Technical note: Evaluation of profile retrievals of aerosols and trace gases for MAX-DOAS measurements under different aerosol scenarios based on radiative transfer simulations

Abstract

Abstract. Ground-based Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) is a state-of-the-art remote sensing technique for deriving vertical profiles of trace gases and aerosols. However, MAX-DOAS profile inversions under aerosol pollution scenarios are challenging because of the complex radiative transfer and limited information content of the measurements. In this study, the performances of two inversion algorithms were evaluated for various aerosol pollution scenarios based on synthetic slant column densities (SCDs) derived from radiative transfer simulations. Compared to previous studies, in our study, much larger ranges of aerosol optical depth (AOD) and NO2 vertical column densities (VCDs) are covered. One inversion algorithm is based on optimal estimation; the other uses a parameterized approach. In this analysis, three types of profile shapes for aerosols and NO2 were considered: exponential, Boltzmann, and Gaussian. First, the systematic deviations of the retrieved aerosol profiles from the input profiles were investigated. For most cases, the AODs of the retrieved profiles were found to be systematically lower than the input values, and the deviations increased with increasing AOD. In particular for the optimal estimation algorithm and for high AOD, these findings are consistent with the results in previous studies. The assumed single scattering albedo (SSA) and asymmetry parameter (AP) have a systematic influence on the aerosol retrieval. However, for most cases the influence of the assumed SSA and AP on the retrieval results are rather small (compared to other uncertainties). For the optimal estimation algorithm, the agreement with the input values can be improved by optimizing the covariance matrix of the a priori uncertainties. Second, the aerosol effects on the NO2 profile retrieval were tested. Here, especially for the optimal estimation algorithm, a systematic dependence on the NO2 VCD was found, with a strong relative overestimation of the retrieved results for low NO2 VCDs and an underestimation for high NO2 VCDs. In contrast, the dependence on the aerosol profiles was found to be rather low. Interestingly, the results for both investigated wavelengths (360 and 477 nm) were found to be rather similar, indicating that the differences in the radiative transfer between both wavelengths have no strong effect. In general, both inversion schemes can retrieve the near-surface values of aerosol extinction and trace gas concentrations well.

Evaluation of MAX-DOAS Profile Retrievals under Different Vertical Resolutions of Aerosol and NO2 Profiles and Elevation Angles

“…The results indicated that the effect of vertical resolutions for the Lidar profiles as aerosol information on NO 2 profile retrieval was insignificant. This finding aligns with previous reports [22], confirming that the retrieved NO 2 profiles were consistent based on the input aerosol profile and the retrieved aerosol profile.…”

“…In this study, the PriAM algorithm, based on the SCIAMACHY radiative transfer model (SCIATRAN), which was used to retrieve trace gas VCDs and profiles, was developed by the Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences (AIOFM, CAS), in cooperation with the Max Planck Institute for Chemistry (MPIC) [24,37,38]. PriAM also consists of a two-step inversion procedure based on the optimal estimation method (OEM) [8,18,22,27,39]. PriAM can retrieve trace gas and aerosol profiles for any arbitrary vertical grid.…”

“…Next, the tropospheric (0-4 km) trace gas profiles, VCDs, and VMRs are retrieved based on the aerosol profiles derived in step 1 and the trace gas dSCDs [18,19]. In order to study the accuracy of the MAX-DOAS inversion algorithm, extensive comparative studies have been carried out based on large-scale comprehensive observation experiments and simulation verification, which mainly included (1) remote-sensing techniques based on synthetic data that tested the performance of the algorithms for the retrieval of the atmospheric state [2,4,[20][21][22];…”

“…[23][24][25][26]; (3) intercomparisons of different algorithms [18,19,23]. In order to study the effect of physical characteristics and the spatiotemporal distributions of aerosols on the remote sensing of trace gases, Tian et al [22] studied the effect of the aerosol extinction and profile shape on the retrieved NO 2 profiles based on synthetic data. The vertical resolution of the profile inversion algorithm used in the previous study was 200 m. In addition, the retrieved aerosol profile in step 1 was also required as the a priori information to retrieve the trace gas profile.…”

Research of NO2 vertical profiles with look-up table method based on MAX-DOAS

“…Then the O 4 DAMF α and the look-up table method are applied to retrieve the aerosol profile information. [12,13] Finally the trace gas profile information is retrieved by the trace gas DSCD α and look-up table method. The look-up table method [5,6] employs finite parameters to represent the aerosol extinction profile and trace gas vertical profile, which include aerosol optical depth (trace gas is VCDs), the boundary layer height, the weight factor of NO 2 in the boundary layer, etc.…”