Sunday, Nov 27, 2022

Explained: Will the odd-even scheme help reduce Delhi pollution?

While announcing the odd-even scheme in September, Kejriwal had said that a 10%-13% reduction in air pollution could be expected as a result of the intervention.

Explained: Will the odd-even scheme help reduce Delhi pollution? A volunteer holds a placard with information regarding restrictions on private vehicles in New Delhi on Monday. (Reuters Photo)

As the odd-even road rationing scheme began in Delhi on Monday, the number of vehicles out in the streets were reported to have come down significantly, and Chief Minister Arvind Kejriwal claimed that the order was seeing near 100 per cent compliance.

It is too early to judge the impact, if any, on the quality of the air in the city. While announcing the odd-even scheme in September, Kejriwal had said that a 10%-13% reduction in air pollution could be expected as a result of the intervention.

According to CPCB data, the air quality index (AQI) reading in Delhi at 2.45 pm was 424; the average of the previous 24 hours that the CPCB recorded at 4 pm on Sunday was, by comparison, 494.

Sunday was among the worst air days of the past few years, but the situation appeared to improve somewhat around the evening.

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Much of the public discourse in recent days has been focussed on stubble-burning in the fields of Punjab. In its advertisements, the Delhi government has sought to present ‘odd-even’ as a step to mitigate this pollution, over which it has no control.

Though not stated explicitly, the premise of ‘odd-even’ is that tailpipe emissions are a significant contributor to air pollution and that taking a large number of vehicles off the road at any given time would be able to improve the quality of the air noticeably.

Is this understanding accurate? What do source apportionment studies say on the relative contribution of sources of particulate matter in Delhi’s air? This is what some major scientific studies showed:

Volunteers on duty on the first day of odd-even road rationing scheme in Delhi on Monday. (Express photo by Abhinav Saha)

2003, Central government

In 2003, the Union Ministry of Environment and Forests published a white paper on air pollution.

It reported that in the three decades between 1970-71 and 2000-01, the contribution of vehicles to particulate matter in Delhi’s air had more than trebled to 72 per cent from 23 per cent.

February 2007, IIT-Delhi


This was “A Detailed Study to Ascertain the Effect of Diesel Operated Trucks, Tempos, Three-Wheelers and Other Commercial Vehicles on the Ambient Air Quality of Delhi”, sponsored by the Department of Environment, Government of the NCT of Delhi. The principal investigator was Prof Pramila Goyal of the Centre for Atmospheric Sciences, IIT-Delhi.

The study noted that the “emission of air pollutants [is] directly proportional to the number of vehicles and concentration of ambient air pollutants is also directly proportional to the emission of air polluting sources”.

It said that “tempos contribute the maximum amount of concentration of NOx and PM (58%) followed by trucks (24.1%), buses (12%), cars/taxis (9.7%), small trucks (3.7%) and tractor, trailer (0.18%).”

It suggested that “control may require[d] on the new registration of diesel tempos and trucks”.

December 2008, NEERI, Nagpur

This study was titled “Air Quality Monitoring, Emission Inventory & Source Apportionment Studies for Delhi”, and was carried out by the Central Pollution Control Board (CPCB) with National Environmental Engineering Research Institute (NEERI).


This study identified road dust as the biggest contributor (52.5%) to particulate matter in Delhi’s air, followed by industries (22.1%). It attributed only 6.6 per cent of particulate emissions to vehicles.

The study found industries contributed 79 per cent of the NOx, and vehicles only 18%. However, vehicles were the main source of carbon monoxide and hydrocarbons (59% and 50% respectively), the study found.

2011, SAFAR


This was the “Emissions Inventory of Anthropogenic PM 2.5 and PM 10 in Delhi during Commonwealth Games 2010”, carried out by Saroj Kumar Sahu, Gufran Beig, and Neha S Parkhi for the System of Air Quality Forecasting And Research (SAFAR) project. It was published in the journal Atmospheric Environment in 2011.

This study reached the conclusion that road dust from paved and unpaved roads contributed the largest share to air pollution (55%). This was followed by residential sources (15%), transport and vehicular pollution (13%), industrial sources (12%), and power (5%).


“Emission from the unattended source like windblown dust from paved and unpaved roads is found to be the major contributor in PM10 and PM2.5 emissions in Delhi and inclusion of this sector helped in better forecasting skill,” the study said.

January 2016, IIT-Kanpur

This was a “Comprehensive Study on Air Pollution and Green House Gases (GHGs) in Delhi (Final Report: Air Pollution Component)”, carried out by Prof Mukesh Sharma and Prof Onkar Dikshit of the Department of Civil Engineering, IIT-Kanpur. It was submitted to the Department of Environment, Government of the NCT of Delhi, and the Delhi Pollution Control Committee (DPCC).

This latest study is the most important and relevant. It carried out sampling during the winter of 2013-14 and the summer of 2014. It had five components: air quality measurements, emission inventory, air quality modelling, control options and an action plan.

This study formed the basis of the Graded Action Response Plan (GRAP) framed by the Environment Pollution (Prevention & Control) Authority (EPCA) for the NCR. The EPCA was set up by the central government in 1998 following a directive of the Supreme Court. The GRAP is acknowledged to be a major contributor to the reduction in air pollution levels over the last four years.

Unlike the 2008 and 2011 studies, this study, while underlining the role of road dust, also stressed on vehicular emissions — moving vehicles, in fact, contributed to over half of Delhi’s air pollution, it said.

For PM2.5, the source apportionment, according to the study, was: road dust (38%), vehicular pollution (20%), domestic sources (12%), industrial sources (11%), concrete batching (6%), hotels and restaurants (3%), municipal solid waste burning (3%), diesel gensets (2%), industrial area sources (2%), and cremation, aircraft and medical incinerators (1% each).

For NOx emissions, industrial point sources (52 per cent) and vehicles (36 per cent) were the biggest contributors, followed by diesel gensets (6 per cent), the study found.

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First published on: 04-11-2019 at 05:04:27 pm
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