Intercomparison of MAX-DOAS vertical profile retrieval algorithms: studies on field data from the CINDI-2 campaign

Abstract

Abstract. Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) is a well-established ground-based measurement technique for the detection of aerosols and trace gases particularly in the boundary layer and the lower troposphere: ultraviolet- and visible radiation spectra of skylight are analysed to obtain information on different atmospheric parameters, integrated over the light path from space to the instrument. An appropriate set of spectra recorded under different viewing geometries ("Multi-Axis") allows retrieval of tropospheric aerosol and trace gas vertical distributions by applying numerical inversion methods. The second Cabauw Intercomparison of Nitrogen Dioxide measuring Instruments (CINDI-2) took place in Cabauw (The Netherlands) in September 2016 with the aim of assessing the consistency of MAX-DOAS measurements of tropospheric species (NO2, HCHO, O3, HONO, CHOCHO and O4). This was achieved through the coordinated operation of 36 spectrometers operated by 24 groups from all over the world, together with a wide range of supporting reference observations (in situ analysers, balloon sondes, lidars, Long-Path DOAS, sun photometer and others). In the presented study, the retrieved CINDI-2 MAX-DOAS trace gas (NO2, HCHO) and aerosol vertical profiles of 15 participating groups using different inversion algorithms are compared and validated against the colocated supporting observations. The profiles were found to be in good qualitative agreement: most participants obtained the same features in the retrieved vertical trace gas and aerosol distributions, however sometimes at different altitudes and of different intensity. Under clear sky conditions, the root-mean-square differences of aerosol optical thicknesses, trace gas (NO2, HCHO) vertical columns and surface concentrations among the results of individual participants vary between 0.01–0.1, (1.5–15) x 1014 molec cm-2 and (0.3–8) x 1010 molec cm-3, respectively. For the comparison against supporting observations, these values increase to 0.02–0.2, (11–55) x 1014 molec cm-2 and (0.8–9) x 1010 molec cm-3. It is likely that a large part of this increase is caused by imperfect spatio-temporal overlap of the different observations. In contrast to what is often assumed, the MAX-DOAS vertically integrated extinction profiles and the sun photometer total aerosol optical thickness were found to not necessarily being comparable quantities, unless information on the real aerosol vertical distribution is available to account for the low sensitivity of MAX-DOAS observations at higher altitudes.

Intercomparison of NO<sub>2</sub>, O<sub>4</sub>, O<sub>3</sub> and HCHO slant column measurements by MAX-DOAS and zenith-sky UV–visible spectrometers during CINDI-2

“…In addition to dSCD measurements, the subsequent steps in MAX-DOAS retrievals, i.e. their conversion into vertical-column and profile information, is also further investigated in a CINDI-2 profiling working group and as part of the ESA FRM 4 DOAS project (Frieß et al, 2019;Tirpitz et al, 2020). Furthermore, other aspects of the campaign measurements are being further exploited, such as mobile car DOAS observations, reference in situ measurements and instrument elevation pointing calibration (Donner et al, 2020).…”

Inter-comparison of MAX-DOAS measurements of tropospheric HONO slant column densities and vertical profiles during the CINDI-2 campaign

“…Multi-axis differential optical absorption spectroscopy (MAX-DOAS) is widely used as a ground-based remote sensing technique for retrieving lower tropospheric vertical profiles of trace gases (e.g. NO 2 , SO 2 , HCHO) and aerosols from sequential measurements of ultraviolet and visible spectra of scattered sunlight recorded at multiple elevation angles (Hönninger and Platt, 2002;Bobrowski et al, 2003;Van Roozendael et al, 2003;Hönninger et al, 2004;Wagner et al, 2004;Wittrock et al, 2004). MAX-DOAS instruments have been developed with different optical and mechanical systems by different research groups and companies in order to meet the requirements of high accuracy and automatic operation.…”

A multi-axis differential optical absorption spectroscopy aerosol profile retrieval algorithm for high-altitude measurements: application to measurements at Schneefernerhaus (UFS), Germany

“…4a. The vertical resolution of our retrieval grid is lower compared to that of many other studies (e.g., Clémer et al, 2010;Tirpitz et al, 2020); however, the vertical gradient of aerosol extinction at such a high-altitude site is expected to be small and this is also proved by the ceilometer measurements. Therefore, the coarse resolution is considered to be sufficient for the retrieval of the UFS MAX-DOAS measurements.…”

“…Next to the MAX-DOAS instrument, a sun photometer is installed at the UFS, which provides measurements of radiances at 12 wavelengths between 340 and 1640 nm with a temporal resolution of 1 s. The instrument was developed at the Meteorological Institute of Ludwig Maximilian University of Munich (LMU) based on a system operated in the framework of the SAMUM campaigns (Toledano et al, 2009(Toledano et al, , 2011 but with improved electronics and data acquisition developed by Physikalische Messsysteme Ltd. In this study, the AODs derived from sun photometer measurements applying the well-established Rayleigh calibration method were used for the intercomparison with the MAX-DOAS retrieval.…”