Ozone levels increased during lockdown: CSE

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New Delhi (NVI): An analysis done by Centre for Science and Environment (CSE) of air pollution levels in 22 cities across India has revealed that ozone levels increased during the nationwide lockdown that is in place since March 25.

According to the CSE analysis, while PM2.5 and NO2 (Nitrogen dioxide) levels plummeted, invisible ozone became the most prominent pollutant in several metropolitan cities.

“While the PM2.5 and NO2 curves fell and flattened dramatically in cities – a phenomenon that hogged the national attention — tropospheric ozone pollution (henceforth ozone) increased and even breached standards in several cities: a fact that was not noticed as widely,” says the report.

According to the researchers at CSE, ozone is primarily a sunny weather problem in India that otherwise remains highly variable during the year. It is a highly reactive gas; even short-term exposure (one hour) is dangerous for those with respiratory conditions and asthma. That is why ozone has a short-term standard – one hour and eight hours, as opposed to 24 hours for other pollutants, as per the report.

Ozone is not directly emitted by any source but is formed by photochemical reactions between oxides of nitrogen (NOx) and other volatile organic compounds (VOCs) and gases in the air under the influence of sunlight and heat. Ozone can be controlled only if gases from all sources are controlled.

Instead of the official method of considering only the fixed eight-hour average (from 8 PM to 4 PM) to assess ozone levels, CSE adapted the global best practice of calculating the maximum eight-hour average of the day. This indicated a higher level of exceedance in cities.

The cities covered by the CSE analysis include Delhi-NCR (including Faridabad, Ghaziabad, Gurugram and Noida), Kolkata, Chennai, Mumbai, Ahmedabad, Ujjain, Bengaluru, Hyderabad, Jaipur, Jodhpur, Patna, Vishakapatnam, Amritsar, Howrah, Pune, Guwahati, Lucknow and Kochi.

While particulate levels hit the lowest possible range during lockdown phases, the share of PM2.5 fraction in the overall particulate concentration increased, the analysis shows.

However, as traffic nearly stopped, the daily NO2 curve flattened in all cities.

Crop fires in April were quite extensive in Punjab and Haryana, but their impact was blunted by the overall very low background and local pollution, said the report.

“This pandemic-led change in air quality has helped us understand summer pollution. Normally, every year, winter pollution is what draws our attention. The characteristics of summer pollution are different: there are high winds, intermittent rains and thunderstorms, and high temperature and heatwaves. This is in contrast to winter — with its inversion, lower mixing height of air, and cold and calm conditions that trap the air and the pollutants in it,” says Anumita Roychowdhury, executive director-research and advocacy, CSE.

CSE has analysed trends in PM2.5, PM10, NO2, and ozone in 22 cities across 15 states and the National Capital Territory of Delhi for the period January 1, 2019 to May 31, 2020. This also includes spatial trend analysis of ozone in selected cities.

Talking about ozone exceedance, if the maximum eight-hour average during 24 hours is considered (as the US Environment Protection Agency does to capture the health risks), then more than two-thirds of the lockdown days in Delhi-NCR cities and Ahmedabad had at least one station that exceeded the standard. In Ahmedabad, the city-wide maximum eight-hour average exceeded the standard on 43 days; in Ujjain, it exceeded on 38 days.

The city-wide maximum average in Gurugram exceeded the standard on 26 days — at least one station exceeded the standard on 57 days. The city-wide eight-hour maximum average in Ghaziabad exceeded the standard on 15 days, with at least one station exceeding on 56 days. In Noida,the city-wide maximum average exceeded the standard on 12 days; at least one station exceeded on 42 days. In Delhi, the maximum eight-hour average exceeded the standard on four days, and at least one station exceeded the standard on 67 days.

Ozone is a problem during spring and summer in the north, central and the arid parts of western India. It can even increase during warmer winters in the southern and coastal cities.