Air Pollution in Delhi - Winter Time Highs and Blame Games

Update (March 2023) -- What Is Polluting Delhi's Air: A Review from 1990 to 2022

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Old blog material

The smog and pollution in Delhi this year is particularly bad given that it is so early in the winter season. Usually the health and economic impacts of the smog become front news headlines later in the winter season when travel is disrupted and there is a permanent haze over the city due to several factors including burning of biomass to keep warm. Read more @ what's polluting Delhi's air? 

In response to the call for action to reduce air pollution in the city, the minister in charge of environment, Sheila Dikshit declared that the source of the pollution in the city is agricultural waste burning from farms in Punjab and Haryana and none of the pollution is from sources within the city. Commentary on air pollution in Delhi, published in EPW - June, 2012  

Delhi's Air Pollution - Emissions vs. Meteorology

As the capital of India with a population of about 20 million – Delhi does have a well-recognized air pollution problem and over the years the government has taken some steps to reduce air pollution (e.g conversion of buses to run on Compressed Natural Gas - CNG). However these measures are not enough, in the wake of the increasing activity within the city and the peculiar meteorological conditions of the city.

Pollution from particulate matter (PM) has been growing steadily in Delhi. As the graph below shows, there were some gains post-implementation of the CNG program, but those have been by far overtaken with time. The benefits of leapfrogging to alternative fuels like CNG is outdone by the increasing number of passenger vehicles on the road, lack of enough public transport buses, the in- crease in freight movement and construction material and debris by trucks passing through the city, the lack of maintenance of trucks and buses, growing demand for electricity leading to the use of in- situ generator sets, and industrial growth.




The above graph is the data presented by the Minister of State, Ms. Jayanti Natarajan, in the Rajya Sabha, in March 2012. The increase in annual averages between 2001-10 is not due to seasonal agricultural burning that happens for a few weeks.

An emissions inventory for Delhi @ 1kmx1km resolution

A source apportionment study of PM2.5 Hydrocarbon analysis of measured samples from 2002 highlights the main contributions to pollution in Delhi. As we see in the graphs, the profiles differ for summer and winter. Additionally, pollution is 2 to 4 times as high in winter as in summer. With summer concentrations approximately 40-80 micro-gm/m3 daily average and winter concentrations 90-320 micro-gm/m3 daily average. The biomass burning which forms a large part of the pollution in winter, includes emissions from both outside, as well as from within the city (waste, cooking, warmth). 

What is source apportionment?

Sources within the city, such as coal burning, vehicle emissions, resuspension of road dust and garbage burning are significant contributors to pollution in Delhi. In winter, in addition to garbage burning, biomass burning for cooking and for keeping warm is a major threat to air quality. To avoid freezing, people without access to electric and gas heaters resort to burning not just wood, but also plastic, rubber, cloth, etc. These are even more hazardous than wood and agricultural wate burning as they release significant toxins and heavy metals in the air.

Meteorology of Delhi

Delhi experiences extremes in climate – very hot summers and cold winters. It is a landlocked city, and hence cannot rely on breeze from the sea to carry away pollutants. A characteristic of these extremes is that the inversion layer is high in summer, but significantly lower in winter. What this means is that, emissions in winter are more concentrated because they cannot get distributed high into the atmosphere. As the graph below shows, on average mixing layer heights are almost twice as high in summer months as compared to winter. Winds are also much lower strength in winter, and hence any pollution that is created tends to stay for longer.

Role of meteorology in the seasonality of air pollution in Delhi

Air pollution emissions tend to be higher in the winters because of increased burning of bio-mass across sectors. Watchmen, pavement dwellers, slum dwellers do not have a choice but to burn wood or any item to keep warm or for cooking, this is the peak season for brick kiln operations on the outskirts of the city, and agricultural waste burning. Black dots in the map below shows the location of brick kilns around Delhi. There are 1,000 large units mapped in graph and there are more in the outskirts. All these emissions are thus concentrated in a small space and stay around for long – thus causing a lot of pollution.

The night time pollution levels in Delhi are worse compared to the daytime. This is primarily due to all the trucks passing through the city at night (as well as meteorological conditions, explained above). And even when the trucks stop operating after 6 AM, it takes time for the pollution dissipate.

Air pollution in the city is extremely high compared to international, and our own ambient, standards. Data on pollution levels over the past years shows that the situation has deteriorated. This has happened despite a fairly successful CNG conversion programme in the early 2000s. Part of the reason for this is because transport accounts for only a fifth of air pollution in the city.

We know the areas that need action. These include emissions from power plants and industrial units, waste burning, re-suspension of road dust and dust from construction activities, etc. But at the end of the day, pollution is an externality (a public bad) that cannot be addressed without concerted action from the city and national authorities. This goes beyond just setting emission and ambient standards, monitoring emissions and pollution but also enforcing these for vehicles, industry, waste management and power plants. Pollution in Delhi is a result of multiple sectors and focusing on one alone or blaming on neighbors will not have an impact on the air we breathe.

Health impacts of air pollution in Delhi

A video on On-road exposure on Delhi roads by Joshua Apte.