Abstract
Abstract. We present different methods for in-field elevation calibration of MAX-DOAS (Multi AXis Differential Optical Absorption Spectroscopy) instruments that were applied and inter-compared during the second Cabauw Intercomparison campaign for Nitrogen Dioxide measuring Instruments (CINDI-2). One necessary prerequisite of consistent MAX-DOAS retrievals is a precise and accurate calibration of the elevation angles of the different measuring systems. Therefore, different methods for this calibration were applied to several instruments during the campaign, and the results were inter-compared. This work first introduces and explains the different methods, namely far- and near-lamp measurements, white-stripe scans, horizon scans and sun scans, using data and results for only one (mainly the Max Planck Institute for Chemistry) instrument. In the second part, the far-lamp measurements and the horizon scans are examined for all participating groups. Here, the results for both methods are first inter-compared for the different instruments; secondly, the two methods are compared amongst each other. All methods turned out to be well-suited for the calibration of the elevation angles of MAX-DOAS systems, with each of them having individual advantages and drawbacks. Considering the results of this study, the systematic uncertainties of the methods can be estimated as ±0.05∘ for the far-lamp measurements and the sun scans, ±0.25∘ for the horizon scans, and around ±0.1∘ for the white-stripe and near-lamp measurements. When comparing the results of far-lamp and horizon-scan measurements, a spread of around 0.9∘ in the elevation calibrations is found between the participating instruments for both methods. This spread is of the order of a typical field of view (FOV) of a MAX-DOAS instrument and therefore affecting the retrieval results. Further, consistent (wavelength dependent) offsets of 0.32∘ and 0.40∘ between far-lamp measurements and horizon scans are found, which can be explained by the fact that, despite the flat topography around the measurement site, obstacles such as trees might mark the visible horizon during daytime. The observed wavelength dependence can be explained by surface albedo effects. Lastly, the results are discussed and recommendations for future campaigns are given.
Highlights
Multi AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) is a well established method of ground-based remote sensing of trace gases and aerosols
MAX-DOAS instruments measure spectra of scattered sunlight at different elevation angles. They have a high sensitivity to trace gases and aerosols located close to the surface (e.g. Hönninger and Platt, 2002; Hönninger et al, 2004; Irie et al, 2008; Van Roozendael et al, 2004; Wagner et al, 2004, 2011; Wittrock et al, 2004). Such measurements allow for the retrieval of vertical profiles of trace gases and aerosol extinction as well as column properties such as vertical column densities (VCDs) and aerosol optical depths (AODs) (e.g. Frieß et al, 2006; Irie et al, 2008; Clémer et al, 2010; Wagner et al, 2011)
An analysis of the CINDI-2 data set shows that, for low elevation angles, wrong pointing has a large impact on the retrieved trace-gas differential slant column densities, which are the basic quantity obtained by MAX-DOAS (Hönninger et al, 2004)
Summary
Multi AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) is a well established method of ground-based remote sensing of trace gases and aerosols. MAX-DOAS instruments measure spectra of scattered sunlight at different (mostly low) elevation angles They have a high sensitivity to trace gases and aerosols located close to the surface The curves were obtained by calculating the derivative of the dSCD curves from panel (a) with respect to the elevation angle and dividing the results by the dSCDs at the corresponding elevations This approach remains qualitative, it shows clearly under which conditions pointing errors can lead to substantial biases in the dSCDs. As an example, an error of 1◦ in the telescope’s elevation close to 0◦ elevation can lead to an error of around 20 % in the retrieved NO2 dSCD, since the sensitivity of MAX-DOAS measurements depends strongly on the elevation angle and NO2 is mainly located close to the surface
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