| In this thesis work, a significant water vapor interference inherent to many and possibly all UV-based, commercially available ozone monitors was positively identified for the first time. The mechanism of this indirect interference, as water itself does not directly absorb 254 nm light, is elucidated: the interference caused by changes in the transmission efficiency of light through the UV absorption cell as a result of adsorption of water molecules to the cell surface. Deviations from true ozone concentrations result from modulations in the water vapor concentration within the cell between I and Io measurements as a result of the ozone scrubber acting as a water vapor reservoir, alternately drying or humidifying the air stream flowing through the absorbance cell.; Consistent with the proposed mechanism, a number of factors were found to reduce the water vapor interference with the use of a NafionRTM tube to equilibrate the humidities of scrubbed and unscrubbed air prior to the absorbance cell proving most successful in reducing the humidity interference to an insignificant level (+/-2 ppbv for rapid cycling between 0% and 90% relative humidity). To date, NafionRTM tubes, for elimination of the water vapor interference, have been installed in more than fifty 2B Tech Ozone Monitors. Furthermore, the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory (NOAA/CMDL) currently utilizes these NafionRTM modified 2B Technology Ozone Monitors on a fleet of light aircraft which obtain vertical profiles of ozone on a weekly basis at numerous sites in the U.S. The complete elimination of the water vapor interference presented in this thesis is critical, not only in making valid ozone measurements on balloons and aircraft where rapid changes in humidity are encountered on time scales of seconds or less, but also in ambient monitoring sites across the globe where large humidity variations often occur as a result of weather fronts and/or calibration methods which can potentially affect the results of ozone monitoring for compliance with environmental standards such as the U.S. Clean Air Act.*; *Please refer to Appendix for complete abstract... |
