Abstract
Abstract. This study examines the long-term variability of UV solar irradiances at 305 nm and 325 nm over selected sites in Canada, Europe and Japan. Site selection was restricted to the availability of the most complete UV spectroradiometric datasets during the period 1990–2011. The analysis includes the long-term variability of total ozone, aerosol optical depth and cloud fraction at the sites studied. The results, based on observations and modeling, suggest that over Canada, Europe and Japan the period under study can be divided into three sub-periods of scientific merit: the first period (1991–1994) is the period perturbed by the Pinatubo volcanic eruption, during which excess volcanic aerosol has enhanced the "conventional" amplification factor of UV-B at ground level by an additional factor that depends on solar elevation. The increase of the UV-B amplification factor is the result of enhanced scattering processes caused by the injection of huge amounts of volcanic aerosols during the perturbed period. The second period (1995–2006) is characterized by a 0.14% yr−1 increase in total ozone and an increasing trend in spectral irradiance by 0.94% yr−1 at 305 nm and 0.88% yr−1 at 325 nm. That paradox was caused by the significant decline of the aerosol optical depth by more than 1% yr−1 (the "brightening" effect) and the absence of any statistically significant trend in the cloud fraction. The third period (2007–2011) shows statistically significant evidence of a slowdown or even a turning point in the previously reported upward UV-B trends over Canada, Europe and Japan.
Highlights
The world “avoided” following the success of the Montreal Protocol, and evidence for recovery of the ozone layer over the Northern Hemisphere (Zanis et al, 2006; Chipperfield et al, 2007; Harris et al, 2008; Newman et al, 2009; Zerefos et al, 2009a; WMO, 2011) suggest that, other factors remaining constant, the observed increases of the harmful solar UV-B doses (Zerefos, 2002) are expected to have levelled off or even decreased in the last decade
This study examines the long-term variability of UV solar irradiances in connection to the observed upward trends in total ozone and downward trends in aerosols
The first period is the period perturbed by the Pinatubo volcanic eruption (1991–1994), for which it is shown that the excess volcanic aerosol might have enhanced by an additional 6 % the “conventional” amplification factor of UV-B at ground level
Summary
The world “avoided” following the success of the Montreal Protocol, and evidence for recovery of the ozone layer over the Northern Hemisphere (Zanis et al, 2006; Chipperfield et al, 2007; Harris et al, 2008; Newman et al, 2009; Zerefos et al, 2009a; WMO, 2011) suggest that, other factors remaining constant, the observed increases of the harmful solar UV-B doses (Zerefos, 2002) are expected to have levelled off or even decreased in the last decade. The 2011 WMO/UNEP Ozone Assessment and a recent paper by den Outer et al (2010) reported the continuation through 2005 of upward trends in UV-B over Europe They found these upward trends to range between +0.3 % yr−1 and +0.6 % yr−1, of which 2/3 they attributed to the combined decrease of cloudiness and aerosol optical depth and 1/3 to the past longterm ozone change through 2005. As is shown in the analysis that follows, UV-B solar irradiances continued their positive trends through about 2005, after which they have levelled-off and seem to have reversed their upward trend. This can only be explained if we consider the opposite effects the aerosols and ozone trends have on UV-B change, since the observed trend in cloudiness is insignificant. Since 2007, the continued upward trend of ozone dominated and overwhelmed the opposing trend imposed by the aerosol decline
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