Why Delhi AQI Remains High Even When Pollution Sources Are Restricted
Every winter, Delhi introduces pollution-control measures under the Graded Response Action Plan (GRAP), often restricting construction activities, certain vehicles, diesel generators and other emission sources. Yet Delhi’s Air Quality Index (AQI) frequently remains in the ‘Very Poor’ or ‘Severe’ category.
Why does Delhi’s air pollution persist despite these restrictions?
Pollution concentrations depend on more than emissions occurring at a particular moment. Pollutants already present in the atmosphere, regional transport, chemical reactions and winter weather all influence Delhi’s air quality. Restrictions can reduce additional emissions, but they cannot immediately clear accumulated pollution.
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Why Does Delhi AQI Remain High
Despite Pollution Restrictions?
Reducing emissions and lowering existing pollution concentrations are two different processes. When construction stops or vehicle restrictions begin, pollutants already present in the atmosphere do not disappear immediately. They remain until they disperse, settle or are removed through rainfall.
Meanwhile, vehicles that continue to operate, industrial activities, residential fuel combustion and pollution sources outside Delhi continue releasing pollutants. During winter, weak winds and limited atmospheric mixing make it harder for these pollutants to disperse.
As a result, restrictions can limit additional emissions without producing an immediate improvement in AQI. Their effectiveness also depends on when they are implemented, how consistently they are enforced and what proportion of total emissions they actually control.
Delhi Air Pollution Extends
Beyond Administrative Boundaries
Delhi’s location within the Indo-Gangetic Plain exposes it to pollution transported from neighbouring regions, including Punjab, Haryana and parts of Uttar Pradesh. Winds carry pollutants across state boundaries, making Delhi’s air pollution a regional challenge that requires coordinated action beyond the city.
A 2026 study published in npj Clean Air examined how the geographical coverage of emission controls affects Delhi’s PM2.5 concentrations.
Researchers found that restricting crop-residue-burning controls to Delhi-NCR reduced modelled PM2.5 concentrations in Delhi by approximately 2–3%. Extending the same controls across the wider airshed, covering Punjab, Haryana and Delhi-NCR, increased the modelled reduction to approximately 10%.
Similarly, a 2026 CSTEP report, Beyond State Boundaries, estimated that approximately 39% of NCR’s annual PM2.5 pollution originates from other regions of India. The report emphasises the need for coordinated, multi-sector emission controls across the Indo-Gangetic Plain, rather than relying exclusively on city-level interventions.
Together, these results underline why controlling emissions within Delhi alone is insufficient. Addressing pollution transported from upwind regions requires coordinated emission-reduction measures across the wider airshed.
How Winter Weather Traps Pollution Over Delhi
Delhi’s winter air pollution depends not only on emission levels but also on the atmosphere’s ability to disperse pollutants.
During winter, lower temperatures, weak winds and atmospheric stability favour pollution accumulation.
| Environmental & Meteorological Factor | Impact on Air Quality |
|---|---|
| Temperature Inversion | Restricts vertical mixing, trapping pollutants near the surface. |
| Low Wind Speed | Limits horizontal dispersion, allowing pollutants to accumulate locally. |
| Planetary Boundary Layer Variability (Mixing Height) | A shallow mixing layer reduces the volume of air available to dilute pollutants, increasing concentrations near the ground. |
| Atmospheric Stagnation | Allows pollution to accumulate over several days due to weak winds and limited atmospheric mixing. |
| Limited Rainfall | Reduces the removal of airborne particles through precipitation. |
| Land Use/Land Cover (LULC) | Dense urban development and lack of vegetation block regional wind corridors, trapping vehicular and industrial emissions close to the ground. |
Under these conditions, even reduced emissions can sustain high PM2.5 concentrations. Conversely, stronger winds or rainfall can improve Delhi AQI without major changes in pollution-control policies.
Weather does not create most emissions, but it strongly influences how long pollutants remain concentrated near the surface.
What Are the Major Sources of Delhi's PM2.5 Pollution?
Delhi’s pollution burden comes from multiple activities, including transport, biomass burning, construction and road dust, industrial operations, power plants and residential fuel combustion.
However, not all PM2.5 is emitted directly into the atmosphere. Gases such as nitrogen oxides (NOx), sulphur dioxide (SO₂), ammonia and volatile organic compounds (VOCs) can undergo atmospheric chemical reactions to form secondary particulate matter, adding to PM2.5 concentrations.
