Council on Energy, Environment and Water Integrated | International | Independent
In 2024, India’s capital city of Delhi breathed some of its cleanest air in recent years due to an above-average monsoon rainfall. The Air Quality Index (AQI) dropped to as low as 50-60 on some days. The city enjoyed 27 consecutive days of satisfactory (AQI < 100) air quality during July-August this year, the longest streak in the last four years. However, with winter approaching, the threat of smoky skies looms large, as the city is notorious for its poor air quality during this season. Delhi has already seen a glimpse of deteriorating air quality, recording ‘Very Poor’ AQI (>300) for seven out of the last eight days. This winter pollution comes from local and cross-border sources such as construction, demolition waste, stubble burning, and transportation. Apart from emission sources, meteorological factors like temperature, wind speed, and planetary boundary layer height – which decides the extent to which pollutants mix vertically in the atmosphere – also affect air quality. A significant proportion of the emissions comes from biomass burning which increases as temperatures drop below tolerable levels. A 2023 study by IIT Kanpur attributed 36 and 21 per cent of the PM2.5 concentration in Delhi in November and December 2022, respectively, to biomass burning.
As the World Meteorological Organization (WMO) forecasts a 55 per cent chance of a La Niña onset this winter and the India Meteorological Department (IMD) warns of a harsher winter, we need to consider its potential impact on the severity of Delhi’s winter and whether it could lead to an increase in biomass burning. But first, what exactly is La Niña?
Figure 1: Effect of meteorology on air quality
La Niña and El Niño are two opposite phases of the El Niño Southern Oscillation (ENSO), a climate phenomenon that occurs in the Pacific Ocean around the Equator. During El Niño, the region of the Pacific Ocean between Indonesia and South America is warmer than usual, while La Niña is marked by cooler-than-average sea surface temperatures in the same region. These phases influence weather patterns around the world, including in India, where La Niña typically brings more rainfall while El Niño leads to dry spells during the monsoon season. Historically, La Niña winters are also colder than El Niño winters in the Indo-Gangetic Plain (IGP), which includes Delhi, and colder winters tend to worsen the trapping of pollutants near the land surface.
At CEEW, we analysed meteorological data from the last 35 winters (November to February) over Delhi, covering 10 El Niño and 14 La Niña years between 1990 and 2024. We focussed on surface temperature, surface wind speed, and the planetary boundary layer (PBL) height, which are the key parameters influencing air quality.
The results show that La Niña winters are colder, on average, with 47 colder days compared to El Niño years between 16 November and 10 January, as shown in Figure 2. Specifically, nighttime temperatures between 7 pm and 9 am are on average 0.3°C lower during La Niña winters as shown in Figure 3. These colder nights could lead to more biomass burning to stay warm. Additionally, the Planetary Boundary Layer Height (PBLH), which is the lowest layer of the atmosphere, and is directly influenced by the Earth’s surface through land-atmosphere interactions, is 33.7 meters lower on average through November and December during La Niña. A lower PBLH means pollutants are more likely to be trapped near the ground. The average wind speeds in La Niña are generally higher by 0.1 m/s throughout the day, as shown in Figure 4. Higher wind speeds could potentially help in dispersing the pollutants generated by biomass burning at night and counter the effects of lower temperatures and PBLH.
Figure 2
Figure 3
Figure 4
Upon examining air quality data from the past four winters, we observed a positive impact of higher wind speeds during La Niña. We tracked the number of ‘Very Poor’ and ‘Severe’ air quality days for each winter month from 2020 to 2024 (Figure 5). While 2020-2021, 2021-2022, and 2022-2023 were La Niña winters, 2023-2024 was an El Niño winter.
Figure 5
The lower number of ‘Very Poor’ and ‘Severe’ days during La Niña winters can be attributed, at least in part, to higher wind speeds. A 2023 study also attributed the reduced levels of PM2.5 in Delhi during winter 2022-2023 to higher wind speeds due to a third consecutive La Niña.
Although La Niña meteorological conditions are somewhat more favourable for pollutant dispersion than El Niño conditions, particulate matter concentrations in Delhi remain significantly above the National Ambient Air Quality Standards (NAAQS) of 40 μg/m³ for PM2.5 and 60 μg/m³ for PM10. Therefore, we cannot rely solely on meteorological factors to improve air quality without taking targeted measures to curb pollution at its sources.
Photo: People keeping themselves warm by burning biomass and waste at Chandni Chowk, Delhi.
As biomass burning during winter months contributes to around 25–30 per cent of PM2.5 pollution, we identified hotspots in Delhi, which will facilitate a more targeted approach towards managing air quality. We identified these hotspots using PM2.5 - CO ratio for the winter of 2023-2024 as a proxy for biomass burning. A high PM2.5-CO ratio is an indicator of secondary PM2.5 which forms from precursor gases released while burning biomass. We observed that CRRI Mathura Road, Nehru Nagar, and ITO CAAQMS (Continuous Ambient Air Quality Monitoring Stations) showed a higher value of the ratio throughout the 2023 winter months (November, December, and January). We estimated the ratio of PM2.5 and CO for each hour of the day, averaging it across all days of the month separately for all available CAAQMS stations in Delhi. We then calculated the maximum value of each station for each month. Finally, we identified the stations with maximum values above the 75th percentile. Figure 6 shows the 18 hotspots identified in Delhi for November, December, and January.
Figure 6: Biomass burning hotspots in Delhi during winter months
The Commission for Air Quality Management in the National Capital Region and Adjoining Areas (CAQM) has proactively taken steps to manage air pollution and issued revised guidelines to implement the Graded Response Action Plan (GRAP) from 1 October. Also, the Government of the National Capital Territory of Delhi (GNCTD) recently announced its 21-point Winter Action Plan. The plan includes measures like promoting work from home for office employees, monitoring hotspots, and prevention of open burning of waste. Alongside vigilance, GNCTD also runs shelter homes to accommodate the homeless, who would otherwise have to resort to biomass burning to combat the cold. Delhi Urban Shelter Improvement Board (DUSIB) currently has 343 permanent and temporary shelter homes, which can accommodate 20,000 people. However, the 2011 Census reports the number of homeless people in Delhi to be 46,724, calling for a requirement for additional shelters.
Based on our analysis, we recommend the following steps to ensure targeted implementation of the pre-existing measures:
While La Niña conditions bring a mix of both challenges and opportunities for Delhi’s air quality, a proactive approach is essential to tackling pollution. The cold will leave the homeless with no option but to burn biomass and waste to get some warmth. Therefore, providing adequate shelters this winter should be a priority. Security guards working during night time will also require open burning to stay warm. Residents Welfare Associations (RWAs) should adhere to the CAQM’s orders and provide them with electric heaters. By combining these measures with the inherent advantages of higher wind speeds during La Niña, Delhi can make meaningful strides towards better air quality this winter. Collective and sustained efforts from the government, community, and other stakeholders are crucial to achieving cleaner, healthier air for all.
Shubhi Verma is a Research Analyst, Shikhar Tiwari and Rishikesh P. are Consultants at the Council on Energy, Environment and Water (CEEW). Send your comments to [email protected]
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