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Weather Conditions and Air Pollution

Impact of Odd-even Policy in Delhi

To understand the impact of the odd-even policy in Delhi researchers will need to take into account prevailing weather conditions of not just Delhi, but also its outskirts and neighbouring states. 

Since 1 January 2016, the odd-even formula has been implemented for private vehicles in Delhi for 15 days. The most visible impact of this scheme has been on road congestion, with noticeable declines at major intersections. However some have claimed that air pollution has gone up during this period, leading to the question whether this scheme worked at all. This article argues that there is a host of benefits even if Particulate Matter (PM)2.5 does not go down, and points out some of these benefits.

Sources of Air Pollution

What leads to air pollution in cities? Air pollution can be caused by the emission of toxic vapours/gases and particles. Particles can be of different sizes, with the ones under 2.5 microns (10-6 m), also called fine particulate matter, that do not settle to the ground under gravity, causing the most damage to human health. The very small size of these particles causes them to be inhaled deep into lungs with our bodies’ natural mechanisms unable to filter them. Greenstone et al (2015) have estimated that 660 million people or half of India’s population live in areas that exceed India’s National Ambient Air Quality standard for fine particulate pollution.[i] The effects of toxic gases such as the oxides of nitrogen, carbon monoxide, sulphur dioxide and ozone, on health are well established too (Central Pollution Control Board 2008).

Several studies undertaken by the Central Pollution Control Board (CPCB) and Centre for Science and Environment (CSE) have shown that for several of the major cities in India, the sources of air pollution include the transport sector, industry and power plants and dust, burning of leaves, etc. The transport angle has been discussed at great length in these studies. Both natural and manmade factors play an important contribution to the PM build up in a city.

World Health Organization (WHO) finds that 13 of the 20 cities with the worst particulate matter PM2.5 are in India. Delhi recorded an average PM10 of 286 mg/ m3 in 2010, and had the world’s highest levels of average PM10 during 2008–10. Similarly, the PM2.5 of level of 153 mg/ m3 in 2013, is also the highest in the world.  (WHO: Ambient Air Pollution database, update 2014).  

Delhi has more cars than Mumbai, Chennai and Kolkata together. Reddy and Balachandra (2010) note that the per capita travel by cars was 331 passenger kilometre (pkm) in 23 urban areas in 2005, with Delhi reporting 894 pkm against the national average of car use per capital of 58.6 pkm.

Consequently, emissions from the transport sector are the highest in Delhi. Ramachandra et al (2015) show that during the year 2009–10, the total number of registered vehicles in Delhi was 6451883, out of which there were about 20 lakh cars and jeeps and 40.5 lakh motor cycles, mopeds and scooters. The emissions from these registered vehicles were 10867.51 gigagrams (Gg) which was about 30% of the total emissions in this sub category.

The number of private cars for which the odd even policy is applicable (2587887 as of 31 December 2013) is about a third of the total registered vehicles (8140695) in the city (based on data from the Transport Department, Government of National Capital Territory)—hence the impact of the policy is limited to that extent. Moreover compressed natural gas (CNG) vehicles were exempt.  Efforts to contain the pollution from other sources need to be pursued as well. 

Fate of Pollutants

When pollutants are emitted, they stay in the air for a while and then get deposited on the ground, leaves—any surface, in fact, or, then are carried elsewhere by air. Some react with other substances in the air to form secondary pollutants (so called because they are not emitted directly). Most of these secondary pollutants are constituents of PM2.5, so when the number of vehicles on the roads is reduced, fewer pollutants are released and fewer secondary particles are formed.

The Effect of Weather on Pollution

Many newspapers have reported that pollution levels—mostly those of the particulate matter of size 2.5 microns, have not declined despite the imposition of the odd-even policy. One of the reasons that particulate matter of the fine kind continues to remain in the air of Delhi, is that such particles are present in the air all over North India. Gupta, et al (2006) report worst air quality in Delhi using satellite and ground measurement over several global locations including Delhi.

The answer lies in the prevailing weather conditions. While the sources remain more or less the same, better circulation in the summer (more sunshine and windier conditions than the winter) allows the pollutants to be carried elsewhere; they do not sit at ground level, but move either higher up in the air or to some other places, by horizontal movements of air, that is, via breeze or winds.[ii] So, rural areas receiving air from Delhi suffer the same effects that Delhi’s residents do—perhaps in a less severe manner.  

