CHENNAI: A new analysis of summer air quality trends during the national
that started on March 25, 2020, reveals a mixed trend. While the PM2.5 and NO2 curves fell and flattened dramatically in cities – a phenomenon that hogged the national attention -- tropospheric
pollution increased and even breached standards in several cities, according to a
conducted by Centre for Science and Environment (
The study has emerged from a new analysis of 22 mega and metropolitan cities in India.
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; 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.
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.
"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 heat waves. 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," said Anumita Roychowdhury, executive director-research and advocacy, CSE.
Studies from the Indian Institute of Technology- Kanpur in 2015 and a joint study by The Energy Research Institute (TERI) and the Automotive Research Association of India (ARAI) in Delhi in 2018 have shown that during summer, Delhi experiences relatively higher contribution of dust compared to winter; also, the share of secondary particulates that form from the gases in the atmosphere, is comparatively lower.
“This pandemic has shown that big reduction is possible only if all regions clean up together and at a scale and with speed across all critical sectors including vehicles, industry, power plants, waste, construction, use of solid fuels for cooking and episodic burning. There is a need for an agenda for a ‘blue sky and clear lungs’ for the post-pandemic period to sustain the gains. This action must also ensure the co-benefit of reducing both particulate and gaseous emissions, including ozone," she said.
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.
The most granular data (15-minute averages) has been sourced from the Central Pollution Control Board’s (CPCB) official online portal,the Central Control Room for Air Quality Management - All India (https://app.cpcbccr.com/). This has analysed over 23 million data points recorded by 116 air quality monitoring stations or about 50 per cent of the existing network under the Continuous Ambient Air Quality Monitoring System (CAAQMS) of the CPCB.
All cities with three or more CAAQMS stations are included in this analysis; more have been chosen to ensure geographical and demographical representation. However, this study does not include modelling to isolate the impacts of lockdown or any other impacts.
The cities covered by the CSE analysis: 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.
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.
Even when PM2.5 and NO2 levels plummeted to the lowest in most cities during the lockdown and caught public attention, invisible ozone raised its ugly head on several days and in several cities. For analysis of ozone trends, CSE has adopted the global best practice of considering the maximum rolling eight-hour average during 24 hours, as opposed to the fixed time slot of 8AM to 4PM daily that the CPCB has adopted. The CPCB’s method fails to capture the worst part of the day that poses a higher health risk -- most of the time, ozone peaks after 4pm.
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.
In Kolkata, the city-wide average exceeded on eight days; at least in one station exceeded on 17 days. Chennai and Mumbai did not register a single day of excess at the city-wide level, but at least one station in both exceeded the standard on 61 days and five days, respectively.