Ground-level ozone highest in New Delhi and South Delhi areas from March to May: CSE

Ozone is produced when oxides of nitrogen, volatile organic compounds, and carbon monoxide, from vehicular emissions and those from industries and power plants, react in the presence of sunlight. With sunlight being key to the formation of ozone, it is found to have a wider spatial spread in hot and sunny weather conditions, particularly in April and May, according to the analysis.

Fewer instances of heatwaves in Delhi-NCR this summer have meant that the geographical spread of ground-level ozone pollution has been the lowest in March and April this year when compared to data over the past five years, from 2019 onwards. An “average of around 10 stations have exceeded the standard daily this summer, which is 33 percent lower” compared to previous summers from 2019 onwards, according to the analysis.

However, ozone levels exceeded the eight-hour standard for longer this summer—an average of 4.9 hours this summer, having increased from 4.6 hours last summer, and decreased from 5.1 hours in the summer of 2021.

High ozone levels can aggravate respiratory issues like asthma, chronic bronchitis, and chronic obstructive pulmonary disease. With ozone being a highly reactive gas, it can also inflame the airways and make the lungs susceptible to infection, the analysis mentioned. The impact of ozone on health means that the National Ambient Air Quality Standard has been set for hourly and eight-hourly exposures to ozone, in contrast to standards for pollutants like particulate matter, where standards are set for daily and annual average exposures.

The analysis noted that ground-level ozone remains a problem at night as well. “Ground-level ozone should ideally become negligible in the night air but Delhi-NCR has been witnessing a rare phenomenon where ozone levels remain elevated hours after sunset. This was found to be very widespread during the lockdowns of the 2020 summer and it continues to linger this summer as well. This May night-time ozone was noted on 28 days with 3 stations on average reporting it every night,” it explained.

Similarly, cold and foggy conditions in the winter can inhibit the formation of ground-level ozone, but ozone levels were found to exceed the standard at multiple stations on 26 days in January this year, according to the analysis, making it a problem that persists all year round.

In 2022, ozone levels exceeded the standard on 334 days, while this was 312 days in 2021, 304 days in 2020 and 286 days in 2019.