Tropospheric ozone trends

Abstract

Anthropogenic emissions of chemical reactive trace gases have substantially altered the composition of the troposphere. These perturbations have caused tropospheric O₃ increases, in particular in the Northern Hemisphere. It remains, however, difficult to accurately establish O₃ trends throughout the troposphere because the number of sites where surface O₃ measurements and O₃ soundings of high quality are performed are small, especially at low latitudes and throughout the Southern Hemisphere. The longest O₃ surface measurements and sounding records are available from Europe where the upward O₃ trend seems largest, 5–20%/decade; the increase occurred, primarily before 1985. Ozone trends for other mid-latitudinal locations are generally smaller. At high latitudes in the Southern Hemisphere a negative O₃ trend is due to a reduced downward flux of ozone associated with stratospheric ozone depletion and to increased UVB levels, resulting in stronger tropospheric photochemical destruction.

After H₂O, of which tropospheric levels are not expected to change by direct emissions of H₂O from anthropogenic activities, and CO₂, tropospheric O₃ is presently the third most important greenhouse gas. Because of its significant consequences for human health and nature, the large-scale increase in tropospheric O₃ levels is to our opinion one of the most crucial environmental problems to solve during the coming decades.