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Maha cities see rise in ozone levels during lockdown: Nat’l Clean Air Programme tracker

By Badri Chatterjee, Mumbai
UPDATED ON MAY 28, 2020 01:27 AM IST

Despite the decline in traffic pollution during the lockdown imposed due to the Covid-19 pandemic, 10 cities in Maharashtra are reporting an increase in urban ground-level ozone, a harmful pollutant, according to a recent study.

The Mumbai Metropolitan Region (MMR), comprising Mumbai, Thane, Navi Mumbai, and Kalyan, witnessed a 24.8% spike in urban ozone during the first three phases of the lockdown over two months (March 24 to May 24), according to an analysis released as part of the National Clean Air Program (NCAP) tracker on Tuesday by Mumbai-based air quality research group Respirer Living Sciences (RLS) and Delhi-based communications initiative, Climate Trends.

The 10 cities in the state with increase in ground-level ozone are Mumbai, Navi Mumbai, Thane, Kalyan, Chandrapur, Aurangabad, Nagpur, Nashik, Pune and Solapur. The Maharashtra Pollution Control Board (MPCB) confirmed the findings were accurate.

Surface ozone (O3) is a photochemical oxidant, which is formed when volatile organic compounds and oxides of nitrogen (NOx) chemically react in the presence of heat and sunlight. Ground-level ozone is harmful to humans as it can aggravate asthma, cardiovascular diseases, and other ailments.

The latest study found a 10.3% rise in ozone across 10 Indian cities during the first two months of the lockdown. Among monitoring locations across the state, Khadakpada in Kalyan has shown the highest average ozone levels at 70 micrograms per cubic metre (μg/m³) against the safe limit of 80μg/m³ (for 24 hours), followed by the Maharashtra Industrial Development Corporation’s Khutala station in Chandrapur at 64μg/m³. While the concentration ranged between 23-37μg/m³ for MMR, a 50% increase in average O3 levels was witnessed when compared to 2019. Comparatively, a 50% reduction for Mumbai and 58% for Thane was witnessed for nitrogen dioxide (NO2) levels. NO2 is a result of burning of coal and vehicular exhaust.

“The analysis indicates that when NO2 levels remain low, surface ozone is higher as photochemical production can become more efficient during summer months,” said Ronak Sutaria, founder and director, RLS. “Breathing air with high ozone poses a risk for people with asthma and other lung ailments, especially for children, older adults, and those active outdoors such as daily wage workers.”

MPCB said it was investigating the spike in the pollutant by comparing different station readings. “Based on our preliminary data collection, ozone is going up across Maharashtra cities,” said SC Kollur, chief scientist, MPCB. “It may be a secondary pollutant arising from the reduction of other major pollutants. Industrial areas do witness such spikes but the increase for major cities comes as a surprise. We have to analyse the source.” For Mumbai, MPCB plans on studying background data from the Borivli station to check reasons for an increase in ozone. “The station data has been unavailable for a week due to a broken cable. The data is crucial as it gives background data, and human-induced emissions are not present,” said Kollur.

All 10 Maharashtra cities are part of the NCAP, where the Centre seeks to reduce pollutant concentrations by 20-30% by 2024 with the target pollutant as particulate matter (PM2.5 and PM10). Data was compiled from state pollution control board monitoring stations over two months. The tracker is a web portal that monitors the achievement of state-specific air quality targets for 122 Indian cities, and whether funds released for each city are being appropriately utilised.

“The rise in ozone comes as an alert at a time when cities are already facing a health crisis. The findings should equip policy-making and encourage more studies to identify the impact of other pollutants apart from PM,” said Aarti Khosla, director, Climate Trends.

A recent study by the Indian Institute of Technology, Delhi (IIT-Delhi), and two Chinese universities also showed a 20% rise in ozone for western parts of India. The study suggested that secondary pollutants such as ozone need to be considered by NCAP. “China witnessed a similar rising trend for ozone and adopted measures to reduce the pollutant. India is already showing an indication of higher O3 when other pollutants reduce. During summer months, the Centre should consider precursors to ozone for non-attainment cities and in addition to PM, O3 has to be kept in mind while preparing action plans,” said Sri Harsha Kota, professor, IIT-Delhi.

NCAP apex committee and professor, Indian Institute of Technology, Kanpur, SN Tripathi said, “Ozone has not been a problem in India over the years, barring occasional spikes, which are regularly observed across cities abroad. An increase has been observed during lockdown due to its function as a precursor to declining NOx concentration but has remained below the safe limit. Ozone should be considered by NCAP as a secondary pollutant. Studies show PM followed by ozone contributes to 85-90% human-related adverse health impacts. The current data is giving us the opportunity to understand the O3 mitigation strategy.”

The NCAP database also calculated average pollutant levels across 10 Indian cities over two months. PM2.5 was 31.5 μg/m³ showing 33.5% decline from 2019; 71.5 μg/m³ for PM10 (35.7% decline), and 15.4 μg/m³ for NO2 (52.6% decline from last year).


The NCAP dashboard for 10 Indian cities highlighted the offline functioning of monitors leading to data gaps. The air quality monitors in Mumbai have shown substantially lower uptime (data provided in a given period) during the lockdown period of March 24 to May 24, with Bandra working only for 56% of the time, Borivli at 26% capacity, Colaba at 52%, and the international airport monitor at 69%. Air quality monitors in Worli, Powai, Vile Parle, and Kurla have all given over 95% uptime, indicating reliable data. “Maintenance of air quality monitoring is essential to understand the long-term impact of city-level air pollution trends. Of particular concern is the long-running monitoring done in Bandra which has shown as 42% fall in data provided during the lockdown as compared to the same period in 2019,” said Sutaria.

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