The impact of vibrational Raman scattering of air on DOAS measurements of atmospheric trace gases
Atmospheric Measurement Techniques Discussions
In remote sensing applications, such as differential optical absorption spectroscopy (DOAS), atmospheric scattering processes need to be considered. After inelastic scattering on N<sub>2</sub> and O<sub>2</sub> molecules, the scattered photons occur as additional intensity at a different wavelength, effectively leading to filling-in of both solar Fraunhofer lines and absorptions of atmospheric constituents. <br><br> Measured spectra in passive DOAS applications are typically corrected for
... corrected for rotational Raman scattering (RRS), also called Ring effect, which represents the main contribution to inelastic scattering. In contrast to that, vibrational Raman scattering (VRS) of N<sub>2</sub> and O<sub>2</sub> has often been thought to be negligible, but also contributes. <br><br> Consequences of VRS are red-shifted Fraunhofer structures in scattered light spectra and filling-in of Fraunhofer lines, additional to RRS. We describe how to calculate VRS correction spectra in analogy to the Ring spectrum. <br><br> We discuss further the impact of VRS cross-sections for O<sub>2</sub> and N<sub>2</sub> on passive DOAS measurements. The relevance of VRS is shown for the first time in spectral evaluations of Multi-Axis DOAS data. This measurement data yields in agreement with calculated scattering cross-sections, that the observed VRS cross-section amounts to 2.2 ± 0.4% of the cross-section of RRS under tropospheric conditions. It is concluded, that this phenomenon has to be included in the spectral evaluation of weak absorbers as it reduces the measurement error significantly and can cause apparent differential optical depth of up to 2.5 × 10<sup>−4</sup>. Its influence on the spectral retrieval of IO, Glyoxal, water vapour and NO<sub>2</sub> in the blue wavelength range is evaluated. For measurements with a large Ring signal a significant and systematic bias of NO<sub>2</sub> dSCDs up to (−3.8 ± 0.4) × 10<sup>14</sup> molec cm<sup>−2</sup> at low elevation angles is observed if this effect is not considered.