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This assessment is based on historical AQI datasets, hourly measurements, seasonal and diurnal patterns, real-time source apportionment studies, institutional research, and global examples. These datasets show that, although Delhi has strengthened its air quality policies over the past two decades, emissions have expanded faster than the response has been able to keep pace. This imbalance has led to a sustained decline in overall air quality.
A central question in understanding the current challenge is when this decline began. Long-term AQI and PM2.5 data indicate a clear inflexion between 2009 and 2011. This period marks the point at which Delhi's air pollution shifted from being episodic to becoming structurally embedded.
Pollution levels were high before 2010 but responsive to major interventions. The early and mid-2000s saw measurable gains following actions such as the transition to CNG in public transportation and the relocation of several polluting industries. Severe episodes were fewer and shorter, and baseline air quality outside winter remained stable enough for atmospheric dispersion to reset conditions periodically.
The pattern shifted around 2009-2011. Regulatory measures continued and were later strengthened, yet pollution continued to show no improvement. Winter peaks intensified; recovery periods shortened, and baseline PM2.5 levels rose. By 2013, deterioration had become persistent, with high pollution lasting from October to January in several years. This period coincided with rapid motorisation, increased construction activity, growth in freight movement, rising energy demand, and accelerated urbanisation across NCR. These shifts significantly expanded emissions, counterbalancing the efficiency gains from cleaner fuels and technologies.
Historical AQI data support this structural change. The number of days categorised as Poor, Very Poor and Severe increased after 2011 and rose sharply after 2014. Before 2010, Severe days were relatively rare and usually limited to short winter episodes. In recent years, the Delhi–NCR region has consistently recorded more than 100 days annually with an AQI above the Poor category.
Winter season data offers further evidence. Between November and January, Delhi now experiences 30 to 40 days of 'Very Poor' or 'Severe' air quality. The early 2000s recorded far fewer such days. December has become the most consistently polluted month, with average AQI levels remaining in the "Very Poor" category for extended periods.
These patterns indicate that Delhi's current air quality situation is linked to systemic changes in emissions, urban activity, and regional pollution movement that began around 2009-2011.
Source apportionment studies show a significant shift in the composition of Delhi's particulate pollution. Secondary inorganic aerosols formed from nitrogen oxides, sulphur dioxide and ammonia now contribute 30 to 35% of winter PM2.5 levels. Local primary emissions drove earlier pollution profiles more heavily.
A significant proportion of these secondary aerosols originates outside Delhi. Regional wind patterns transport precursor emissions into the city, and studies show that during winter, 80 to 85% of secondary aerosol formation affecting Delhi comes from outside its boundary. This shift explains why improvements in fuel quality and vehicle emissions standards have not resulted in proportional improvements in AQI. These reforms mainly address local emissions, while precursor gases continue to form secondary particulates at the regional level.
Across multiple datasets, winter particulate pollution in Delhi is driven by a wide range of sources. These include secondary aerosols, transport emissions, biomass and stubble burning, road dust, construction dust, industrial activity, coal and fly ash, waste burning, residential fuels and pollution transported from neighbouring NCR regions. No single source dominates strongly enough to act as a sole intervention point. Coordinated action across sectors and states is essential.
Governments have launched several measures over the past decade. These include cleaner fuel standards, tighter vehicle norms, electric mobility incentives, construction and demolition of waste rules, bans on waste burning, stubble management schemes, and graded response action plans. The Supreme Court's desulphurisation mandate sought to address regional sulphur emissions linked to secondary pollution. Delays in implementation limited its impact on winter outcomes.
Despite these measures, several gaps remain. Enforcement is uneven, particularly for diffuse sources such as construction, road dust, and informal waste burning. Many interventions operate at a scale too small to match the volume of emissions. Regional coordination is limited, despite the fact that cross-border pollution significantly contributes to Delhi's poor air quality.
Global examples show the impact of sustained and large-scale efforts. Beijing improved particulate pollution levels through coal-to-gas transitions, industrial relocation and coordinated regional action. London has recorded improvements through the implementation of low-emission zones, supported by effective enforcement. Both examples demonstrate the importance of behavioural change in conjunction with regulation and technology.
In the Delhi–NCR region, individual and institutional behaviours influence several major pollution drivers. These include the use of private vehicles, burning waste, and improper construction practices. Strong outcomes require consistent public participation. A mass civic mobilisation for clean air would strengthen the effectiveness of policy and technology-led interventions.
A recent enabling development is the World Bank's approval of nearly USD 600 million for clean air programmes in Uttar Pradesh and Haryana in December 2025. These programmes support airshed-level action that targets sources outside Delhi but have a strong influence on its winter pollution.
Delhi's data and global experience indicate that substantial AQI improvement is possible when interventions focus on high-impact levers and are executed in parallel. These include the rapid electrification of public transport and freight, large-scale mechanised dust management, strong technology-based enforcement at thermal plants and construction sites, accelerated reduction of coal and diesel generator use across NCR, decentralised waste processing and formal regional airshed governance.
Delhi's air pollution challenge is identifiable and addressable. The data shows when the deterioration intensified, why measures have had limited impact and which actions can deliver improvement. With coordinated, region-wide interventions, visible AQI improvements are achievable within two to three years, with long-term benefits following.
A data-driven point of view
