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 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.
Highlights
High-resolution reference data for ozone absorption in the UV are widely called for, as this region is used for remote and in situ measurement of atmospheric ozone concentrations, and new measurements are under way in the framework of the ESA TROPOMI/Sentinel 5 precursor mission that aims at establishing an improved atmospheric spectroscopy database (SEOM-IAS)
This was highlighted in the last report of the “Absorption Cross-Sections of Ozone” (ACSO, http://igaco-o3.fmi.fi/ACSO) from the joint initiative of the International Ozone Commission (IO3C), the World Meteorological Organisation (WMO) and the Integrated Global Atmospheric Chemistry Observations (IGACO) O3/UV subgroup, which was dedicated to studying, evaluating and recommending the most suitable cross-section data to be used in atmospheric ozone measurements (Orphal et al, 2016)
Remote sensing of tropospheric ozone by joint retrieval of UV and IR satellite instruments is another emerging application (e.g. Cuesta et al, 2013) that strongly depends on unbiased UV spectroscopic data as most of the ozone resides in the stratosphere, but accurate knowledge of the ozone spectrum is required for the retrieval of other, less abundant trace gases that absorb in spectral ranges where ozone acts as an interfering species
Summary
High-resolution reference data for ozone absorption in the UV are widely called for, as this region is used for remote and in situ measurement of atmospheric ozone concentrations, and new measurements are under way in the framework of the ESA TROPOMI/Sentinel 5 precursor mission that aims at establishing an improved atmospheric spectroscopy database (SEOM-IAS). The demands for augmented quality of these atmospheric measurements have been increasing continuously over the last decades in order to fulfil the requirement of reliably detecting small atmospheric changes. This was highlighted in the last report of the “Absorption Cross-Sections of Ozone” (ACSO, http://igaco-o3.fmi.fi/ACSO) from the joint initiative of the International Ozone Commission (IO3C), the World Meteorological Organisation (WMO) and the Integrated Global Atmospheric Chemistry Observations (IGACO) O3/UV subgroup, which was dedicated to studying, evaluating and recommending the most suitable cross-section data to be used in atmospheric ozone measurements (Orphal et al, 2016). Janssen et al.: Absolute ozone absorption cross section at the 325 nm HeCd laser wavelength
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