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

Abstract

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 cm 2 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 cm 2 , corresponding to a relative level of 2 ‰. The measurements are most compatible with a relative temperature coefficient c T = σ −1 ∂ T σ = 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.

Ab initio predictions and laboratory validation for consistent ozone intensities in the MW, 10 and 5 μm ranges

“…Together with a recent high accuracy measurement of ozone at 325 nm [12] , these new revised sets of strong line intensities should provide a decisive step forward to solving the long-standing UV-IR discrepancy problem. The evaluation of uncertainties for a new line list is subject of a HITRAN Work group including the teams of [50,28,29,47,57].…”

“…To this end, we used on the theoretical side several versions of the dipole moment surfaces (DMS) computed by external electric field approach [34] utilizing the multireference configuration interaction MRCI-SD method ( [4,5,35] and references therein) in an "internally contracted" version as implemented in the MOLPRO code of electronic structure calculations [34] . The MRCI-SD electronic wave functions were built on the top of the CASSCF (18,12) full valence active space involving the 2s and 2p atomic orbitals of oxygen as described in [36] including relativistic corrections. The correlation consistent quintuple atomic basis set of Dunning [37] , augmented by diffuse functions aug-cc-pV5z was employed [36] , as it provided a large scale qualitatively correct picture for ozone band intensities [32] .…”

“…A very recent absorption cross section measurement at 325 nm actually indicates that even after the new recommendations a discrepancy of about 2%-3% remains when measurements in this wavelength region are compared to results from the 10 μm range [12] . Other comparisons, between the 10 and 5 μm ranges [13,14] show that relative discrepancies at the 1 to 2 % level exist between some data sets.…”

Quality assessment of Dobson spectrophotometers for ozone column measurements before and after automation at Arosa and Davos

“…In the past 10 years, the EMRP-ATMOZ project has contributed to an improved understanding of the sun spectrophotometer's measurement principle (ATMOZ, 2018). Thus, measurements of the Dobson slit functions (Köhler et al, 2018), of the ozone cross-sections and their temperature dependencies (Bass and Paur, 1985;Serdyuchenko et al, 2014;Malicet et al, 1995;Janssen et al, 2018), of the stray-light effect (Christodoulakis et al, 2015;Karppinen et al, 2015;Moeini et al, 2019) and their implications on the ozone column retrieval for different instruments (Redondas et al, 2014) are now available. An adaptation of the processing algorithm with these recent findings would certainly improve the absolute accuracy of the ozone observations.…”

First pressure shift measurement of ozone molecular lines at 9.54 μm using a tunable quantum cascade laser

“…Uncertainties of pressure and wavenumber shifts were determined independently and the uncertainty has then been obtained from weighted fits with errors in both coordinates, using a total least squares algorithm that allows to introduce correlated uncertainty values [35]. The implementation of the fit algorithm and its performance have been discussed elsewhere [6].…”