Delhi, Patna face worst PM2.5 pollution in winter as weather traps emissions: Report

Cities located in the Indo-Gangetic Plain, including Delhi and Patna, continue to record the highest PM2.5 pollution levels in the country, with winter emerging as the most critical season due to low wind speed, high humidity, and atmospheric stagnation that trap pollutants near the surface, according to a new analysis by Climate Trends.

The study, based on CPCB air quality data for 2024–2025, highlights that meteorological conditions play a major role in determining pollution intensity and can significantly worsen air quality even without any increase in emissions. 

Between January 2025 and January 2026, the Graded Response Action Plan (GRAP) was imposed 17 times in Delhi to control severe air pollution, with stricter curbs remaining in force for long periods during the winter season.

Stage III restrictions, which are applied when air quality reaches the “severe” category, were in place for about 53 days, while the most stringent Stage IV measures lasted for around 15 days. GRAP is not enforced for a fixed duration but is triggered by the Commission for Air Quality Management (CAQM) based on air quality forecasts.

Under the system, Stage I applies when AQI is between 201–300, Stage II for 301–400, Stage III for 401–450, and Stage IV when AQI crosses 450, with higher stages involving construction bans, curbs on vehicles and other emergency steps.

Winter stagnation

Researchers found that northern cities experience severe pollution episodes during winter because of weak winds, temperature inversions, and reduced atmospheric mixing. These conditions prevent pollutants from dispersing, leading to prolonged periods of hazardous air quality.

Delhi continues to record the highest annual PM2.5 levels among major cities, while Patna remains the second-most polluted, with both cities frequently exceeding national air quality standards during winter months. The report notes that the Indo-Gangetic Plain is particularly vulnerable because of its geography and seasonal weather patterns that favour pollutant accumulation. 

Weather change

According to the analysis, meteorological conditions alone can shift pollution levels by as much as 40%, meaning air quality trends cannot be assessed solely on the basis of emission changes.

Low wind speed, high relative humidity, and stable atmospheric conditions are identified as the main factors that intensify PM2.5 levels, especially in winter and post-monsoon periods. These conditions allow pollutants from vehicles, industries, construction dust, and biomass burning to remain concentrated near the ground for longer durations. 

Northern cities

The report states that a large share of days in Delhi and Patna fall under low-ventilation weather regimes, making these cities structurally prone to high pollution levels. Even when emissions fall, stagnant weather can still result in severe air quality episodes.

In contrast, coastal and southern cities such as Bengaluru and Chennai generally record lower and more stable PM2.5 levels because stronger winds and marine influence help disperse pollutants more effectively. However, the study notes that some of these cities are also showing signs of winter deterioration, indicating a wider seasonal risk. 

NCAP Phase-III

The findings have significant implications for the upcoming Phase-III of the National Clean Air Programme (NCAP). The report recommends that air-quality targets should be evaluated using meteorology-adjusted methods instead of relying only on annual averages.

Researchers have called for winter-specific action plans, dynamic weather-triggered interventions, and regional airshed-level coordination to address pollution in stagnation-prone regions such as the Indo-Gangetic Plain.

According to the study, without incorporating the role of weather into policy planning, improvements in annual pollution data may continue to hide the severe winter exposure faced by millions of people in northern India.