Secondary aerosols and regional sources driving Delhi

Secondary aerosols and regional sources driving Delhi–NCR pollution: Study

By A Representative

Delhi–NCR’s air pollution crisis has shifted from a seasonal challenge to a structurally embedded, year-round problem, according to a new data-driven analysis released by Grant Thornton Bharat. The report states that long-term air quality data show a clear inflexion point between 2009 and 2011, after which pollution levels stopped responding to incremental policy interventions and began deteriorating persistently.

“Delhi's air pollution challenge is no longer episodic or seasonal; it has become structural, persistent, and increasingly resistant to incremental interventions,” the report notes, based on an analysis of historical AQI trends, high-frequency PM2.5 data, and real-time source apportionment studies. While earlier measures such as the conversion of public transport to CNG and the relocation of polluting industries delivered measurable improvements in the early 2000s, the report finds that these gains were overwhelmed after 2010 by rapid motorisation, construction activity, freight movement, and urban expansion across the National Capital Region.

According to the analysis, winter pollution episodes have intensified and expanded in duration, with high pollution now often beginning in October and persisting into January. December has emerged as the most consistently polluted month, with average AQI levels frequently remaining in the “Very Poor” category for extended periods rather than showing brief spikes followed by recovery. The report adds that Delhi now routinely records more than 100 days annually with AQI levels exceeding the “Poor” category, a sharp increase compared to the period before 2010.

The study highlights a significant change in the composition of pollution, noting that secondary inorganic aerosols now account for approximately 30–35% of winter PM2.5 concentrations. It further states that “as much as 80–85% of secondary aerosol formation affecting Delhi can be attributed to sources beyond the city,” indicating that regional emissions play a dominant role during peak pollution months. This shift, the report explains, helps clarify why tighter vehicle emission norms and cleaner fuels have not translated into proportional improvements in overall air quality.

Despite a wide range of policy measures introduced over the past decade, including graded response action plans, bans on waste burning, stubble management schemes and stricter construction norms, the report identifies persistent gaps in enforcement, scale and regional coordination. “Policies underperform when public participation is inconsistent, highlighting the limits of enforcement-only approaches in a dense urban region,” the analysis states.

Drawing comparisons with international case studies such as Beijing and London, the report argues that sustained air quality improvement requires large-scale structural interventions implemented simultaneously, along with behavioural change and strong public participation. It notes that recent multilateral financing for clean air programmes in neighbouring states reflects growing recognition that Delhi’s air quality challenge must be addressed at an airshed level rather than through city-centric measures alone.

The report concludes that Delhi’s air pollution problem is not irreversible, stating that “visible AQI improvements are achievable within two to three years” if high-impact interventions are executed in parallel and supported by coordinated regional governance. However, it cautions that success will depend on aligning the scale of action with the scale of the problem and sustaining momentum beyond short-term emergency responses.

Commenting on the findings, Amit Kumar, Partner and Climate Ecosystem Leader at Grant Thornton Bharat, said sustained improvement requires focused action executed at scale. “Sustained gains in air quality come from doing fewer things at much larger scale and doing them together. Rapid electrification of public transport, disciplined dust management, strict control of waste burning and tighter compliance at high-emission sites must advance in parallel,” he said, adding that “behavioural consistency strengthens these measures, and organisations that convert policy direction into integrated execution frameworks can deliver results within defined timelines and maintain momentum over the longer term.”

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