At the end of the first lockdown period, from March 24 to April 14, the level of criteria pollutants in Delhi had declined to different extents – but the daytime average for the amount of ozone in the air had begun to increase. As The Wire Science reported, this was the result of atmospheric chemistry. NOx, the generic term for the nitrogen oxides important in atmospheric chemistry, comprises nitric oxide (NO) and nitrogen dioxide (NO2). NO eats ozone and NO2 makes ozone.

Vehicular emissions are responsible for more than 50% of Delhi’s NOx emissions. With only a limited amount of NO present, ozone accumulates in the atmosphere.

The data showed the same thing happening during the subsequent lockdown phases as well, in Delhi as well as around the country. Other datasets showed similar trends around the planet.

Ozone in the stratosphere, above 10 km, is good: it blocks ultraviolet radiation from the Sun from reaching the ground. However, ozone in the troposphere, under 1-2 km, where we live, is not good for human health. A global burden of disease study estimated that from 1990 to 2017, 146,000 people had died prematurely due to ozone pollution, in addition to 674,000 premature deaths due to PM2.5 pollution.

According to the US Environmental Protection Agency, inhaling ozone can cause respiratory difficulties ranging from shortness of breath to bronchitis and chronic obstructive pulmonary disease.

Like particulate matter – PM2.5 and PM10 – there are ambient air quality standards for ozone as well.

Changes in ozone pollution levels are more sensitive to the NOx to VOC ratios in urban environments. VOC stands for volatile organic compounds. And NOx levels of course have distinct diurnal cycles.

The lockdowns gave us an opportunity to test and better understand these concepts.