Articles | Volume 11, issue 3
Research article
27 Mar 2018
Research article |  | 27 Mar 2018

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

Christof Janssen, Hadj Elandaloussi, and Julian Gröbner

Abstract. The room temperature (294.09 K) absorption cross section of ozone at the 325 nm HeCd wavelength has been determined under careful consideration of possible biases. At the vacuum wavelength of 325.126 nm, thus in a region used by a variety of ozone remote sensing techniques, an absorption cross-section value of σ = 16.470×10−21 cm2 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) = 31×10−24 cm2, corresponding to a relative level of 2 ‰. The measurements are most compatible with a relative temperature coefficient cT = σ−1 ∂ 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 4 % discrepancy between UV and IR remote sensing data and indicates that further studies will be required to reach the accuracy goal of 1 % in atmospheric reference spectra.

Short summary
Monitoring ozone layer recovery at a rate of few percent per decade requires dedicated instrumentation and spectroscopic data of the highest quality. Highly accurate absorption cross sections of ozone are rare, especially in the important UV region between 300 and 340 nm. Our measurement provides the first reference point with permil level of accuracy in this range. Interestingly, our value is lower than currently used data. This might resolve an inconsistency between UV and IR measurements.