Impact of two low ozone events on surface solar UV radiation over the northeast of Spain

Total ozone column (TOC) abundance over northeastern Spain follows a strong seasonal pattern with higher values in spring and lower values in fall. The annual cycle has weakened since the late 1970s, due to a decrease in the ozone annual maximum monthly mean in spring. The long-term trend and the seasonal variability of the TOC suggest that thresholds of ozone 'mini-holes' should be defined as deviations from local monthly mean levels. Ozone mini-holes are synoptic scale regions of short-term, large TOC reductions. The climatology of ozone mini-holes over Barcelona shows a seasonal pattern, with a higher frequency in winter and early spring. Nevertheless, many of these events are detected in September, when the UV irradiance reaching the Earth's surface is still high. This may enhance negative effects such as erythema. The deepest ozone 'mini-hole' ever reported over the area was on 10 January 2004, when the daily TOC was about 40% lower than the climatological mean for January. A high-pressure system induced cloud-free skies, and the small temporal variations in aerosol concentrations indicated that low TOC was responsible for the enhanced UV radiation at ground level: the erythemal irradiance at noon was 36% higher than the day before. However, in absolute terms, this increase is relatively small as UV levels in winter are low. Nevertheless, the low ozone events observed in late spring could be biologically harmful, even though the TOC anomaly was rather less than the winter values. On 20 April 2004, low TOC and high solar elevation favoured the increase that was detected in UVB radiation, especially at shorter wavelengths. The UV Index at noon was 1 unit higher than the previous day's value, due to an uplift of the tropopause and a reduction in the ozone mixing ratio in the upper troposphere-lower stratosphere (UTLS) region. Copyright (C) 2010 Royal Meteorological Society