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.
Highlights
In recent years, several large-scale aerosol pollution incidents in China (Hu et al, 2014; Huang et al, 2014; Wang et al, 2014; Zhang and Cuo, 2015) have drawn increasing attention due to their effects on atmospheric visibility and health
The fitting error for all O4 differential slant column densities (DSCDs) is set as 0.03 × 1043 molecules2 cm−5 and that for NO2 DSCDs to 1 % of the NO2 DSCDs in the PriAM and Mainz profile algorithm (MAPA) retrievals
In order to limit the number of investigated profiles, first a sensitivity study with PriAM was carried out for the selected profile shapes in Table 1
Summary
Several large-scale aerosol pollution incidents in China (Hu et al, 2014; Huang et al, 2014; Wang et al, 2014; Zhang and Cuo, 2015) have drawn increasing attention due to their effects on atmospheric visibility and health. The physical and chemical properties and the spatial–temporal distributions of aerosols can both affect remote sensing measurements of trace gases in the atmosphere (Seinfeld and Pandis, 1998; Quinn et al, 1998; Bond et al, 2001; Sheridan et al, 2001). The ground-based Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) technique can be performed with a relatively simple setup and very low power consumption in the ultraviolet (UV) and visible (Vis) spectral range to synchronously measure the vertical distributions of aerosol optical extinction and concentrations of several trace gases (e.g., NO2, SO2, HCHO, HONO, and CHOCHO) in the troposphere (Hönninger and Platt, 2002; Hönninger et al, 2004; Wittrock et al, 2004; Wagner et al, 2004; Frieß et al, 2006). The vertical profiles and vertical column densities (VCDs) of trace gases can be retrieved from the EA dependence of DSCDs using the result of the aerosol profile inversion from MAX-DOAS (Irie et al, 2008a, 2009; Li et al, 2010; Clémer et al, 2010; Hartl and Wenig, 2013; Hendrick et al, 2014; Vlemmix et al, 2015; Frieß et al, 2006)
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