A new photometric ozone reference in the Huggins bands: The absolute ozone absorption cross section at the 325 nm HeCd laser wavelength

Abstract

The room temperature (294.09K) absorption cross section of ozone at the 325ĝ€nm HeCd wavelength has been determined under careful consideration of possible biases. At the vacuum wavelength of 325.126ĝ€nm, thus in a region used by a variety of ozone remote sensing techniques, an absorption cross-section value of σ =16.470×10-21cm2 was measured. The measurement provides the currently most accurate direct photometric absorption value of ozone in the UV with an expanded (coverage factor k =2) standard uncertainty u(σ) =31×10-24cm2, corresponding to a relative level of 2% The measurements are most compatible with a relative temperature coefficient cT =σ-1 a Tσ =0.0031 K-1 at 294-K. The cross section and its uncertainty value were obtained using generalised linear regression with correlated uncertainties. It will serve as a reference for ozone absorption spectra required for the long-term remote sensing of atmospheric ozone in the Huggins bands. The comparison with commonly used absorption cross-section data sets for remote sensing reveals a possible bias of about 2%. This could partly explain a 4ĝ€% discrepancy between UV and IR remote sensing data and indicates that further studies will be required to reach the accuracy goal of 1ĝ€% in atmospheric reference spectra.