Carolina Camilion

Ozone and UV Radiation over Southern South America: Climatology and Anomalies

Abstract Ozone and UV radiation were analyzed at eight stations from tropical to sub-Antarctic regions in South America. Ground UV irradiances were measured by multichannel radiometers as part of the Inter American Institute for Global Change Radiation network. The irradiance channels used for this study were centered at 305 nm (for UV-B measurements) and 340 nm (for UV-A measurements). Results were presented as daily maximum irradiances, as monthly averaged, daily integrated irradiances and as the ratio of 305 nm to 340 nm. These findings are the first to be based on a long time series of semispectral data from the southern region of South America. As expected, the UV-B channel and total column ozone varied with latitude. The pattern of the UV-A channel was more complex because of local atmospheric conditions. Total column ozone levels of <220 Dobson Units were observed at all sites. Analysis of autocorrelations showed a larger persistence of total column ozone level than irradiance. A decreasing cross-correlation coefficient between 305 and 340 nm and an increasing cross-correlation coefficient between 305 nm and ozone were observed at higher latitudes, indicating that factors such as cloud cover tend to dominate at northern sites and that ozone levels tend to dominate at southern sites. These results highlight the value of long-term monitoring of radiation with multichannel radiometers to determine climatological data and evaluate the combination of factors affecting ground UV radiation.

Simulation of Ozone Depletion Using Ambient Irradiance Supplemented with UV Lamps

Abstract In studies of the biological effects of UV radiation, ozone depletion can be mimicked by performing the study under ambient conditions and adding radiation with UV-B lamps. We evaluated this methodology at three different locations along a latitudinal gradient: Rimouski (Canada), Ubatuba (Brazil) and Ushuaia (Argentina). Experiments of the effect of potential ozone depletion on marine ecosystems were carried out in large outdoor enclosures (mesocosms). In all locations we simulated irradiances corresponding to 60% ozone depletion, which may produce a 130–1900% increase in 305 nm irradiance at noon, depending on site and season. Supplementation with a fixed percentage of ambient irradiance provides a better simulation of irradiance increase due to ozone depletion than supplementation with a fixed irradiance value, particularly near sunrise and sunset or under cloudy skies. Calculations performed for Ushuaia showed that, on very cloudy days, supplementation by the square-wave method may produce unrealistic irradiances. Differences between the spectra of the calculated supplementing irradiance and the lamp for a given site and date will be a function of the time of day and may become more or less pronounced according to the biological weighting function of the effect under study.

Multichannel radiometer calibration: a new approach.

The error in irradiance measured with Sun-calibrated multichannel radiometers may be large when the solar zenith angle (SZA) increases. This could be particularly detrimental in radiometers installed at mid and high latitudes, where SZAs at noon are larger than 50 degrees during part of the year. When a multiregressive methodology, including the total ozone column and SZA, was applied in the calculation of the calibration constant, an important improvement was observed. By combining two different equations, an improvement was obtained at almost all the SZAs in the calibration. An independent test that compared the irradiance of a multichannel instrument and a spectroradiometer installed in Ushuaia, Argentina, was used to confirm the results.

Calibration improvement of the IAI Network for the measurement of UVR: multichannel instruments

The IAI Network for the measurement of ultraviolet radiation in Chile, Argentina and Puerto Rico is composed of ten multi-channel radiometers (GUV 511, Bisopherical Instruments Inc.), which are periodically sun calibrated with a traveling reference GUV (RGUV). The RGUV is calibrated under solar light against a SUV100 spectroradiometer. This calibration is then transferred to each instrument in the network through the RGUV. A previous multi-regression model proved to be suitable to derive narrowband irradiance from broadband irradiance, ozone column and solar zenith angles (SZA). A recent modification of the existing multi-regression model improved the multi-channel instrument sun calibration against spectroradiometers. In this approach, the narrowband irradiance is the SUV spectral irradiance and the broadband is the multi-channel GUV irradiance. We included the azimuth angle as a parameter into the multi-regression equation and we applied a non-linear function, instead of a single coefficient, to correct for SZA. In this paper, the new multi-regression approach is applied to both steps of a GUV calibration: SUV - RGUV and RGUV - GUV and the results are compared with traditional calibration methods. Important improvements are observed in the calibration, in particular for SZA larger than 50°.

Improving multichannel radiometer calibration and data quality

When a multi-channel radiometer is calibrated using the sun as calibration source, the error in the calculated calibration constants may be large when solar zenith angles increase. A multi-regression model has proved to be suitable to derive narrowband irradiance from broadband irradiance, ozone column and solar zenith angles (SZA). In this paper, a modification of this model is being proposed to improved the multi-channel instrument sun calibration against spectroradiometers, considering a channel of a multi-channel radiometers as a broadband instrument. The errors in the GUV irradiance, compared to the spectroradiometer irradiance, diminished considerably at all channels for SZA larger than 50a, then, this technique could be particularly beneficial to calibrate radiometers installed at high latitudes, where SZA during winter, even at noon, are larger than 50a.

Determination of spectral irradiances from broadband instrument measurements

After the discovery of the ozone hole the use of spectral and multiband instruments in UV radiation measurements became more common, but time series for these instruments are still relatively short to determine trends. Nevertheless, systematic measurements of ozone column have been performed since late fifties at several stations and with worldwide coverage since late seventies, and long time series of broadband instruments (Pyranometers, UV and erythemally weighted) are also available at stations all over the world. In this paper we introduce a multi- regressive model that allows inference of spectral or narrowband irradiances from ozone total column and broadband irradiances, in places where a relatively short time series of spectral irradiances is available. To test the model, measurements under all weather, solar zenith angle and ground conditions performed at three of the stations of the NSAF UV Radiation Monitoring Network were used. The model generated very good results over a wide variety of situations. Broadband pyranometer data from the NOAA/CMDL surface radiation budget database for the South Pole Station were used in the model to estimate the daily-integrated narrowband irradiances. A time series of monthly means for the narrowband 303.030-307.692 nm were then computed, dating back to 1978.