Development and intercalibration of ultraviolet solar actinometers

Ultraviolet (UV) sunlight actinometers were developed based on the photolysis of nitrate and nitrite. Photon exposures (i.e. time-integrated irradiances) were quantified from the photochemical production of salicylic acid (SA) or p-hydroxybenzoic acid (pHBA) formed by the reaction of the hydroxyl radical with benzoic acid. The solar response bandwidth for the nitrate actinometer in quartz tubing was 322 +/- 11 nm during the Spring of 1999, while the response bandwidth of the Mylar D-filtered nitrite actinometer was 355 +/- 25 nm. Intercomparisons of the nitrate and nitrite actinometers with a Smithsonian Environmental Research Center SR-18 scanning UV-B radiometer (SERC SR-18) and an Optronics OL-754 spectroradiometer (OL-754) were performed during the summer of 1998, and the winter and spring of 1999. Photon exposures determined by the nitrate actinometer were in excellent agreement with the SERC SR-18, with a slope (95% confidence interval [CI]) of 0.98 +/- 0.01 based on SA production and 0.94 +/- 0.02 based on pHBA production. Excellent agreement was also found between the nitrite actinometer and the OL-754, with a slope (95% CI) of 1.00 +/- 0.01 using SA production and 1.00 +/- 0.02 using pHBA production. These actinometers are well suited for use in the water column and are sufficiently sensitive to determine photon exposures below the 0.1% UV lightlevel.