Formation

UTC3 March 25, 2016

While stratospheric ozone (“good” ozone) blocks the sun’s ultraviolet (UV) rays and prevents them from reaching the earth’s surface, tropospheric (ground-level) ozone (O3) (“bad” ozone) is an air pollutant that damages human health, vegetation, and ecosystems (http://www.eea.europa.eu/themes/air/ozone).

Stratospheric ozone occurs naturally in the upper atmosphere, at 20 to 30 km height, where it forms a protective layer that shields us from the sun’s harmful UV rays. This beneficial ozone has been partially destroyed by man-made chemicals, causing what is sometimes called a “hole in the ozone.”

Differences

Tropospheric ozone is a secondary pollutant, which means that it is not emitted directly into the atmosphere. Instead, it is formed from complex chemical reactions of nitrogen oxides (NOX) and volatile organic compounds (VOCs), carbon monoxide (CO) or methane (CH4), in the presence of sunlight. These gases are called ozone precursors (http://www.ccacoalition.org/en/slcps/tropospheric-ozone).

Ozoneformation

The major sources of NOX are combustion processes (e.g. in fossil-fuelled vehicles and power plants). Carbon sources have both natural (biogenic) and anthropogenic origin. As a result, the sources of ozone precursors are mostly found in urban areas. Tropospheric ozone is a major component of urban photochemical smog, but O3 concentrations are generally higher at rural locations than at urban and traffic sites (EEA Report No 5/2015 http://www.eea.europa.eu/publications/air-quality-in-europe-2015#tab-data-references). The reason for this is that ozone is formed from its gaseous precursors during transport, as the polluted air masses generated in the urban areas are transported towards suburban and rural areas.

In this sense, ozone is sometimes referred to as a “forgotten pollutant“, given that it is formed in rural areas through chemical reactions from precursor gases emitted mainly in urban environments. Therefore, the polluters (the urban population) often do not suffer from the effects of the degraded air quality generated by their emissions to the same extent whereas the rural population has limited influence on the emissions which degrade the air they breathe.