Experts warn about rise in NO2 levels across Delhi

While Delhi on Friday recorded the fourth consecutive “severe” day under the air quality index (AQI) – a scale that uses a weighted formula biased towards particulate matter (PM) – experts warned that levels of another dangerous toxin in the air, nitrogen dioxide (NO2), have been consistently soaring, even during periods of relatively “low pollution”.

The primary sources of NO2, which causes wheezing, shortness of breath and can lead to respiratory infections, include vehicles, biomass burning, and industrial emissions, according to scientists. Traffic congestion is particularly significant, as it results in idling vehicles that release high levels of NO2.

Data analysed by HT shows that NO2 concentrations have frequently breached safety limits in several regions across the city, especially high-density traffic zones like ITO and Anand Vihar.

Data from Central Pollution Control Board (CPCB) showed the hourly NO2 concentration touched a peak of 494 µg/m3 at ITO, which is among Delhi’s busiest traffic intersections. This was followed by an hourly peak of 489µg/m3 at east Delhi’s Anand Vihar, another traffic hub comprising of an ISBT, a metro station and a railway station.

Not too far behind was west Delhi’s Pusa, which touched a peak of 447µg/m3.

These figures are nearly six times the national safety standard of 80µg/m3, with the World Health Organization’s (WHO) limit being far stricter at 25µg/m3. In other words, if you’re at ITO or Anand Vihar during poor traffic, you are inhaling as much as six times the safe limit of NO2.

December’s average NO2 level at ITO was 165µg/m3, while Anand Vihar recorded 161µg/m3 — both double CPCB’s safety threshold. Even areas with moderate traffic, such as North Campus DU and Punjabi Bagh, showed peaks exceeding 200µg/m3, underscoring the widespread nature of the problem.

Data indicates that the location of monitoring stations and traffic patterns play a significant role in NO2 distribution. At least 17 more stations out of Delhi’s 40 ambient air quality stations crossed a peak of 200µg/m3, including North Campus DU (350µg/m3), Punjabi Bagh (298µg/m3), Dwarka’s Sector 8 (261µg/m3), Wazirpur (257µg/m3) and CRRI Mathura road (253µg/m3).

Experts said ITO was exhibiting high NO2 levels even before Delhi saw its first smog spell from November 13 onwards, indicating NO2 peaks don’t necessarily correlate with PM2.5 and PM10 levels.

“We have seen ITO consistently record high NO2 levels, a fairly busy intersection. The station is also located next to the road and is likely to capture a significant chunk of the emission released by vehicles, which is what people on the roadside are also likely inhaling,” said Sunil Dahiya, an analyst at the think-tank Envirocatalysts, stating in addition to PM2.5 and PM10, agencies need to monitor NO2 levels and take action, particularly at hotspots.

Experts point to traffic congestion as a primary driver of NO2 pollution.

“The most common reason is traffic congestion, which is a source that can easily be fixed. Long-term action will however only occur when we reduce the vehicles on the road and make a transition not just to cleaner fuels, but also to cleaner public transport,” he adds, stating with Stage-3 and Stage-4 restrictions of the Graded Response Action Plan (Grap) rolling out, should help bring down NO2.

Immediate solutions like reducing congestion and expanding public transportation could mitigate the crisis while transitioning to cleaner fuels remain a long-term goal.

Delhi’s road network, burdened by 7.9 million registered vehicles and an influx from neighbouring cities, underscores the scale of the challenge.

Anumita Roychowdhury, executive director, of research and advocacy at the Centre for Science and Environment (CSE) said that vehicles contribute up to 81% of Delhi’s total NO2 pollution, making them primary sources that need tackling. “While we see congestion through the year, particularly at intersections like ITO, calm wind conditions in the winter period mean the gas can stay trapped and build up over time,” she said.

While transitioning to cleaner fuels and expanding public transportation are necessary long-term strategies, immediate interventions like reducing road congestion could yield quicker results. Without urgent action, Delhi’s residents will continue to bear the brunt of this harmful pollutant, compounding the city’s already dire air quality crisis.