Why ozone prominent pollutant over past two days in Delhi

At Dr Karni Singh Shooting Range in South Delhi, ozone level at 12 pm on May 5 was 190.4 µg/m3 while at Nehru Nagar, also in South Delhi, it was even higher at 201.1 µg/m3 at the same time. Ozone concentration at Nehru Nagar remained above 180 µg/m3 from noon to 6 pm, when it fell to 168 µg/m3, hourly data from Delhi Pollution Control Committee shows. Similarly, at Patparganj in East Delhi, ozone levels remained above 180 µg/m3 from 1 pm to 5 pm on Friday, peaking at 190.8 µg/m3 at 4 pm, DPCC data shows.

Ozone is formed when oxides of nitrogen and volatile organic compounds react in the presence of sunlight, making it a key pollutant in Delhi in the summer. As per the WHO, “Exposure to excessive ozone can cause problems breathing, trigger asthma, reduce lung function and lead to lung disease.” Oxides of nitrogen are released by combustion of fuels such as in vehicles and industries.

On Friday, among places in NCR, the CPCB bulletin identified ozone as a prominent pollutant in Ghaziabad and Bulandshahr. Similarly, on Thursday, when Delhi recorded an AQI of 113 (moderate), the bulletin identified ozone as the prominent pollutant in Greater Noida and Ghaziabad, among locations in NCR.

The National Ambient Air Quality Standards sets hourly and 8-hourly standards for only 2 out of 12 air pollutants — ozone and carbon monoxide — as opposed to 24-hour and annual standards for other pollutants. Dipankar Saha, former head of CPCB’s air laboratory, said, “Short-term standards are measured as short-term exposure can also sometimes be harmful on account of oxidative potential of ozone, and possible impact on respiratory system. In summer, ozone is a prominent pollutant, but only during the day, as it depends on solar radiation.” Adding that as there was a good quantity of rain recently, Saha said ozone formation is unlikely: “But when sunlight is direct, it allows reactions with oxides of nitrogen contained in emissions.”

Clear skies are likely in Delhi over the week ahead, and the temperature is set to rise to around 39 degrees Celsius by May 12.

On five days in April this year, the CPCB’s daily bulletin identified ozone as a prominent pollutant in Delhi, either by itself or along with other pollutants like PM2.5. There were seven such days in March.