractors, etc.

To what extent are vehicular emissions responsible for Delhi’s bad air?

There are multiple reasons behind the current crisis. Delhi is situated in a shallow atmospheric ‘bowl’, in which pollutants are trapped easily. When combined with falling temperatures and slow wind speeds, pollutants in the air are not dissipated, and tend to hang low over the surface, causing Delhi’s infamous smog.

Adding to this is the smoke from the countless fires that burn across Punjab and Haryana at this time as farmers, rushing to prepare their fields for the rabi crop, set fire to the stubble that is left in the fields after the kharif crop has been harvested. While sources of pollution like vehicular emissions or road dust are present round the year, the farm fires and atmospheric conditions around Diwali act as force multipliers.

A study by the environment and climate change think tank Centre for Science and Environment, (CSE) found that at their peak in past years, farm fires contributed to 40% of the PM 2.5 concentration in the air. This year, on November 3, 4, and 5, the daily mean contribution from biomass burning to the PM 2.5 concentration in Delhi was 35%, 20%, and 21% respectively, according to Indian Institute of Tropical Meteorology data.

Farm fires, however, burn in a small window — and taken for the year as a whole, their contribution to Delhi’s air pollution is only around 3% or less. Several studies have suggested that the bulk of Delhi’s air pollution is caused by local sources, and vehicles contribute up to 30% of the city’s annual PM 2.5 concentration.

Vehicles also emit other pollutants such as NO2, and the CSE estimates that levels of the gas this year are up by as much as 60% compared to the same time last year. Certain high traffic locations have reported levels as high as three-four times the 24-hour standard.

So, does the odd-even scheme make sense, given the inevitable hardships it will cause?

Odd-even as a way to bring down air pollution has been tried in some form in cities in China, Mexico, and France as well. Its efficacy has been hotly debated almost everywhere.

After the 2019 scheme was implemented, The Indian Express analysed its efficacy, comparing AQI levels in Delhi with those in Gurgaon, Ghaziabad, and Noida — also in the NCR region (thus sharing similar environmental factors) but where the odd-even scheme was not implemented.

The results were clear: the average AQI decreased most significantly in the national capital, where odd-even was in place.

The average AQI in Delhi between October 23 and November 3, 2019, before the implementation of odd-even, was 369.5. During odd-even, from November 4-15, the average AQI was 328.5 — a 41-point reduction. In the same period, AQI in Gurgaon went up by 7.6 points, and showed small reductions in Noida (12.3 points) and Ghaziabad (13.6 points).

However, despite this data, experts say determining the exact impact of an individual measure on air pollution is difficult.

“As air quality depends on so many different factors, it is not easy to say what the effect one action such as odd-even has. For instance, on the first day that odd-even was implemented, strong winds were seen in the city… A significant reduction in AQI was noticed… To say odd-even works or doesn’t will be akin to ignoring all other factors at play,” a senior official in the Environment Ministry had told The Indian Express in 2019.