Delhi suffers from high levels of ground-level ozone, finds study
According to the study, between January 1 and July 18 — a period of 200 days — Delhi-NCR recorded 176 days of ground-level ozone exceedance
New Delhi
Delhi-NCR topped a list of 10 cities across the country that were found to have high concentrations of ground-level ozone (O3), according to a study released by the Centre for Science and Environment (CSE) on Tuesday.
According to the study, between January 1 and July 18 — a period of 200 days — Delhi-NCR recorded 176 days of ground-level ozone exceedance. The second highest was 138 days each, in Mumbai-MMR and Pune, followed by Jaipur, at 126 days. CSE used data from official stations under the Continuous Ambient Air Quality Monitoring System (CAAQMS) of the Central Pollution Control Board.
“Ground-level ozone, a highly reactive gas, has serious health consequences. Those with respiratory conditions, asthma and chronic obstructive pulmonary disease, as well as children with premature lungs and older adults, are at serious risk. This can inflame and damage airways, make lungs susceptible to infection, aggravate asthma, emphysema and chronic bronchitis, and increase the frequency of asthma attacks leading to increased hospitalisation,” Anumita Roychowdhury, executive director, research and advocacy, CSE, said.
The study looked at ozone data from January 1 to July 18, with the summer period considered as April 1 to July 18. Comparative data was assessed from 2020 till 2023, covering Bengaluru, Chennai, Delhi-NCR, Greater Ahmedabad, Greater Hyderabad, Greater Jaipur, Kolkata, Greater Lucknow, Mumbai Metropolitan Region and Pune.
Ground-level ozone is not directly emitted, but is produced from a complex interaction between nitrogen oxides (NOx) and volatile organic compounds (VOCs) emitted from vehicles, power plants, factories, and other combustion sources. This makes it a highly volatile gas that has one-hour and eight-hour standards, as compared to PM (particulate matter) 2.5, which has a 24-hour standard.
Dangerous levels
Delhi-NCR recorded excess ozone levels on 103 of 109 days this summer, followed by Jaipur, at 75 days, and Pune, at 58 days. However, based on concentration levels, Jaipur’s air was more toxic than that of Delhi-NCR.
“The regional average of Jaipur for April 1 to July 15 is 81.2 µg/m3 compared to Delhi-NCR’s 73.3µg/m3 and Mumbai-MMR’s 34.8µg/m3. Chennai has the lowest regional average of 24.3µg/m3 but the peak recorded in the southern metro was 192.7µg/m3, third highest among the 10 metropolitan areas -- only exceeded by Jaipur (205.9µg/m3) and Delhi-NCR (194.2µg/m3),” the study said.
“While analysing the data, it was also noted that the ozone data available on the CPCB portal never exceeds 200 microgramme per cubic metre (μg/m3) while data for the corresponding time provided by the Delhi Pollution Control Committee may show higher levels. Due to this capping of data, it is not possible to understand the nature of peaking in the city,” Avikal Somvanshi, programme manager, Urban Lab, CSE, who was also part of the study, said.
According to CPCB, the one-hour standard for ozone is 180µg/m3, and the eight-hourly standard is 100µg/m3.
Night ozone
CSE said that although ground-level ozone should ideally become negligible in the night air, the 10 metropolitan areas have been witnessing a rare phenomenon, where ozone levels remain elevated for hours after sunset.
The report said Mumbai-MMR reported most instances of night-time ground-level ozone, with 171 nights of exceedance, followed by Delhi-NCR, at 161, and Pune, at 131 nights. The least number of exceedances was recorded in Kolkata, on 17 nights.
Year-on-year increase
Comparing the number of exceedances this year with the same period from last year shows an increase in seven of 10 metropolitan areas, the study found. Smaller metropolitan areas had the biggest jumps, with Ahmedabad registering a 4,000% rise in number of exceedances, followed by Pune, with a 500% rise and Jaipur, with a 152% rise.
“Hyderabad registered a 115% rise in the number of exceedance days; Mumbai-MMR and Bengaluru saw an 18% and 1% increase, respectively. Less than 5% change has been noted in Delhi-NCR and Lucknow. Meanwhile, Kolkata-KMA and Chennai have seen a drop in the number of exceedances by 19% and 40%, respectively,” the study said.
Green areas worst hit
The study found that green or high-end neighbourhoods were worst impacted, indicating that though these areas may have lesser PM 2.5, ozone remains a threat.
According to the study, Dr Karni Singh Shooting Range and Lodi Colony in Delhi, Navy Nagar in Mumbai, Fort William in Kolkata, Kapra in Hyderabad, BMT Layout in Bengaluru, Velachery in Chennai, Savitribai Phule University in Pune, Police Commissionerate in Jaipur, Gomti Nagar in Lucknow and Maninagar in Ahmedabad are all hot spots of ground-level ozone pollution.
“This is consistent with the science that ozone drifts and gathers in areas where comparatively lesser amount of gaseous pollutants are present to further mop it up. Thus, relatively cleaner areas, including urban peripheries, can have comparatively higher ozone concentrations,” Roychowdhury said.
The spatial distribution of ozone was also found to be inverse of NO2 and PM2.5.
“This bears out the fact that while ozone is created in polluted areas with nitrogen oxide being the catalyst, it also gets mopped up in high NO2 areas as it further reacts. But the ozone that escapes to cleaner areas with less NO2 builds up faster, as unavailability of NO2 hampers its dissipation,” the study said.