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. Ground-based Multi-AXis Differential Optical AbsorptionSpectroscopy (MAX-DOAS) is a state-of-the-art remote sensing technique forderiving vertical profiles of trace gases and aerosols. However, MAX-DOASprofile inversions under aerosol pollution scenarios are challenging becauseof the complex radiative transfer and limited information content of themeasurements. In this study, the performances of two inversion algorithmswere evaluated for various aerosol pollution scenarios based on syntheticslant column densities (SCDs) derived from radiative transfer simulations.Compared to previous studies, in our study, much larger ranges of aerosol optical depth (AOD) andNO2 vertical column densities (VCDs) are covered. One inversion algorithm is based on optimalestimation; the other uses a parameterized approach. In this analysis, threetypes of profile shapes for aerosols and NO2 were considered:exponential, Boltzmann, and Gaussian. First, the systematic deviations ofthe retrieved aerosol profiles from the input profiles were investigated.For most cases, the AODs of the retrieved profiles were found to besystematically lower than the input values, and the deviations increasedwith increasing AOD. In particular for the optimal estimation algorithm and forhigh AOD, these findings are consistent with the results in previous studies.The assumed single scattering albedo (SSA) and asymmetry parameter (AP) have a systematicinfluence on the aerosol retrieval. However, for most cases the influence ofthe assumed SSA and AP on the retrieval results are rather small (comparedto other uncertainties). For the optimal estimation algorithm, the agreementwith the input values can be improved by optimizing the covariance matrix ofthe a priori uncertainties. Second, the aerosol effects on the NO2 profileretrieval were tested. Here, especially for the optimal estimationalgorithm, a systematic dependence on the NO2 VCD was found, with astrong relative overestimation of the retrieved results for low NO2VCDs and an underestimation for high NO2 VCDs. In contrast, thedependence 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 theradiative transfer between both wavelengths have no strong effect. Ingeneral, both inversion schemes can retrieve the near-surface values ofaerosol extinction and trace gas concentrations well.

atmospheric chemistry and physics

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