Abstract
Abstract. 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 multi-axis differential optical absorption spectroscopy (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, direct-sun DOAS, Sun photometer and meteorological instruments). 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, with the focus on aerosol optical thicknesses (AOTs), trace gas vertical column densities (VCDs) and trace gas surface concentrations. The algorithms are based on three different techniques: six use the optimal estimation method, two use a parameterized approach and one algorithm relies on simplified radiative transport assumptions and analytical calculations. To assess the agreement among the inversion algorithms independent of inconsistencies in the trace gas slant column density acquisition, participants applied their inversion to a common set of slant columns. Further, important settings like the retrieval grid, profiles of O3, temperature and pressure as well as aerosol optical properties and a priori assumptions (for optimal estimation algorithms) have been prescribed to reduce possible sources of discrepancies. The profiling results were found to be in good qualitative agreement: most participants obtained the same features in the retrieved vertical trace gas and aerosol distributions; however, these are sometimes at different altitudes and of different magnitudes. Under clear-sky conditions, the root-mean-square differences (RMSDs) among the results of individual participants are in the range of 0.01–0.1 for AOTs, (1.5–15) ×1014molec.cm-2 for trace gas (NO2, HCHO) VCDs and (0.3–8)×1010molec.cm-3 for trace gas surface concentrations. These values compare to approximate average optical thicknesses of 0.3, trace gas vertical columns of 90×1014molec.cm-2 and trace gas surface concentrations of 11×1010molec.cm-3 observed over the campaign period. The discrepancies originate from differences in the applied techniques, the exact implementation of the algorithms and the user-defined settings that were not prescribed. For the comparison against supporting observations, the RMSDs increase to a range of 0.02–0.2 against AOTs from the Sun photometer, (11–55)×1014molec.cm-2 against trace gas VCDs from direct-sun DOAS observations and (0.8–9)×1010molec.cm-3 against surface concentrations from the long-path DOAS instrument. This increase in RMSDs is most likely caused by uncertainties in the supporting data, spatiotemporal mismatch among the observations and simplified assumptions particularly on aerosol optical properties made for the MAX-DOAS retrieval. As a side investigation, the comparison was repeated with the participants retrieving profiles from their own differential slant column densities (dSCDs) acquired during the campaign. In this case, the consistency among the participants degrades by about 30 % for AOTs, by 180 % (40 %) for HCHO (NO2) VCDs and by 90 % (20 %) for HCHO (NO2) surface concentrations. In former publications and also during this comparison study, it was found that MAX-DOAS vertically integrated aerosol extinction coefficient profiles systematically underestimate the AOT observed by the Sun photometer. For the first time, it is quantitatively shown that for optimal estimation algorithms this can be largely explained and compensated by considering biases arising from the reduced sensitivity of MAX-DOAS observations to higher altitudes and associated a priori assumptions.
Highlights
The planetary boundary layer (PBL) is the lowest part of the atmosphere, whose behaviour is directly influenced by its contact with the Earth’s surface
For clear-sky conditions, we find that the differences in the measured differential slant column densities (dSCDs) are responsible for approximately 40 %, 85 % (HCHO vertical column densities (VCDs)), 70 % (HCHO surface concentrations), 50 % (NO2 VCDs), 40 % (NO2 UV surface concentrations) and 20 % (NO2 Vis surface concentrations) of the total variance observed among the participants
15 participants used nine different profiling algorithms with three different technical approaches (optimal estimation (OEM), parameterized (PAR) and analytical (ANA) approach) to retrieve aerosol and trace gas (NO2, HCHO) vertical profiles from a common set of dSCDs which was recorded during the CINDI-2 campaign
Summary
The planetary boundary layer (PBL) is the lowest part of the atmosphere, whose behaviour is directly influenced by its contact with the Earth’s surface. Profiles can be retrieved from this information by applying numerical inversion algorithms, typically incorporating radiative transfer models. This study compares MAX-DOAS profiles of NO2 and HCHO concentrations as well as the aerosol extinction coefficient (derived from O4 observations) from 15 of the 24 groups. In a recent publication by Bösch et al (2018), CINDI-2 MAX-DOAS profiles retrieved with the Bremen Optimal estimation REtrieval for Aerosols and trace gaseS (BOREAS) algorithm were already compared against supporting observations but regarding a few days only. It shall be mentioned that already in the course of the precedent CINDI-1 campaign in 2009, there were comparisons of MAX-DOAS aerosol extinction coefficient profiles, e.g. by Frieß et al (2016) and Zieger et al (2011); these are over shorter periods and a smaller group of participants.
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