CNR-26 “A Methodology to Extend Spectral Time Series and Improve Multichannel Radiometers Calibration” R. Amstrong, R. Booth, S. Cabrera, C. Cassiccia, S. Diaz, H. Fuenzalida, C. Lovengreen, D. Nelson, A. Paladini, J. Pedroni, A. Rosales, H. Zagarese, C. Brunat, C. Camilion, G. Deferrari, M. Vernet INTRODUCTION After the discovery of the ozone hole, many efforts have been developed to measure UV radiation with spectral or multi-channel instruments but, time series for these measurements are still relatively short for the determination of, for example, trends applied in geophysical and biological studies. However, systematic measurements of total column ozone have been performed since the late 1950s at several stations, and global coverage has been available since the late 1970s. Long-term time series of broadband instruments (Pyranometers, UV and erythemally weighted) are also available from many sites around the world. A multi-regressive model, which allows estimation of spectral and biologically weighted irradiances from broadband measurements has been developed and tested using data of the NSF UV Radiation Monitoring Network and NOAA CMDL broadband insturments. Applying this model, biologically weighted irradiances back to 1979 were calculated and analyzed for Barrow and South Pole.   Also, the IAI Network for the measurement of ultraviolet radiation in Chile, Argentina and Puerto Rico is composed of ten multi-channel radiometers (GUV 511, Biospherical Instruments Inc.), which are periodically sun calibrated with a traveling reference GUV (RGUV). The RGUV is calibrated under solar light against a SUV100 spectroradiometer of the NSF UV Radiation Monitoring Network. This calibration is then transferred to each instrument in the network through the RGUV. A modification of the above-mentioned multi-regression model was applied to improve the multi-channel instrument sun calibration. These results have been published at: S. Díaz, D. Nelson, G. Deferrari, C. Camilión. A Model to Extend Spectral and Multi-wavelength UV Irradiances Time Series. Model Development and Validation. Journal of Geophysical Research, Atmospheres, (In press). These results have been presented and published at: S. Diaz, D. Nelson, G. Deferrari, C. Camilión. Determination of Spectral Irradiances from Broadband Instruments Measurements. Proceeedings of SPIE´s 46th Annual Meeting, San Diego, California, USA, 30 July – 3 August, 2001. The International Symposium on Optical Science and Technology. S. Diaz, C. Booth, G. Deferrari, C. Camilion and J. Robertson. Improving Multichannel Radiometer Calibration and Data Quality. Proceedings of SPIE´s 47th Annual Meeting, Seattle, Washington, USA, 7-11 July, 2002. The International Symposium on Optical Science and Technology. (In press) S. Diaz, R. Booth, R. Amstrong, S. Cabrera, C. Cassiccia, H. Fuenzalida, C. Lovengreen, A. Paladini, J. Pedroni, A. Rosales, H. Zagarese, C. Brunat, G. Deferrari, C. Camilion, M. Vernet. Calibration improvement of the IAI Network for the measurement of UVR: Multi-channel instruments. Proceedings SPIE's Third International Asia-Pacific Symposium on Remote Sensing of the Atmosphere, Ocean, Environment, and Space, Hangzhou, China, 23-27 October 2002. (In press) These results have been presented at: S. Diaz, C. Booth, R. Amstrong, S. Cabrera, C. Cassiccia, H. Fuenzalida, C. Lovengreen, D. Nelson, A. Paladini, J. Pedroni, H. Zagarese, C. Brunat, G. Deferrari, C. Camilion, M. Vernet. Two Spectral and Multi-channel Instruments Network. A methodology to Extend Time Series and Improve Calibration. I Congreso Latinoamericano sobre Radiación Ultravioleta: su Medición y sus Efectos. Oficina Ejecutora de Programas de la Universidad de Panamá, Instituto Panamericano de Geografía e Historia (IPGH) y Centro Latinoamericano de Física (CLAF). Panama City, Panama, 15 al 19 de julio de 2002 EXTENDING SPECTRAL TIME SERIES Measurements of spectral and multi-channel UV radiation are important in determining the effects of atmospheric ozone depletion at the earth’s surface. Since these types of measurements became more common only after the discovery of the “ozone hole”, time series are still relatively short and many investigators have attempted to extend them in order to obtain databases suitable to study the effects of ozone depletion in the biosphere. Ground-level radiation is a function of several factors: sun-earth distance, atmospheric gases and aerosols, solar zenith angle, clouds, altitude and surface albedo. To infere past UV spectral irradiances is problematic, in some cases because of the complexity of the relation between the UV and the parameter, and in others because of the lack of information on the variability of the parameter itself. We developed a multi-regressive model, which can be used to estimate spectral, narrowband or biologically weighted irradiances in places where long time series of broadband measurements (UV-B, UV-A, UV-B biologically weighted, pyranometer), are available. The model is very simple and, in spite of that, it showed very good results. Since the same methodology can be applied for different types of broadband instruments, it would also be possible to convert time series of several independent well calibrated broadband instruments of different type into a network, obtaining a database of spectral or narrow-band irradiances, just adding a spectral or multi-channel travelling instrument, which would be used to determine the regression coefficients. The proposed equation is:  ln Es = a1 ln Eb + a2 O3 + a3 f + b (1) where: Es: spectral, narrowband or biologically weighted irradiance; Eb: broadband irradiance; O3: total column ozone; f: solar zenith angle, a1, a2, a3 and b are the regression coefficients, determined using least squares method. Figure 1 shows the monthly-integrated irradiance estimated from broadband instruments, for October at South Pole and April at Barrow. Gaps in TOMS data were filled with spectral measurements of the SUV 100. Ozone total column monthly average is also shown. At South Pole the irradiance increase in accordance to ozone depletion is well reflected, while at Barrow the effect is less pronounced. Figure 1. Estimated monthly-integrated irradiance (columns dark red) and measured (columns white) at 303.030-307.692 nm, for South Pole (left) and Barrow (right). Lines with diamonds show total column ozone. IMPROVING MULTI-CHANNEL INSTRUMENTS CALIBRATION The usual techniques to calibrate multi-channel radiometers are lamp and sun calibrations. The first type is performed in dark laboratories, and they present some inconveniences, as differences between lamps and sun spectrum and angular incidence of the light on the collector. Sun calibration seems to present a more realistic situation. This type of calibration can be performed following two different procedures. One of them is to install the multi-channel radiometer side by side with a spectroradiometer, and the other is to install it closed to a reference multi-channel radiometer, which has already been calibrated against a spectroradiometer. The second procedure is used to calibrated the instruments of the IAI network. Since the radiometer GUV can be considered a broadband instrument, compared to the spectroradiometer SUV then, the above-described multi-regressive approach was applied to improve the calibration. In this case, at the first step of the calibration, the narrowband irradiance is the 1 nm bandwidth irradiance measured by the SUV and the broadband is the 10 nm bandwidth irradiance measured by the GUV. Empirically, we determined that, including the azimuth angle as a parameter into the multi-regression equation and applying a non-linear function, instead of a single coefficient, to correct for SZA, better results were obtained. When calibrating the site GUV against the RGUV, the same procedure was applied, replacing the spectral measurement by the RGUV and the broadband by the site GUV values. In both cases important improvements were observed in the calibration, in particular for Solar Zenith Angle (SZA) larger than 50o. Figure 2 shows the RMS relative error in absolute value (in the sense that the sign of the error was not taken into account) in the calibration of the site GUV against the RGUV9287 for all sites of the network, for SZA larger than 50o. Figure 2. RMS relative error for the calibration of the site GUV against the RGUV9287. Green single regression (traditional calibration), dark red multi-regression method.