Assessing the feasibility of a New Solid State Instrument for Remote Sensing of Stratospheric Ozone and Other Atmospheric Species.

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Abstract

Column ozone (predominantly located in the stratosphere) has been measured at distinct ground sites since the 1920s, and globally from satellites from the late 1970s. The importance of ozone was highlighted by recognition of the Antarctic ozone hole in 1985, leading to international action (Montreal Protocol) to protect the ozone layer. Monitoring the success of this action and the future ozone layer is currently reliant on the “gold standard” represented by Dobson and Brewer spectrophotometers.The manually operated Dobson instrument is no longer manufactured, although it is still in operation, leaving the Brewer as the only ground-reference spectrophotometer commercially available. While these instruments provide the standard for column ozone measurement, improvements in technology over the past 40 years provide possibilities for alternative approaches to column ozone measurement with advantages that might include increased speed of data acquisition, reduced size and weight, improved portability and lower cost. The key elements of any spectrophotometer are a dispersion system to isolate different wavelengths (traditionally diffraction gratings), and a suitably sensitive detector for those wavelengths (traditionally a photomultiplier tube PMT). Any replacement technology should achieve an accuracy comparable to measurements obtained with Dobson and Brewer spectrophotometers. Most of the recent alternative spectrometers found in the literature use CCD or photodiode arrays to replace the PMT, but retain the diffraction grating. They have to overcome stray-light and sensitivity issues to achieve an accuracy similar to the Brewer spectrophotometer measuring ozone. Here a radically different design is proposed for a UV ground-based spectrometer to measure column ozone. An Acousto-Optical Tuneable Filter (AOTF) replaces the traditional diffraction grating, and an avalanche photo-diode detector (APD) is used instead of the photomultiplier tube, with a lock-in amplifier algorithm to perform the digital signal processing. This new spectrometer design has the potential to diminish moving parts in the instrument (improving reproducibility and decreasing maintenance), increase the measurement rate and reduce production costs without affecting the reliability of the measurements. The aim of this thesis is to prove the feasibility of a new spectrometer that uses this solid state technology and the lock-in amplifier algorithm. Experimental results suggest that the AOTF and APD are feasible options to replace the diffraction grating and the photomultiplier tube respectively in the traditional spectrometer design.

Keyword(s)

Spectrophotometer, spectrometer, AOTF, lock-in amplifier, ozone

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