Abstract
Abstract. A method is developed for the calibration of the colour index (CI) and the O4 absorption derived from differential optical absorption spectroscopy (DOAS) measurements of scattered sunlight. The method is based on the comparison of measurements and radiative transfer simulations for well-defined atmospheric conditions and viewing geometries. Calibrated measurements of the CI and the O4 absorption are important for the detection and classification of clouds from MAX-DOAS observations. Such information is needed for the identification and correction of the cloud influence on Multi AXis (MAX-)DOAS profile inversion results, but might be also be of interest on their own, e.g. for meteorological applications. The calibration algorithm was successfully applied to measurements at two locations: Cabauw in the Netherlands and Wuxi in China. We used CI and O4 observations calibrated by the new method as input for our recently developed cloud classification scheme and also adapted the corresponding threshold values accordingly. For the observations at Cabauw, good agreement is found with the results of the original algorithm. Together with the calibration procedure of the CI and O4 absorption, the cloud classification scheme, which has been tuned to specific locations/conditions so far, can now be applied consistently to MAX-DOAS measurements at different locations. In addition to the new threshold values, further improvements were introduced to the cloud classification algorithm, namely a better description of the SZA (solar zenith angle) dependence of the threshold values and a new set of wavelengths for the determination of the CI. We also indicate specific areas for future research to further improve the cloud classification scheme.
Highlights
One major aim of this study is to provide a universal cloud classification scheme for Multi AXis (MAX-)differential optical absorption spectroscopy (DOAS) measurements based on the new calibration procedures for the colour index (CI) and the O4 absorption and the updated threshold values
We developed methods for the calibration of the colour index (CI) and the O4 absorption derived from MAX-DOAS measurements of scattered sunlight, which are an important step towards a universal cloud classification scheme for MAXDOAS observations
Both calibration methods are based on the comparison of measurements and radiative transport simulations for well-defined atmospheric conditions and limited solar zenith angle (SZA) ranges
Summary
Multi AXis differential optical absorption spectroscopy (MAX-DOAS) measurements are a widely used remote sensing technique for the measurement of atmospheric trace gases and aerosols (e.g. Hönninger and Platt, 2002; Wittrock et al, 2004; Hönninger et al, 2004; Heckel et al, 2005; Frieß et al, 2006; Irie et al, 2008; Clémer et al, 2010; Li et al, 2010; Wagner et al, 2011; Ma et al, 2013; Hendrick et al, 2014; Wang et al, 2014, 2015; Vlemmix et al, 2015). Since MAX-DOAS instruments are usually not radiometrically calibrated, a direct quantitative comparison of measured and simulated quantities is not possible, which hampers the direct application of threshold values derived from radiative transfer simulations. To overcome this limitation, in this study we develop calibration procedures for the CI and the O4 absorption and apply them to MAX-DOAS observations. One major aim of this study is to provide a universal cloud classification scheme for MAX-DOAS measurements based on the new calibration procedures for the CI and the O4 absorption and the updated threshold values.
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