Ground-level, or tropospheric ozone (O₃), is a strong oxidizing agent that reacts with living tissue. Consequently, tropospheric ozone is considered an air pollutant and a respiratory irritant at low concentrations. At higher concentrations, tropospheric ozone is toxic to plant and animal life. Tropospheric ozone is also a greenhouse gas that contributes to global warming. In the troposphere [the lowest 10–12 kilometers (6–7.5 miles) of the atmosphere], ozone is produced through chemical reactions involving volatile organic compounds (VOCs; primarily reactive hydrocarbons) and nitrogen oxides (NO_x). In spite of pollution controls to limit the emissions of NO_x and of VOCs, particularly in urban areas, researchers reporting in the journal Science Advances (August 2020) found that ozone levels have increased in the lower troposphere of the Northern Hemisphere throughout the past 20 years. See also: Air pollution; Atmosphere; Atmospheric chemistry; Global warming; Greenhouse effect; Nitrogen oxides; Oxidation-reduction; Oxidizing agent; Ozone; Troposphere

Ozone occurs in both the Earth's upper atmosphere (stratosphere) and lower atmosphere (troposphere). In the stratosphere ozone protects plant and animal life from the Sun's harmful ultraviolet rays. In the troposphere, ozone can damage lung tissue in humans and other animals as well as vegetation. (Credit: Zack Frank/Shutterstock)

The ozone data came from a European initiative known as the In-service Aircraft for a Global Observing System (IAGOS), which used internationally flying commercial passenger aircraft with onboard instruments to measure atmospheric ozone for 11 regions worldwide from 1994 to 2016. The Northern Hemisphere regions studied included Europe, Eastern North America, Western North America, Southeast United States, South America, Northeast China/Korea, Persian Gulf, Southeast Asia, India, Gulf of Guinea (off the West African coast), and Malaysia. Over the 22 years studied, tropospheric ozone increased in all 11 regions, based on 34,600 ozone measurements. See also: Africa; Asia; Europe; North America

A few regions in North America and Europe showed a decline in lower-tropospheric ozone. However, these regions still showed an increase in upper-tropospheric ozone—high enough to result in an overall increase in ozone throughout the troposphere. The largest ozone increases occurred in the troposphere above India, Southeast Asia, and Malaysia—that is, in the Northern Hemisphere lower latitudes. According to the researchers, the likely cause of increasing Northern Hemispheric tropospheric ozone worldwide can be attributed to an increase in NO_x emissions in regions closer to the tropics, resulting from increased fossil fuel emissions. Further research is needed to better understand the increase in tropospheric ozone. However, data collection has nearly halted in 2020 because of decreased air travel caused by the coronavirus pandemic. See also: Fossil fuel; Novel coronavirus is declared a global pandemic