A comparision of direct and global UV-B X Ozone anticorrelations

E. Echer and V.W.J.H.Kirchhoff

• Introduction
• Instrumentation
• Results and Discussion
• Summary
• References

FIGURES

• Figure 1
• Figure 2
• Figure 3
• Figure 4

A study of ozone X UV-B anticorrelation is made for October 1999, using data from a Brewer Spectrophotometer at Punta Arenas (53.2^oS, 70.9^oW). The correlation coefficients between ozone and spectral direct and global UV-B were calculated, at a fixed solar zenith angle, and for daily maximum of UV-B radiation. Larger correlations for direct radiation and for measurements at a fixed solar zenith angle were observed, as expected. A comparison was made with GUV data from Ferraz Station (62^oS, 58^oW) from the same month, and higher correlation coefficients were observed for the irradiance ratio 305¤320 nm.

Introduction

The ozone hole phenomenon, discovered in the 1980s (Chubachi, 1984; Farman et al., 1985) has created the opportunity to study UV-B variations associated to large ozone variations. Typical values of ozone in austral spring at Antarctica should be of about 350-400 DU. However, during ozone hole conditions, very low ozone is observed, and the threshold of 220 DU was defined as the ozone hole condition. Observations by satellites, mainly the Total Ozone Mapping Spectrometer ­ TOMS, have shown the occurrence and the extension of ozone hole (Herman et al., 1993). Very large ozone variations can be observed in austral spring in Antarctic region, between 140 ­ 400 DU (Galtier et  al., 1994; Chubachi, 1997).  It implies in a variation of 65% in a few days. A large UV-B variability is also observed in Antarctic region (Frederick and Snell, 1988; Booth and Madronich, 1994; Frederick and Lubin, 1994; Herman et al., 1996). 

Punta Arenas (53.2^o S, 70.9^o W) is located at the extreme south of South America, and ozone disturbances are best seen in October, when the dynamics of ozone hole make it stretch over the city, and strong depletion is observed during a few days, with ozone reaching values lower than 220 DU (Kirchhoff et al., 1997a). As a result, strong UV-B enhancements are also observed, of about 10 at 297 nm (Kirchhoff et al., 1997b).

The Brazilian Antarctic Station Comandante Ferraz (62^o S, 58^o W) is located in the King George Island, Southern Shetland, and it shows strong ozone depletion during a large part of spring (Kirchhoff et al., 1997a).

Large UV-B enhancements have been observed (Kirchhoff et al., 2000; Paes Leme et al., 2000). 

In this paper, an anticorrelation study ozone X UV-B is made for the month of October 1999, using direct and global UV-B measurements at Punta Arenas, and global UV-B at Ferraz.

Instrumentation

Observations at Punta Arenas with a Brewer spectrophotometer have been made since 1992 by INPE ­ National Institute of Space Research of Brazil, and Universidad de Magallanes (Kirchhoff et al., 1997a, 1997b). Total ozone is determined from direct UV-B observations at 5 wavelengths ­ 306.3, 310.0, 313.5, 316.7 and 320. 1 nm. Global UV-B measurements are also made, in the range 290-325 nm.  At Ferraz Station, a Ground-based ultraviolet radiometer ­ GUV, was used, which measures global narrowband UV radiation at 305, 320, 340 and 380 nm. Observations and typical variations are reported by Paes Leme et al. (2000). Total ozone over Ferraz Station was obtained from TOMS observations.

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.

Summary

An anticorrelation ozone UV-B study at Punta Arenas and Ferraz, during October 1999, have shown that direct radiation is more correlated with total ozone than global radiation. The correlation is higher for measurements at a fixed solar zenith angle than for the daily maximum. It was observed also the use of a irradiance ratio improves the anticorrelation, by eliminating the common factors, no related to ozone, in different wavelengths. A depletion of 25% in total ozone implies in UV-B increases, at 305 nm, of 200% at Ferraz (solar zenith angle = 60^o) and 160% at Punta Arenas (solar zenith angle = 50^o).

References

Booth, C. R. and Madronich, S.  Radiation Amplification Factors: Improved formulation    accounts for large increases in ultraviolet radiation associated with Antarctic ozone depletion, Ultraviolet Radiation in Antarctica: Measurements and Biological Effects, Antarctic Research Series, vol. 62, Amer. Geophys. Union, 39-42, 1994.

Chubachi, S., Preliminary result of ozone observations at Syowa station from February 1982 to January 1983, Mem. Natl. Inst. Polar Res., Spec. Issue Jpn., 34, 13-19, 1984.

Chubachi, S. Annual variation of total ozone at Syowa Station, Antarctica, J. Geophys. Res., 102, 1349-1354, 1997.

Farman, J. C., B. G. Gardiner and J. D., Shanklin, Large losses of total ozone in Antarctica reveal seasonal ClO_x/NO_x interaction, Nature, 315, 207-210, 1985.

Frederick, J. E. and H. E. Snell, Ultraviolet radiation levels during the Antarctic spring, Science, 241, 438-440, 1988.

Frederick, J. E. and Lubin, D.  Solar Ultraviolet Irradiance at Palmer Station, Antarctica, in Ultraviolet Radiation in Antarctica: Measurements and Biological Effects, Antarctic Research Series, vol. 62, Amer. Geophys. Union, 43-52, 1994.

Galtier, C., G. He, S. Yang and D. Lubin,  Role of clouds and ozone on spectral ultraviolet-B radiation and biologically active UV dose over Antarctica,     in Ultraviolet Radiation in Antarctica: Measurements and Biological Effects, Antarctic Research Series, 62, 83-91, 1994.

Herman, J. R., R. McPeters and D. Larko, Ozone depletion at northern and      southern latitudes derived from January 1979 to December 1991 Total Ozone Mapping Spectrometer data, J. Geophys. Res., 98, 12783-12793, 1993.

Herman, J. R., P. K. Bhartia, J. Ziemke, Z. Ahmad and D. Larko, UV-B increases (1979-1992) from decreases in total ozone, Geophys. Res. Lett., 23, 2117-2120, 1996.

Kirchhoff, V.W.J.H., Casiccia S, C.A.R., and Zamorano B, F., The Ozone Hole over Punta Arenas, Chile, J. Geophys. Res., 102, 8945-8953, 1997a.

Kirchhoff, V.W.J.H., Zamorano B, F., and Casiccia S, C.A.R. UV-B Enhancements at Punta Arenas, Chile, J. Photochem. Photobiol., B: Biology, 38, 174-177, 1997b.

Kirchhoff, V. W. J. H., E. Echer,  N. Paes Leme, C. Casiccia, F. Zamorano and V. Valderrama, UV-B enhancements during ozone hole disturbances at the brazilian antarctic station (62^o S, 58^o W) and Punta Arenas (53.2^o S, 70.9^o W),  Quadrennial Ozone Symposium 2000, Sapporo, Japão, 3-8 Jul, Proceedings, p. 475-476, 2000.

Lubin, D. and Frederick, J. E., Column ozone measurements from Palmer Station,  Antarctica: Variations during the austral springs of 1988 and 1989, J. Geophys. Res., 95(D), 13883-13889, 1990.

Paes Leme, N., V. W. J. H. Kirchhoff, E. Echer, C. Casiccia and F. Zamorano  B., UV-B x Ozone anti-correlations at the Brazilian Antarctic Station, Quadrennial Ozone Symposium 2000, Sapporo, Japão, 3-8 Jul., Proceedings, p. 577-578, 2000.