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Introduction

A network of 21 Brewer MKIV spectroradiometers (Kipp & Zonen Inc, Canada) is measuring UV spectral irradiances throughout the United States. The database from these sites extends for periods ranging from two to seven years.

Corrections to the raw data have now been implemented. These corrections include (1) the cosine errors associated with the full sky diffuser, (2) the temperature dependence of the response of the instruments and (3) the temporal variation in the instrument response due to optical changes in the characteristics of the instruments. The dark count, dead time and stray light corrections of the program RD_UX and the UVA correction and erythemal action spectrum used in the latest version of the program UVSUM, were also used for this paper (Sci-Tec 1999). When referring to the uncorrected UV data it is implied that dark-count, dead time and the UVA correction have been applied.

Corrected irradiances at four sites are compared with a clear sky UV model. These sites are Boulder (#101, 40.12 ° N, 105.24 ° W, 1689 m a.s.l.), Rocky Mountain (#146, 40.03 ° N, 105.53 ° W, 2891 m a.s.l.), Gaithersburg (#105, 39.08 ° N, 77.22 ° W, 20 m a.s.l.) and Research Triangle Park (RTP) (#087, 35.89 ° N, 78.88 ° W, 134 m a.s.l.). Details of these sites are available at the EPA web address: http://www.epa.gov/uvnet/

Intercomparisons of the daily-integrated damaging UV (DUV) measurements from the Brewers at these sites to DUV values inferred from satellite overpass data, corresponding to these sites, are made in order to validate the estimates from the satellite data. Available satellite data ranges from July 1996 until February 2000. Brewer and model DUV data will be presented from July 1996 until May 2000.

It should be noted that the erythemally-weighted irradiances for the Brewer, model and TOMS data all used the action spectrum of McKinlay and Diffey (1987).