Previous: Instrumentation Next: Summary Up: Ext. Abst.

Results and Discussion

Figure 1 shows ozone and UV-B variability during October 1999, upper panel for Punta Arenas (solar zenith angle = 50^o), and lower panel for Ferraz (solar zenith angle = 60⁰). The dotted line is the threshold of ozone hole conditions, 220 DU. It is observed that at Punta Arenas some low ozone values are observed, but not lower than 220 DU. At Ferraz some days show values lower than 220 DU. UV-B increases are also observed.

Fig. 1 - Ozone and UV-B radiation on October 1999 at Punta Arenas (upper panel) and Ferraz Station (lower panel).

Relative deviations were calculated for ozone and UV-B. The method used to calculate the percentage deviations is the same reported by Booth and Madronich (1994). The maximum ozone content of the data set was chosen as reference (with a nominal value of 0% deviation), and ozone deviations were calculated from this reference point. The value of 0% variation was then also attributed to the corresponding UV radiation, for the same day of maximum ozone chosen, and the deviations were calculated as for ozone. The results, in percentage, have been calculated by the expressions:

In these expressions, DO₃ and DUV-B are the relative deviations for ozone and UV-B, in percentage, O₃ⁱ is the daily TOMS average, and UV-Bⁱ is the daily value at a given solar zenith angle, and O₃^max and UV-B^O3max are the values for the day with the maximum ozone content.

At Ferraz, during October 1999, ozone variability was of about 50% and UV-B has a variation non-linear with total ozone (Kirchhoff et al., 2000). In order to study linear anticorrelation, the ozone variability here considered for Ferraz was the same observed at Punta Arenas, 35%. Figure 2 shows the anticorrelation at Ferraz, for the 305 nm nd for the irradiance ratio, 305/320.

Fig. 2 ­ UV-B and ozone anticorrelations at Ferraz station during October 1999. Upper panel, UV at 305 nm. Lower panel, irradiance ratio 305/320.

UV-B observations were measured at a solar zenith angle near 60^o. It is observed that the irradiance ratio correlation is better than the 305 nm, with larger correlation coefficients. It is caused because the use of an irradiance ration eliminates noisy variability caused, by example, by clouds, and ozone X UV-B relation is clearly seen (Frederick and Lubin, 1994).

At Punta Arenas, the anticorrelation was studied by using global and direct UV-B radiation, with maximum daily and at a solar zenith angle of 50^o irradiances. In the Figure 3 the anticorrelations for 305 nm and the irradiance ratio 305/320 are presented.

Fig. 3 ­ UV-B and ozone anticorrelations at Punta Arenas during October 1999. Upper panel, maximum daily observations for 305 nm (left) and irradiance ratio 305/320 (right). Lower panel, observations at sza=50^o  at 305 nm (left) and irradiance ratio (right).

As can be seen in Figure 3, the anticorrelation with daily maximum  UV-B  is weak, but at a fixed solar zenith angle it increases, because the geometrical variability is then eliminated. Similar results were obtained for Ferraz, and the correlation coefficients are higher for the irradiance ratio than for a single channel.

Table 1 presents the results obtained for Punta Arenas. In Figure 4 are showed the correlation coefficients r² for direct and global UV-B, daily maximum and solar zenith angle = 50^o observations. It is observed the correlation is higher for lower wavelenghts, as ozone absorbs more strongly in these. Also direct radiation has higher  correlation than global radiation, because in global radiation there is a diffuse component and it is more affected by scattering than direct radiation. Observations at a solar zenith angle = 50^o have more correlation than at daily maximum.

Fig. 4 ­ Correlation coefficient r² versus wavelength for Punta Arenas, direct and global UV-B radiation, with  daily maximum and sza-50^o  observations.

The variability of -25% on total ozone had implied in UV-B variations of: 200% at 305 nm for Ferraz, and for sza=50^o of 350% at 300 nm, 160% at 305 nm and 95% for Erythemal UV-B, at Punta Arenas.