Measurement of Atmospheric Ozone by Cavity Ring-down Spectroscopy

Ozone plays a key role in both the Earth's radiative budget and photochemistry. Accurate, robust analytical techniques for measuring its atmospheric abundance are of successfully used for sensitive and accurate measurements of many atmospheric species. However, this technique has not been used for atmospheric measurements of ozone, because the strongest ozone absorption bands occur in the ultraviolet spectral region, where Rayleigh and Mic scattering cause significant cavity losses and dielectric mirror reflectivitics are limited. Here, we describe a compact instrument that measures O-3 by chemical conversion to NO2 in excess NO, with subsequent detection by cavity ring-down spectroscopy. This method provides a simple, accurate, and high-precision measurement of atmospheric ozone. The instrument consists of two channels. The sum of NO2 and converted O-3 (defined as O-x) is measured in the first channel, while NO2 alone is measured in the second channel. NO2 is directly detected in each channel by cavity ring-down spectroscopy with a laser diode light source at 404 nm. The limit of detection for 03 is 26 pptv (2 sigma precision) at 1 s time resolution. The accuracy of the measurement is 2.2%, with the largest uncertainty being the effective NO2 absorption cross-section. The linear dynamic range of the instrument has been verified from the detection limit to above 200 ppbv (r(2) > 99.99%). The observed precision on signal (2 sigma) with 41 ppbv 03 is 130 pptv in 1 s. Comparison of this instrument to UV absorbance instruments for ambient O-3 concentrations shows linear agreement (r(2) = 99.1%) with slope of 1.012 +/- 0.002.