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Abstract Tropospheric ozone is also highly variable in time, both seasonal and daily scales. Owing to the photochemical mechanisms that synthesize ozone, minimum ozone concentration is usually found early in the morning, while the maximum value is set in late afternoon. Changes in tropospheric ozone from day to day can be as large as a factor of 2 or 3. Regarding long-time trends, the WMO (1994) estimates an enhancement of boundary layer ozone levels of up to 50% in some populated Northern Hemisphere regions; Janach (1989) estimates a global averaged increase of 1–2% for tropospheric ozone between the 1960s and 1989; and the Intergovernmental Panel on Climate Change, IPCC (2001) assigns a small but significant positive radiative forcing to increased tropospheric ozone concentration, which is estimated to be some 8 DU between 1750 and current days. Although tropospheric ozone is very variable both in time and space, it has been suggested that in industrial areas of the Northern Hemisphere increased tropospheric ozone can overcompensate (as far as filtering UV-B is concerned) for stratospheric ozone depletion (Brühl and Crutzen, 1989). The sensitivity of erythemal ultraviolet (EUV), will be discussed later, to atmospheric ozone is frequently expressed with the radiation amplification factor (RAF), also called sensitivity by Madronich (1992) or magnification factor by Bais et al. (1993a), defined as the percentage increase in EUV that would result from a 1% decrease in the column amount of atmospheric ozone (McKenzie et al., 1991; Madronich, 1995a ; Madronich et al., 1995b; Durzan and Smertenko, 2005). For that aim, the RAF has become a widely used standard index during last years (McKenzie et al., 1991; Madronich, 1993b; Bodhaine et al., 1997; Madronich et al., 1998; Dubrovsky, 2000; Zerefos, 2002, Serrano et al., 2008). |