A CAQM-reviewed expert meta-analysis released in February 2026, covering source-apportionment studies conducted between 2015 and 2025, identified the following major contributors to Delhi’s winter PM2.5 pollution:
Secondary particulates form when precursor gases released by several sectors undergo atmospheric reactions. Their contribution is therefore linked to emissions from transport, industries, power plants and biomass burning. They should not be interpreted as a completely separate emission source.
The analysis underlines why controlling visible dust or smoke alone cannot eliminate Delhi’s PM2.5 pollution. Effective control also requires reducing gaseous emissions that contribute to secondary particulate formation.
Why Reduced Stubble Burning
Does Not Always Lower Delhi AQI
Stubble burning contributes to seasonal pollution, but declining farm-fire activity does not guarantee cleaner air.
A December 2025 analysis by the Centre for Science and Environment (CSE) found that average PM2.5 concentrations increased from 163 µg/m³ during October–November to 210 µg/m³ during December 1–28, a rise of approximately 29%.
During the same period, the estimated contribution of farm fires to Delhi’s PM2.5 declined from an average of 4.2% to just 0.2%.
This shows how other emissions, secondary particulate formation and stagnant winter conditions can sustain severe pollution even after a major seasonal contributor diminishes.
How GRAP Works and Why Restrictions Alone Are Not Enough
The Graded Response Action Plan (GRAP) is an emergency pollution-control framework implemented across Delhi-NCR by the Commission for Air Quality Management (CAQM).
CAQM coordinates its implementation with state governments, pollution control boards and other agencies. The plan has four stages, with restrictions becoming progressively stricter as Delhi’s air quality deteriorates. Decisions to invoke stages also take weather and air-quality forecasts into account.
The revised GRAP schedule approved in September 2026 includes four stages:
| GRAP Stage | Delhi AQI | Major Measures |
|---|---|---|
| Stage I – Poor | 201–300 | Mechanised road sweeping, water sprinkling, waste management and construction-dust control |
| Stage II – Very Poor | 301–400 | Staggered office timings, stricter DG-set regulation and measures to discourage private vehicle use |
| Stage III – Severe | 401–450 | Restrictions on specified vehicles and construction and demolition activities |
| Stage IV – Severe+ | Above 450 | Restrictions on entry of specified trucks and additional emergency measures |
Measures introduced at earlier stages continue when a higher stage is invoked. The September 2026 revision also brought certain pollution-control measures into effect at earlier stages, strengthening the response as air quality begins to deteriorate.
However, GRAP primarily addresses pollution during deteriorating air-quality episodes. It cannot replace sustained reductions in emissions from transport, industry, road dust, waste burning and other year-round sources.
Delhi’s pollution-control strategy therefore needs to go beyond emergency restrictions. It requires consistent emission reductions, coordination across the wider airshed and regular evaluation of whether pollution-control measures are delivering measurable improvements.
How Aurassure Supports Data-Driven
Air Quality Intelligence
AQI indicates pollution severity but cannot independently explain why concentrations are rising or whether restrictions are working. Authorities need continuous, location-specific air quality data supported by meteorological observations and historical trends.
Aurassure Infra supports outdoor air quality monitoring through configurable sensors for PM2.5, PM10, gaseous pollutants (NO₂, SO₂, NO, CO, O₃ and CO₂) and environmental parameters such as temperature, humidity, wind speed and direction.
Through its connected monitoring platform, real-time data, historical trends and alerts can help authorities:
- Identify pollution hotspots: Track variations across traffic corridors, industrial zones and residential areas.
- Assess interventions: Compare pollution concentrations before and after restrictions.
- Understand pollutant dispersion: Examine pollution trends alongside local wind and weather conditions.
- Prioritise action: Identify locations requiring closer investigation or targeted pollution-control measures.
When combined with emissions inventories, source-apportionment studies and meteorological analysis, hyperlocal monitoring can support more informed decisions on managing Delhi’s air pollution.
Explore Aurassure Infra – Real-Time Ambient Air Quality Monitoring
Conclusion:
Delhi’s persistent AQI spotlights a critical limitation of short-term pollution controls: restricting emissions during severe episodes cannot substitute for reducing pollution throughout the year.
A more effective approach requires coordinated action across the Delhi-NCR airshed, sustained reductions in major emission sources and continuous assessment of pollution-control measures. By combining air quality data with meteorological insights, authorities can better understand changing pollution patterns and evaluate the impact of their interventions.
Progress should be measured not by the number of restrictions imposed, but by sustained improvements in air quality and reduced public exposure to pollution.
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Author
Mrinali Behera
Designer
Soumyajyoti
