Beyond Stubble Burning: Why Delhi’s Pollution Peaks in Winter Months

In our previous post*, we examined the major contributors to Delhi's air crisis, and traced how the Green Revolution's success story - increasing India's cereal yield from 947 kg/hectare in 1960 to over 3,500 kg/hectare by 2022 - inadvertently led to the practice of stubble burning. Today, we delve deeper into how stubble burning, coupled with co-occurring factors and seasonal weather patterns, jointly impact Delhi's air quality.

Stubble burning in Punjab and Haryana occurs during October-November, following the Kharif season’s rice harvest. This prepares fields for sowing wheat, but the smoke from burning 20-25 million tonnes (annually) of paddy residue travels to Delhi, adding to its pollution woes.

Data from the Ministry of Environment, Forest and Climate Change shows a clear spike in Delhi’s monthly average AQI during October-November. Between 2018 and 2023, October averaged an AQI of 219, while November's average rose to 340, with multiple days falling into the severe category (400+).

All pollution sources - domestic heating, vehicles, industries, construction, and stubble burning - see their impact amplified during winter months.

Co-occurring Factors

While stubble burning contributes significantly, other factors amplify the pollution. Diwali celebrations, for instance, cause an average AQI increase of 92 points (34%) between pre- and post- festival days. However, a more revealing comparison arises when we analyze the October-November block (average AQI: 284) against the January-February block (average AQI: 278)—highlighting remarkably similar pollution levels, even though stubble burning does not occur during the latter period.

January 2024 saw Delhi experience its coldest month in 13 years, with average maximum temperatures at 17.7°C and minimums at 6.2°C. The month recorded three severely polluted days, including its worst AQI of 447 on January 14, and 26 very poor air quality days. These statistics are strikingly comparable to those observed during the peak stubble-burning season in October-November, underscoring the multifaceted nature of Delhi’s air pollution problem.

The cold air trap, combined with near-zero rainfall, creates perfect conditions for pollution accumulation.

Understanding Pollution Persistence

While pollutants from stubble burning typically persist for about a week, they can trigger secondary aerosol formation - a process where gaseous pollutants like sulphur dioxide transform into particulate matter. Research shows these secondary aerosols contribute approximately 25% to PM2.5 levels across the year.

Such gases are, in fact, released more extensively from thermal power plants - which emit about 7 times more sulphur dioxide annually compared to stubble burning in the neighboring states. These secondary pollutants, formed through atmospheric reactions, typically persist for only up to a month, and thereby do not exert any influence on January-February AQI levels.

The Winter Amplification Effect

The key to understanding Delhi's sustained winter pollution lies in "mixing height" - the vertical space available over the ground for pollutant dispersion. During summer, mixing heights are significantly higher, providing more room for pollutants to spread. For instance, in July, daytime mixing heights reach approximately 2000 meters, with nighttime levels around 100 meters.

In contrast, January sees daytime heights of about 900 meters and nighttime levels of 80-90 meters, while November shows daytime heights of 1200 meters but drops to just 40-50 meters at night. This creates a lid effect during the cooler months, trapping pollutants close to the ground.

Consequently, all pollution sources - domestic heating, vehicles, industries, construction, and stubble burning - see their impact amplified during winter months. The cold air trap, combined with near-zero rainfall (Delhi recorded 100% rain deficit in this past Dec-Jan), creates perfect conditions for pollution accumulation.

As Delhi grapples with toxic air during its cooler months, it is vital to address the interplay of multiple pollution sources and seasonal weather factors. The health of millions depends on effective interventions, from curbing emissions at the source to implementing long-term urban planning strategies. In upcoming posts, we’ll explore potential solutions—both medium- and long-term—that can help Delhi breathe easier.

#DelhiAirCrisis #StubbleBurning #WinterPollution #AirPollution #PM25 #PublicHealth #CleanAir #SustainableLiving #AQI #GRAP4 #DelhiAirPollution

*Also see https://x.com/swaminathankp/status/1859205337934672073