Ozone emerges as lead air pollutant in Delhi

Ground-level ozone (O3) has started to emerge as Delhi’s lead pollutant during the day regularly over the last few days, particularly after good spells of rain, according to data from the Central Pollution Control Board (CPCB). The presence of ozone has prevented Delhi’s air from touching the “good” category during this period.

CPCB data over the last three weeks (from March 17 to April 6) showed that out of the 21 days, ozone was Delhi’s lead or prominent pollutant on eight days, indicating its concentration was higher than the usual pollutants which are PM 2.5 and PM 10.

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On March 31, when Delhi recorded its lowest 24-hour average AQI for the year at 73, it was still “satisfactory”, with both O3 and carbon monoxide (CO) emerging as the day’s prominent pollutants. The day after, when Delhi’s AQI rose to 106 (moderate), O3 was the sole lead pollutant. Most recently, O3 was also the prominent pollutant alongside PM 2.5 on April 4, when Delhi’s AQI was 109 (moderate).

Meanwhile, PM 2.5 and PM 10 emerged as the prominent pollutants on days when the 24-hour average AQI was high, for instance on April 3, when it was 179 (moderate), or March 24, when it was 205 (poor).

The highly reactive gas is not emitted from any direct source but gets formed in the air when oxides of nitrogen (NOx) combine with other reactive gases. This happens particularly when temperatures are high and direct sunlight is available. Owing to its reactive nature, the gas has 1-hour and 8-hour standards compared to particulate matter (PM) which has a 24-hour standard, because even a short term exposure to the gas can worsen respiratory conditions. These standards are prescribed by the CPCB.

Experts said that while ozone levels remain fairly high throughout the year, its concentration starts to rise from March onwards with levels breaching the safe limit on most days between March and June. Though ozone is present in winter as well, the levels are lower owing to low temperatures, as it needs sharp sun to aid its formation.

“The data shows that when pollution levels are high, we have particulate matter (PM), which is a concern. However, the background emissions of other gases like O3 and CO are still fairly high. After a spell of rain, there is a washout effect where PM settles down and we really begin to see the impact of these gases,” said Anumita Roychowdhury, executive director, research and advocacy, Centre for Science and Environment (CSE).

She said that to achieve clean or good air, a multi-pollutant strategy is required that tackles gases as well and most action plans in Delhi-NCR currently focus on PM alone. “Only once we move beyond particulate matter can we achieve cleaner air, as these gases continue to remain a problem. Ozone in particular does not have a direct source, so we first need to tackle NOx and we can indirectly control ozone formation too.”

An AQI of 50 or lower is classified as “good” by the CPCB, between 51 to 100 is “satisfactory”, between 101 and 200 is “moderate”, between 201 and 300 is “poor”, between 301 and 400 is “very poor” and over 400 is “severe”.

CSE had also analysed Delhi’s ozone levels in March and April 2022 where they found that, on average, 16 air quality monitoring stations were exceeding the O3 norms in Delhi-NCR on a daily basis. The 1-hour limit for O3 in Delhi is 180 micrograms (mcg) per cubic metre (pcm), while the 8-hour limit is 100 mcg pcm. HT looked at the Delhi Pollution Control Committee’s (DPCC) real-time air pollution data and found that since April 1, the highest 1-hour O3 value at Dr Karni Singh Shooting Range was 190.8 mcg pcm on April 3. The highest 8-hour O3 value at the same station was 100.3 mcg pcm on April 5, which is almost at par with the safe standard.

However, the highest value recorded in Delhi over this period was at Nehru Nagar, where the hourly ozone readings touched 320 mcg pcm on April 2, while the 8-hour average peaked at 131.6 mcg pcm on April 5.

The average O3 levels also saw a 33% rise in 2022 compared to the corresponding period from 2021. The analysis also found that neighbourhoods in south and New Delhi districts were the most affected.

“Dr K S Shooting Range in south Delhi is the most chronically affected in Delhi-NCR. This is followed by JLN Stadium, RK Puram and Nehru Nagar in New Delhi,” CSE said in its analysis in June 2022.

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Professor Mukesh Khare from the department of civil engineering at IIT Delhi said while PM 2.5 has been considered a pollutant based on which most policies and pollution-mitigation strategies have been designed in the last decade, it is now time to focus on other pollutants too, particularly different gases. “We need to control oxides of nitrogen first which emanate from vehicles and industries. CO is a combustion source that is equally dangerous. While we know the impact PM 2.5 has on human health, both CO and O3 can cause dangerous, respiratory problems and even suffocation in an open environment,” said Prof Khare.