A glance at any satellite image taken during the winter months of 2015 and 2016 will show a brown haze over Uttar Pradesh, Punjab, Bihar and West Bengal. What is being experienced in Delhi is only part of a large-scale, regional problem. In fact, the odd-even experiment is showing quite clearly that the whole of North India (and probably other parts of the region as well) is polluted. So, the regional concentrations of PM2.5 contribute a large amount, possibly more than 50%, to what is observed in Delhi—controlling air quality in this large, densely populated urban area, will necessitate controlling pollution over all states north, south east, and west of Delhi, which can only be good news for humans and crops (Air Quality Expert Group 2012).

Benefits of the Odd-even Policy

To summarise, weather and climate cannot be changed, but controlling the sources of pollution is in our hands. The odd-even policy has had the following excellent spin-offs. First, it has raised awareness of air quality among Delhi’s residents and suggests that the mostly compliant population recognises air pollution as a problem and is willing to cooperate even though they (people) are put to some inconvenience. Second, it has taken half the private cars off the roads, reducing traffic and congestion. As cars move relatively faster, they use less fuel and burn it more efficiently. Idling engines at traffic lights and start-stop traffic are associated with greater pollution from exhaust pipes. Commuters and other kinds of travellers reach their destinations faster, wasting less time and, it may be said, reducing mental and physical stress. Two-wheeler drivers, cyclists, pedestrians and anyone outside an enclosed area are exposed less to pollutants, too.

There are reports over the last three to four decades, containing measurements from Mumbai, Delhi and occasionally other cities, too, but these are sporadic and do not cover all aspects of air quality. It is very important to measure the levels of PM2.5 systematically all over India and establish how much sources outside Delhi contribute to its observed levels. Apart from knowing how good or bad air quality is, it is important to know the sources of each of the components mentioned above. This should be the first step towards any action seeking to control pollution of any kind. It is hard to find systematic studies reporting emission sources and their strengths for any region in India.

If one source, that is, vehicular exhaust, is reduced, the others still remain. The odd-even policy is a temporary measure to curb pollution. Other more permanent measures need to be taken to make the quality of air in Delhi liveable.


[i] This includes Delhi, and can be seen on page 42, Greenstone et al (2015).

[ii] A study of particulate air pollution in six Asian cities including Chennai, Oanh et al (2006) found that the levels of particulate pollution were higher in the dry season compared to the wet season. 


Air Quality Expert Group (2012): “Fine Particulate Matter in the United Kingdom,” 150_AQEG_Fine_Particulate_Matter_in_the_UK.pdf.

Central Pollution Control Board (2008): “Epidemiological Study on Effect of Air Pollution on Human Health (adults) in Delhi,” Environmental Health Series, EHS/1/2008, August, Delhi: Ministry of Environment and Forests, Government of India,

Gupta, P, Christopher, S A, Wang, J, Gehrig, R, Lee, Y, and Kumar, N (2006):  “Satellite Remote Sensing of Particulate Matter and Air Quality Assessment over Global Cities,” Atmospheric Environment, Vol 40, Issue 30, pp 5880–5892.

Greenstone, M, Nilekani, J, Pande, R, Ryan, N, Sudarshan, A, and A Sugathan (2015): “Lower Pollution, Longer Lives: Life Expectancy Gains if India Reduced Particulate Matter Pollution,” Economic and Political Weekly, Vol 50, No 8,

Oanh, Kim, N T, Upadhyay, N, Zhuang, Y H, Hao, Z P, Murthy, D V S, Lestari, P, Villarin, J T, Chengchua, K, Co, H X, Dung, N T, and  E S Lindgren (2006): “Particulate Air Pollution in six Asian Cities; Spatial and Temporal Distributions, and Associated Sources,” Atmospheric Environment, Vol 40, No 18, pp 3367–3380.

Ramachandra, T V, Aithel, B H, and K Sreejith (2015): “GHG Footprint of Major Cities in India,” Renewable and Sustainable Energy Reviews, Vol 44, pp 473–495.

Reddy, S and P Balachandra (2010): “Dynamics of Urban Mobility: A Comparative Analysis of Megacities of India,” Indira Gandhi Institute of Development Research Working Paper, WP-2010-023, Mumbai.

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