Lower levels of suspended particulate matter (PM2.5) and nitrogen dioxide (NO2) levels during the lockdown cleared air quality across India. But a new analysis of 22 mega and metropolitan cities in India, done by the Centre for Science and Environment (CSE) of summer air quality trends during the lockdown reveals other details.

While the PM2.5 and NO2 curves fell and flattened dramatically in cities, tropospheric ozone pollution (henceforth ozone) increased and even breached standards in several cities: a fact that was not widely noticed. Invisible ozone became the most prominent pollutant in several cities.

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

The 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.  

Anumita Roychowdhury, executive director-research and advocacy, CSE said, “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.”

The Central Pollution Control Board (CPCB) publishes the daily Air Quality Index (AQI) bulletin to inform about the severity of daily pollution. With all pollutants down, ozone, even at a comparatively lower level, became the most prominent pollutant of the day in several cities and led the daily AQI.

 “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,” Roychowdhury stressed.

The ozone layer is a natural, protective layer of gas in the stratosphere (the second major layer in the Earth’s atmosphere). It lies approximately 25 miles above Earth’s surface. As NASA explains, 90% of the ozone in the atmosphere sits in the stratosphere, the layer of atmosphere between about 10 and 50 kilometers altitude. The natural level of ozone in the stratosphere is a result of a balance between sunlight that creates ozone and chemical reactions that destroy it. Ozone in the stratosphere absorbs most of the ultraviolet radiation from the Sun. Without ozone, the Sun’s intense UV radiation would sterilize the Earth’s surface.

In 1976, atmospheric research revealed that the ozone layer was being depleted by chemicals containing chlorine and bromine. These include chlorofluorocarbons (CFCs), which were commonly found in air conditioners, refrigerators and spray cans; halons, which are found in fire extinguishers; and methyl bromide, which is used to kill weeds, insects and other pests. In 1985, researchers discovered an ozone hole above Antarctica (later known as the ozone hole).

The adoption of the Montreal Protocol in 1987 banned the production of CFCs, halons and other ozone-depleting chemicals. The ban came into effect in 1989 and led to ozone levels stabilizing by the mid-1990s and recovering in the 2000s. Recovery is projected to continue over the next century, and the ozone hole is expected to reach pre-1980 levels by around 2075.In 2019, NASA reported that the ozone hole was the smallest ever since it was first discovered in 1982.