Better characterisation of the underwater solar ultraviolet environment using a high-exposure dosimeter

Better characterisation of the underwater solar ultraviolet environment using a high-exposure dosimeter

[Abstract]This dissertation presents the development, testing and application of a chemical film UV dosimeter based on the polymer Poly (2, 6-dimethyl-1, 4-phenylene oxide) (PPO) prepared especially for long – term high – exposure underwater use. Initial testing of the dosimeter was performed in a water tank within a controlled laboratory environment with an artificial UV source in which various optical and physical properties of the film were rigorously tested such as UV dose and depth response, cosine response, interdosimeter variation, dose rate independence, dark reaction, watermarking effect, exposure additivity and visible and UVA wavelength response. In each of these tests the PPO dosimeter displayed results proving that it could be reliably used in aquatic environments at a level of accuracy only slightly lower than what could be expected for in – air dosimetric measurements. The use of a polyethylene derived neutral density filter (NDF) was then employed with the PPO dosimeter in order to extend exposure time. Results from this investigation showed that the polyethylene NDF could extend the effective life time of the PPO dosimeter by as much as five days in early autumn. Following this the PPO dosimeter was calibrated in the field to the solar erythemal action spectrum in – air and to the solar UVB spectrum in clear water, creek water, sea water and dam water over the duration of a year. In both the in – air and underwater calibrations it was found that the response of the PPO dosimeter lasted over a much greater amount of time when compared to the more commonly used polysulphone dosimeter and also varied with the modulation of the incident solar spectrum brought on by changing SZA and fluctuations in atmospheric column ozone. Additionally, it was discovered that in – air and underwater calibration regimes could not be interchanged and that the PPO dosimeter response underwater is dependent upon water type, but only when transmission spectra differed between two water types by a difference on average of more than 5% across the UVB waveband. As a final test, the PPO dosimeter was deployed over a year to take UVB exposure measurements with the use of a custom built submersible float in three different real – world field environments that included a creek, a sea water tank and a stagnant dam. Exposures could be measured reliably up to a depth of 5 cm in the creek water and the dam water and up to and possibly beyond a depth of 35 cm in the sea water. From the sea water PPO dosimeter measurements a series of attenuation coefficients were estimated for each season. These coefficients showed reasonable agreement when compared to attenuation coefficient calculations made using a calibrated spectrometer in the same sea water, further proving the usefulness of the PPO dosimeter.