Intercomparison of MAX-DOAS vertical profile retrieval algorithms: studies using synthetic data

Atmos. Meas. Tech., 12, 2155–2181, 2019 https://doi.org/10.5194/amt-12-2155-2019 © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. Intercomparison of MAX-DOAS vertical profile retrieval algorithms: studies using synthetic data Udo Frieß1 , Steffen Beirle2 , Leonardo Alvarado Bonilla3 , Tim Bösch3 , Martina M. Friedrich4 , François Hendrick4 , Ankie Piters5 , Andreas Richter3 , Michel van Roozendael4 , Vladimir V. Rozanov3 , Elena Spinei6,a , Jan-Lukas Tirpitz1 , Tim Vlemmix5 , Thomas Wagner2 , and Yang Wang2 1 Institute of Environmental Physics, University of Heidelberg, Heidelberg, Germany 2 Max Planck Institute for Chemistry, Mainz, Germany 3 Institute of Environmental Physics, University of Bremen, Bremen, Germany 4 Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Brussels, Belgium 5 Royal Netherlands Meteorological Institute (KNMI), De Bilt, the Netherlands 6 NASA-Goddard, Greenbelt, Maryland, USA a now at: Virginia Tech, Blacksburg, Virginia, USA Correspondence: Udo Frieß () Received: 3 December 2018 – Discussion started: 17 December 2018 Revised: 22 March 2019 – Accepted: 26 March 2019 – Published: 10 April 2019 Abstract. Multi-axis differential optical absorption spectroscopy (MAX-DOAS) is a widely used measurement technique for the detection of a variety of atmospheric trace gases. Using inverse modelling, the observation of trace gas column densities along different lines of sight enables the retrieval of aerosol and trace gas vertical profiles in the atmospheric boundary layer using appropriate retrieval algorithms. In this study, the ability of eight profile retrieval algorithms to reconstruct vertical profiles is assessed on the basis of synthetic measurements. Five of the algorithms are based on the optimal estimation method, two on parametrised approaches, and one using an analytical approach without involving any radiative transfer modelling. The synthetic measurements consist of the median of simulated slant column densities of O4 at 360 and 477 nm, as well as of HCHO at 343 nm and NO2 at 477 nm, from seven datasets simulated by five different radiative transfer models. Simulations are performed for a combination of 10 trace gas and 11 aerosol profiles, as well as 11 elevation angles, three solar zenith, and three relative azimuth angles. Overall, the results from the different algorithms show moderate to good performance for the retrieval of vertical profiles, surface concentrations, and total columns. Except for some outliers, the root-meansquare difference between the true and retrieved state ranges between (0.05–0.1) km−1 for aerosol extinction and (2.5– 5.0) ×1010 molec cm−3 for HCHO and NO2 concentrations. 1 Introduction The planetary boundary layer (PBL) is the part of the atmosphere that is in direct contact with the terrestrial biosphere. Its chemical composition is determined by anthropogenic and natural emissions. Monitoring of both chemical composition and aerosol content of the PBL is crucial for the understanding of the chemical and physical processes and the spatio-temporal evolution of PBL composition. A versatile tool for the monitoring of atmospheric trace gases and aerosol content of the PBL is the well-known multi-axis differential optical absorption spectroscopy (MAX-DOAS) (e.g. Hönninger et al., 2004; Wagner et al., 2004; Heckel et al., 2005; Frieß et al., 2006; Platt and Stutz, 2008; Irie et al., 2008; Clémer et al., 2010; Wagner et al., 2011; Vlemmix et al., 2015b). It relies on the spectral analysis of scattered sunlight and enables the simultaneous detection of numerous trace gases, such as nitrogen dioxide (NO2 ), formaldehyde (HCHO), nitrous acid (HONO), water vapour (H2 O), sulfur dioxide (SO2 ), ozone (O3 ), and halogen oxides. Measurements along different lines of sight, with eleva- Published by Copernicus Publications on behalf of the European Geosciences Union. 2156 U. Frieß et al.: MAX-DOAS vertical profiling comparison tion angles (EAs) ranging from near the horizon to the zenith, allow for the reconstruction of vertical profiles of the measured trace gases and, using the oxygen collision complex O4 as a proxy for the light path, also of aerosol extinction. Using suitable inverse models, trace gas and aerosol profiles can be retrieved in the lowermost ≈ 2 km with a vertical resolution of about 50–100 m at the surface and a lower resolution above. Up to four independent pieces of information can be retrieved. Algorithms for the retrieval of vertical profiles from MAXDOAS measurements can be separated into those that retrieve vertical profiles on a finite vertical grid (usually with layers of 50–200 m in thickness) using the optimal estimation method (OEM) (Rodgers, 2000) and parametrised algorithms that use a small number of parameters (typically two to four) to describe the shape of the atmospheric profile. Parametrised algorithms are typically faster than OEM algorithms since they are usually based on precalculated lookup tables (LUTs), while OEM algorithms rely on online radiative transfer modelling (RTM). Being based on Bayesian statistics, OEM algorithms have the advantage of providing a thorough error analysis as well as a quantitative characterisation of the vertical resolution and the information content (Rodgers, 2000). However, the results of OEM algorithms critically depend on the appropriate choice of a priori constraints, which are in many cases difficult to assess. In addition to OEM and parametrised approaches, the present study also includes a fast algorithm developed by NASA, which relies only on geometrical considerations and only invokes radiative transfer modelling for a pure Rayleigh atmosphere. Testing the performance of algorithms for the retrieval of the atmospheric state using remote-sensing measurements on the basis of synthetic data is a method that has been widely used in the scientific community. In particular, numerous synthetic studies that investigated the performance of MAX-DOAS retrieval algorithms were published in the past (Wagner et al., 2004; Frieß et al., 2006; Hay, 2010; Vlemmix et al., 2011; Yilmaz, 2012; Hartl and Wenig, 2013; Holla, 2013; Zielcke, 2015). This paper presents the first intercomparison of eight state-of-the-art algorithms for the retrieval of vertical profiles of aerosols and trace gases using synthetic MAX-DOAS measurements. Synthetic measurements have the advantage over ambient measurements that the true atmospheric state is exactly known, and thus a quantitative comparison of true and retrieved atmospheric states is straightforward. This study is part of the Fiducial Reference Measurements for Ground-Based DOAS Air-Quality Observations (FRM4 DOAS) project funded by the European Space Agency (see http://frm4doas.aeronomie.be, last access: 8 April 2019). One of the main objectives of this project is the development of a community algorithm for a harmonised near-real-time processing of MAX-DOAS data, (...truncated)