Why is Delhi’s air quality deteriorating? | Explained

What factors contribute to the deterioration of air quality in Delhi during the winter months? How does stubble burning affect PM 2.5 levels? What role do urban emissions from vehicles play in determining air quality? Why is a coordinated approach necessary to tackle the air pollution crisis?

The story so far:

With the withdrawal of the southwest monsoon and the onset of winter, the air quality in Delhi has started to nose-dive. This week, the city and its adjoining territories have consistently recorded particulate matter (PM 2.5) levels exceeding 300, or ‘very poor’ air quality, and forecasts suggest that this could worsen in the coming days. As is now an established pattern, the decline in air quality coincides with the burning of farm stubble, primarily from Punjab.

What is the contribution of stubble burning to air pollution?

Stubble burning refers to a traditional practice of farmers burning the remnants of paddy stalks after harvesting. This method is often the quickest way, as farmers in Punjab and Haryana have a narrow window of October and November to clear their fields and sow wheat for the winter. The environmental impact of stubble burning has been known since the 1990s. Agricultural researchers, while analysing the economics of rice-wheat cropping pointed out that rising labour costs made it expensive for farmers to collect rice stalks strewn across the field that resulted from the use of mechanical devices such as rice shredders and combine harvesters. Though the burning of rice stalk was initially condemned as a waste of valuable manure, concerns were also raised about its harm to farmers’ health. However, its link to worsening air quality in Delhi was quantified only over the last decade and a half. Today, the use of sophisticated instruments, modelling studies, and computational methods have enabled the estimation, almost daily, of stubble burning’s contribution to air quality in Delhi.

A study by the research and advocacy group Climate Trends of winter pollution trends in 2023 found a “strong correlation” between wind direction originating from Punjab and Haryana and the resulting pollution levels in Delhi. In the case of Punjab, during winter, 54% of the time the wind from the State blew towards Delhi, it led to a spike in air pollution; when the wind originated from Haryana, the figure stood at 27%. Every additional fire incident was correlated with an increase in PM2.5 levels of 12.44 units. Studies over the years, most recently in 2023 by a consortium of IIT Kanpur, IIT Delhi, TERI, and Airshed, Kanpur, found that from mid-October to the end of November 2022, the role of stubble burning to air quality was on average 22% and peaked to as much as 35%. This is fairly consistent with previous studies that have estimated the contribution of stubble burning to range from 20%- 40%. Based on these measurements, the Indian Institute of Tropical Meteorology-Pune (IITM-Pune) maintains an air quality forecast system that models the flow of airborne pollutants through cities. It shows the dynamic nature of stubble burning’s impact on Delhi’s pollution. For instance, from October 8 to 19 this year, farm fires accounted for less than 1.2% of the PM 2.5 load in Delhi. During this period, the average AQI stayed from 130 -198 (or the ‘moderate’ pollution category). However, on October 21, when stubble burning’s relative contribution rose to 3.2%, Delhi’s AQI immediately plummeted to ‘very poor’ (310). On October 23, when the relative contribution of burning reached the seasonal high of 16%, the index deteriorated to 364, still in the ‘very poor’ region. On October 26, the stubble burning contribution slightly dipped to 14.5%, and the AQI improved to 270 or ‘poor’ quality.

What is the inference from these observations?

The transitioning period from the withdrawal of the monsoon to the onset of winter causes a sharp drop in windspeed, and cause air pollutants to hover closer to the ground rather than being flushed away to the higher realms of the atmosphere. In this situation, any additional source of pollutants — such as from stubble burning — can dramatically spike the pollutant load in Delhi. Also, nearly 55% of the pollution in Delhi originates outside its territorial borders, as studies have shown. Thus, relatively small spikes can push the index as much as 100 points and change categories anywhere from ‘poor’ to ‘very poor.’

So is stubble burning the sole villain in Delhi’s pollution?

Urban Emissions, a research outfit that tracks air pollution trends nationally, reports that from 2016-23, the improving air quality reflected in the index going from a high of 285 in 2017 to a low of 173 in 2021. However, in six of these seven years, the index stayed above 216 and therefore within the AQI categorisation of ‘poor’ air quality. However, in Punjab, the farm fires reduced by over an order of magnitude from 17,467 in 2018 (as of October 25, that year) to 1,749 (October 25) this year. Fire incidents in Haryana too have halved since 2020. The paddy harvested in Punjab is more than twice that in Haryana and yet, this has only improved the index by 65 points at the most. Significantly, December, January, and February are officially considered the winter months by the India Meteorological Department and by this time, the atmospheric conditions that create a trap for pollutants and prevent them from being flushed out, grow stronger. However, stubble burning almost entirely ceases. Despite that, data compiled by Urban Emissions says, the air quality index has consistently remained in the ‘very poor’ and ‘severe’ (400+) category from 2016-23. This suggests that sources other than stubble burning contribute significantly to air pollution.

Which are these factors?

On October 25 this year, stubble burning was responsible for nearly 15% of Delhi’s air pollution. On the same day, ‘Delhi transport’ which includes particulate matter from vehicles and vehicles crossing into Delhi was responsible for about 18% of the PM 2.5 load, according to the IITM’s air quality forecast system. The IIT Kanpur, IIT Delhi, and Teri consortia analysis of the sources of pollution in Delhi found that the real-time source apportionment of PM 2.5 results show secondary inorganic aerosols (SIA), which travel from beyond Delhi, contribute the highest to Delhi’s pollution load. The average of winter pollution source apportionment shows SIA (32% ) and biomass burning within and outside Delhi (24%) contribute the most followed by vehicles at 17%. The SIAs form when gaseous precursors like sulfur dioxide (SO2), nitrogen oxides (NOx), and ammonia (NH3) react to form ammonium sulfate or ammonium nitrate. In winter, the mean contribution of SIA from the sources within Delhi is 16% and the rest 84% from outside Delhi. In the last few years, policymakers have realised that the sources of air pollution can be tackled only via an airshed approach that requires coordinated action by multiple States including those beyond Delhi to evolve a joint response to the pollution crisis.

Published - October 27, 2024 10:53 pm IST