Extending differential optical absorption spectroscopy for limb measurements in the UV

Abstract

Abstract. Methods of UV/VIS absorption spectroscopy to determine the constituents in the Earth's atmosphere from measurements of scattered light are often based on the BeerLambert law, like e.g. Differential Optical Absorption Spectroscopy (DOAS). While the Beer-Lambert law is strictly valid for a single light path only, the relation between the optical depth and the concentration of any absorber can be approximated as linear also for scattered light observations at a single wavelength if the absorption is weak. If the light path distribution is approximated not to vary with wavelength, also linearity between the optical depth and the product of the cross-section and the concentration of an absorber can be assumed. These assumptions are widely made for DOAS applications for scattered light observations. For medium and strong absorption of scattered light (e.g. along very long light-paths like in limb geometry) the relation between the optical depth and the concentration of an absorber is no longer linear. In addition, for broad wavelength intervals the differences in the travelled light-paths at different wavelengths become important, especially in the UV, where the probability for scattering increases strongly with decreasing wavelength.However, the DOAS method can be extended to cases with medium to strong absorptions and for broader wavelength intervals by the so called air mass factor modified (or extended) DOAS and the weighting function modified DOAS. These approaches take into account the wavelength dependency of the slant column densities (SCDs), but also require a priori knowledge for the air mass factor or the weighting function from radiative transfer modelling.Correspondence to: J. Puķīte (janis.pukite@mpic.de)We describe an approach that considers the fitting results obtained from DOAS, the SCDs, as a function of wavelength and vertical optical depth and expands this function into a Taylor series of both quantities. The Taylor coefficients are then applied as additional fitting parameters in the DOAS analysis. Thus the variability of the SCD in the fit window is determined by the retrieval itself.This new approach provides a description of the SCD the exactness of which depends on the order of the Taylor expansion, and is independent from any assumptions or a priori knowledge of the considered absorbers.In case studies of simulated and measured spectra in the UV range (332-357 nm), we demonstrate the improvement by this approach for the retrieval of vertical profiles of BrO from the SCIAMACHY limb observations. The results for BrO obtained from the simulated spectra are closer to the true profiles, when applying the new method for the SCDs of ozone, than when the standard DOAS approach is used. For the measured spectra the agreement with validation measurements is also improved significantly, especially for cases with strong ozone absorption.While the focus of this article is on the improvement of the BrO profile retrieval from the SCIAMACHY limb measurements, the novel approach may be applied to a ...

Investigating differences in DOAS retrieval codes using MAD-CAT campaign data

“…6, bottom left). The main reason for this is probably the larger NO 2 slant columns as associated effects like the wavelength depen-dence of the NO 2 slant column (Pukite et al, 2010) were not compensated in this intercomparison exercise as discussed in Sect. 1.…”

Evaluation of tropospheric SO<sub>2</sub> retrieved from MAX-DOAS measurements in Xianghe, China

“…A fifth-order polynomial is applied to fit the low-frequency spectral structure due to Rayleigh and Mie scattering and instrumental effects. Attempts to further adjust these settings, e.g., by adding BrO cross section or by including additional ozone correction terms according to Puķīte et al (2010), were not successful (less stable retrievals with larger noise on the SO 2 DSCDs). Figure 4 shows a typical example of a DOAS fit for SO 2 at 43 • SZA.…”

Sulfur dioxide retrievals from TROPOMI onboard Sentinel-5 Precursor: algorithm theoretical basis

“…It should be noted that the O 3 slant column is dependent on the wavelength when applying the approach of Pukīte et al (2010):…”

“…3). Specifically, in the first two intervals, absorption cross sections of O 3 at 228 and 243 K are included in the fit and, to better cope with the strong (nonlinear) ozone absorption at short wavelengths, the retrieval also includes two pseudocross sections following the approach of Pukīte et al (2010): λσ O 3 and σ 2 O 3 calculated from the O 3 cross-section spectrum at 228 K. The correction for the Ring effect is based on the technique outlined by Vountas et al (1998). This technique involves a PCA of a set of Ring spectra, calculated for a range of solar zenith angles.…”