PM pollution down but ozone levels cross permitted limit during peak lockdown in Dehi-NCR

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Published on Jun 25, 2020 10:56 PM IST
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New Delhi:

Concentration of particulate matter and gases such as nitrogen dioxide reduced significantly during the nationwide lockdown but ozone pollution kept increasing and even breached the safe standards in many metro cities, including the national capital, according to an analysis of summer air quality trends by Centre for Science and Environment (CSE).

The organisation studied pollution trends of 22 mega cities -- Delhi-NCR (including Faridabad, Ghaziabad, Gurugram and Noida), Kolkata, Chennai, Mumbai, Ahmedabad and Bengaluru, among others -- for five months (spring to summer) during January 1-May 31 this year and compared it with the corresponding period of 2019. The focus of the study has been on the period of the lockdown from March 25-May 31.

In Delhi, even though overall the severity of ozone pollution was comparatively lower than summer months of 2019 because of the lockdown, the levels still increased and exceeded the standards. This was when other major pollutants such as PM 2.5 and NO2 curves had flattened during the lockdown period, the study said.

The CSE studied the number of days when the ozone concentrations exceeded between March 22 and May 31 (lockdown period) this year and for the same period in 2019. The permissible standards (maximum 8 hours average) for ozone are 100 micrograms per cubic metres.

“In Delhi, as compared to 2019 when the ozone levels (citywide average) breached the standards on 21 days, in 2020 the levels were breached on four days. However, what is important to note is that the number of days when the levels were breached at least at one monitoring station is high and remains similar for both --- this year (67 days) and in 2019 (68 days). The worst ozone pollution day in Delhi was recorded on May 17,” said Anumita Roychowdhury, executive director, research and advocacy, CSE.

In NCR towns, 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 average in Ghaziabad exceeded the standard on 15 days, with at least one station exceeding on 56 days, the study showed.

In Noida, the citywide average exceeded the standard on 12 days; at least one station exceeded on 42 days.

In Kolkata, the citywide average exceeded on eight days; at least in one station exceeded on 17 days. Chennai and Mumbai did not register a single day of breached standards at the citywide level, but at least one station in both exceeded the standard on 61 days and five days, respectively. In Ahmedabad, the citywide average exceeded the standard on 43 days.

The analysis is based on data sourced from the Central Pollution Control Board (CPCB) website. The study showed that with all pollutants down during lockdown, ozone, even at a comparatively lower level, became the most prominent pollutant of the day in several cities and led the daily air quality index (AQI).

The AQI was led by ozone in Delhi on 10 days (8 of these days were categorised as ‘moderately polluted’) and Gurugram on 14 days (7 days were categorised ‘moderately polluted’ and one as ‘poor’).

Vehicular pollution and industrial emissions are major contributors to creation of ozone. It 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, it added.

CSE researchers point out that ozone is primarily a sunny weather problem in India that otherwise remains highly variable during the year. It is a highly reactive gas and 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.

Also, the study found that ozone pollution is the highest in stations with the lowest NO2 pollution – ozone levels build up in the greenest parts of the city where the NO2 levels are very low. “This is because ozone is formed when NOx, VOCs and gases react with each other under the influence of sunlight and temperature,” she said.

She said, for instance, Nehru Nagar near the Lodhi Garden area in Delhi; Colaba in Mumbai, among others, are ozone pollution hot spots.

“This pandemic-led change in air quality helped us understand summer pollution. Usually, every year, winter pollution is what draws our attention. This means we need to reduce overall pollution to reduce ozone pollution by intervention such as introduction of BS-VI norms for vehicles, augmentation of public transport, cleaner fuel transition in industries and promoting of activities such as cycling and walking,” she added.

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Friday, December 03, 2021