CRN-026: “A network for the measurement of ultraviolet radiation”

Maria Vernet, R. Armstrong, C.R. Booth, S. Cabrera, S. E. Diaz, H. Fuenzalida, C. Lovengreen, A. Paladini, J. Pedroni, H. Zagarese, F. Zamorano

INTRODUCTION

 

Since discovery of the "ozone hole” in Antarctica, monitoring of long-term trends and geographical variability of UV irradiance (UVR) has become increasingly important. A continuing difficulty in judging these impacts is the lack of coordinated, world-wide UV monitoring networks that produce results that can be compared at the level of precision and accuracy needed to detect trends in irradiance that are likely to be occurring. We have established a network of instruments in South America based on a "nested-network" approach (Corell, 1995) where a collection of different UV networks composed of different instruments is linked by standardized protocols, inter-comparisons, and calibrations. The main objective is to bring together researchers from several countries in North and South America to combine efforts to understand the impact of ozone depletion on a regional level.

NETWORK DESCRIPTION

 

UV radiation on a regional level is provided by a network of ground-based instruments. The surface radiometer network includes 9 instruments (Table 1). Location varies with latitude as well as in altitude. Common data processing is done in Ushuaia, Argentina and produces high quality regional data necessary for ecosystem research, modeling and prediction.

 

Compared to high-resolution scanning spectroradiometers, filter radiometers such as the GUVs deployed in South American network have the potential advantages of reduced initial expense (by a factor of 10), reduced operating costs, and reduced data loads. The two most critical elements necessary to proceed in the fielding and coordination of international networks of GUV radiometers aimed at detecting trends in UV irradiance are: (1) instrument stability and accuracy; and (2) the ability of the instrument to yield both total column ozone and biologically weighted irradiances.

The instruments included at this stage of the network are the GUV-511 (Biospherical Instruments Inc. Figure 1), temperature stabilized radiometer with filters centered at 305, 320, 340, and 380 nm and Photosynthetically Available Radiation AR (PAR, 400-700 nm). From the data obtained by the GUVs it is possible to model the solar irradiance spectrum (Dahlback, 1996) and derive a valid biologically weighted UV dose irradiance (Booth at al., 1995).

LITERATURE CITED

 

Booth, C.R., J.H. Morrow and J.P. Schmidt. 1995. Strategy for a UV-B Monitoring Network: Instrument selection, calibration and data products. Final Report to NOAA, Phase I. Biospherical Instruments, San Diego, CA.

Corell, R.W. 1995. The US Interagency UV-monitoring plan. U.S. Global Change Research Program, UV Panel Observations Working Group, USGCRP-95-01.

Dahlback, A. 1996. Measurements of biologically effective UV-doses, total ozone abundance and cloud effect with multi-channel moderate bandwidth filter instruments. Appl.ied Optics 35: 6514-6521.

Diaz, S., D. Nelson, G. Deferrari y C. Camilion. A model to extend spectral and multi-wavelength UV irradiances time series. Journal Geophysical Research (submitted).

Lovengreen, C., J.L. Alvarez, H. Fuenzalida and M. Aritio: “Radiación ultravioleta eritémica en Valdivia: caracterización y predicción”, Revista Médica de Chile (submitted).

TABLE 1 - The UVR instrument network:

OBJECTIVES

 

The network main objectives to obtain regional UVR are:

 

(a) To bring disparate individual instruments and networks together with centralized data processing and quality control in Ushuaia, Argentina.

(b) To bring together researchers from Argentina, Chile, Canada, Puerto Rico, and the US to understand the impact of ozone depletion on a regional level.

(c) To couple in networks in other Latin American countries (i.e. Brazil, Colombia, Costa Rica, Puerto Rico) and elsewhere (i.e. Norway) by extending the quality control and data processing tools already developed.

 

These objectives are achieved by the activities carried out by the UVR instrument network that:

 

(1) Convert instruments from national efforts into a regional network by consistency in instrument maintenance, data collection and storage, instrument calibration and data processing. Comparisons of GUV and spectroradiometers (e.g. SUV-100 from Biopherical Instruments Inc.) at 2 locations (Ushuaia and Valdivia) guarantee data of equal quality and validate the utility of the lower-cost filter radiometers.

(2) Provide data to create a regional UVR climatology (Figure 2).

(3) Estimate UV effects over organisms on a regional scale.

RADIOMETER CALIBRATION

 

The Reference GUV (RGUV) is calibrated against the spectroradiometer SUV-100 (NSF UV Radiation Monitoring Network) installed at San Diego. A set of RGUV´s is located on the roof of Biospherical Instrument Inc. running side by side with the spectroradiometer SUV-100. Synchronized time series of RGUV-SUV are obtained, for the calibration of the RGUV. Time series used for calibration of the RGUV9287 to be applied in the calibration of the GUV network year 2000 have been restricted to 10 days before delivery of the instrument to South-America and 10 days after arrival from the trip. The procedure to calculate the calibration constants of the RGUV against the SUV are, in principle, similar to that used to calibrate GUV against RGUV. Each channel of the GUV is compared against the SUV-100 data corresponding to the nominal wavelength of the channel. Nevertheless, it has been detected that the calibration of channel 305 is sensitive to ozone and solar zenith angle, as a consequence of the bandwidth differences between instruments and the strong dependence of the slope of the solar irradiance curve with these parameters at those wavelengths. We consider then, that calibration in this channel would be improved by introducing additional parameters and some methodologies are being developed and tested at present to accomplish with this goal.

 

DATA ANALYSIS

 

Data analysis is mainly performed using a modified set of Access queries developed by Biospherical Instruments Inc. Raw data from the GUV is processed following a number of fixed steps, with some control nodes, where the quality of the data is checked and possible sources of error are identified, allowing, in some cases, correction of the data.

Most common problems are related to the temperature and the time drift in the computer that collects the data. The instruments are stabilized at temperature, commonly 40oC, using a heater but, sometimes the ambient temperature is too high, and then the temperature in the instrument exceeds 40oC. In those cases, it has been recommended to run the instruments at 50oC. To compensate the error produced by the temperature increase in the data already collected, a correction. was automatically introduced when applying the calibration constants. Computer time drifts are identified analyzing data at sunrise and sunset and drifts larger than 1 minute are corrected. When a drift of several minutes is observed at certain period, it is corrected in 1-minute steps.

Site Latitude Longitude Altitude Location

*Ushuaia 54.49 S 68.19W SL CADIC

Punta Arenas 53.09 S 70.55W SL Univ. de Magallanes

Trelew (2) 42.47 S 65.01 W SL Univ. de la Patagonia

Bariloche 41.32 S 71.62 W ~300 m CRUB

*Valdivia 39.48 S 73.14 W SL Univ. Austral Chile

Buenos Aires 34.35 S 58.29W SL INGEBI

Santiago 33.27 S 70.40 W SL Univ. de Chile

Jujuy 24.10 S 65.01W 1200 m Univ. Nac.de Jujuy

Lajas 17.58 N 67.02 W SL Univ. Puerto Rico

*Sites with spectro-radiometers (SUV-100) that will be used for GUV-511 calibration with BSI. }

Figure 2: 305 nm annual cycle and anomalies therefrom at the network sites in 1995.

Figure 1: Calibration at Valdivia (Chile): four GUV’s (front) and SUV-100 (in the back)