FIGURES

• Figure 1
• Figure 2
• Figure 3
• Figure 4
• Figure 5
• Figure 6

Within the activity of the Italian Antarctic Project (PNRA) since 1994 is running in Ushuaia (Tierra del Fuego, Argentina), 54.82° S - 68.32° W, a spectrophotometer Brewer, Fig. 1, to study during the austral springtime, the ozone depletion over the periantarctic region and its relationships with the Antarctic ozone hole, Rafanelli et al. . The same place is a node of the NSF UV Radiation Monitoring Network (USA) where a spectroradiometer SUV 100, fig 2, is present since 1988 to study the solar UV radiation levels, Frederick et al.. Moreover, Ushuaia is a very suitable site to study the aerosols and cloud cover effects on the UV, with special care on UV-B region, for its climate situation.

During the spring 1999, the activity of the two instruments was included into the APE-GAIA Project campaign, Carli et al., as ground-based measurements for the solar UV and ozone sampling. Therefore, the goal of the paper is to show the results of the intercomparison irradiance obtained integrating over the interval 290 ? 320 nm the spectral irradiance respectively by the Green model, Brewer and SUV 100 measurements. The SUV 100 is used as reference for the UV-B levels and the Brewer as reference for the total ozone.

In many situations, both for whether conditions or instrumental out of order, it is no possible the sampling of UV-B radiation. In other cases, it is necessary to control the instrumental outputs, or to evaluate the effects that some atmospheric components produce on the solar radiation reaching the ground. Therefore, to have an analytical model to compare the experimental data or to evaluate a filling in a time series is very helpful.

The semi-empirical Green model is an efficient algorithm to calculate the solar ultraviolet radiation at ground, Green et al..Then the spectral irradiance as Green model output, G(_), is described by:

where

is the solar direct irradiance, and the diffuse one is

Where the parameter O₃ is the total ozone content and __i(_) & __i(_) are the tabled absorption coefficients and relative air mass for the i absorber.

The solar radiation at ground, as known, is affected also by the albedo. The correction for the albedo, R, modifies G(_, _, O₃) in G(_, _, O₃, R) where

with R(») = {A₀ • (1+ß) • exp [(» ? »₀) / ´] } / { exp[(» ? »₀) / ´] + ß};

A₀, ß and _ are tabled values for various surface types and _₀ is the reference wavelength at 300 nm.

The time is the way to drive the SUV 100, Booth et al; instead the Solar Zenith Angle, SZA, drives the Brewer; in the paper only the data synchronous are considered.

The comparison of all synchronous sampled data of Brewer and SUV 100 during APE-GAIA campaign are examined, clear and cloudy days both, fig. 3. The linear correlation coefficient shows a good agreement of the two samples, but the 0.7 values for the slope can be explained by the different hardware windowing.

In the intercomparison between Green outputs vs. SUV 100 and Brewer, fig. 4, are used all sampled irradiances and, both scattered data and low values of slopes can be due to the cloud effect not considered in the model. This is because it is not known the cloud coverage and however the a simple cloud factor that do not consider the relative position of clouds and sun it is not sufficient for a real cloud effect evaluation, Lubin et. al..

The cloudiness influence is obtained by the comparison day by day between modeled and sampled data, fig. 5. The low levels of slope are according with the cloudy condition over Ushuaia during the period Sep. 22 ? Oct. 03, fig. 6.

Concluding the Green model seems to be a good model for the environmental studies when is used linked with an action spectrum, for example to compute the erythemal dose rate. It has efficacy for SZA over 67°, therefore at high latitudes is useful only in full summer. The UV by Brewer, having SUV 100 as reference, exhibits an underestimate of irradiance, probably for a different shape of the slit and its effect on data management; this, of course not affects the ozone measurements because it is a relative sampling.

This different response will be a future work for the authors.

The authors are grateful to NSF and PNRA Programs for funding the research.

They thank also M.L. Moriconi for the study on Green model and the implementation of the albedo and instrumental windowing algorithms.

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