Mega cities in India see high concentration of ozone pollution this summer

All mega cities in India, including Kolkata, Bengaluru, Mumbai, Hyderabad and Chennai, saw high concentration of ground-level ozone pollution this summer season, according to a new analysis by think tank Centre for Science and Environment (CSE).

A previous analysis had shown that ozone, instead of particulate matter, was the main pollutant in Delhi’s daily Air Quality Index (AQI) on several days this summer.

The new analysis by the CSE’s Urban Lab highlighted that ozone, unlike primary pollutants, is not released directly from any source.

It forms through complex chemical reactions involving nitrogen oxides (NOx), volatile organic compounds (VOCs) and carbon monoxide (CO), pollutants emitted by vehicles, power plants, industries and other sources of combustion.

These substances react in the presence of sunlight, leading to the formation of ozone at the ground level.

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Anumita Roychowdhury, executive director at the CSE, said: “If unchecked, this can become a serious public health crisis as ozone is a highly-reactive gas and can be harmful even with short-duration exposures.” She added that while cities in north India experience high ozone levels in summer due to intense heat and strong sunlight, cities in warmer climates are also seeing consistent ozone exceedance in other seasons.The current policy focus must be expanded to include improved monitoring, the mitigation of this toxic gas and a reduction in high local exposures.

Ozone mitigation needs drastic control over a range of gases from vehicles, industry and all combustion sources, Roychowdhury said.

Sharanjeet Kaur, deputy programme manager at the CSE’s Urban Lab, said: “Inadequate monitoring, limited data and inadequate methods of trend analysis have weakened the understanding of this growing public health hazard across cities of India. Instead of merely averaging out the levels for the city, which is the standard practice to estimate the AQI, it is also important to adequately capture the high levels of local build-up and exposures in the hotspots and design mitigation strategies accordingly.”

Between March 1 and May 31 this year, Mumbai recorded ozone exceedance on 32 out of 92 days across its monitoring stations, a 42-per cent drop compared to the same period last year. The worst day in terms of spread was March 29, when eight out of 31 stations reported exceedance. The highest ground-level ozone concentration recorded was 90 micrograms per cubic metre.

In Kolkata, ozone levels crossed the safe limit on 22 out of 92 days, a 45-per cent decline compared to last summer. The city also saw an overall improvement, with the average hourly ozone peak dropping by 22 per cent.

Bengaluru saw ozone exceedance on 45 of the 92 days, a 29-per cent increase from last year. The worst day was March 31, when four of 14 stations recorded levels above the standard.

Hyderabad recorded 20 ozone exceedance days, a 55-per cent drop compared to last summer. The highest ozone level recorded in the city was 51 micrograms per cubic metre.

Chennai saw ozone exceedance on 15 days this summer. No such exceedance was reported during the same period last year. In comparison, the city had three exceedance days in 2023 and 19 in 2022. The highest level this year was 64 micrograms per cubic metre.

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