Tropospheric ozone trends

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. (C) 2000 Elsevier Science Ltd