The Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) is widely used as an advanced model dataset for the understanding of global climate change. However, independent validation and comparison of MERRA-2 are both insufficient and always desired. Therefore, in this study, the quantitative evaluation of MERRA-2 aerosol products was conducted over China for Aerosol Optical Depth (AOD), Angstrom exponent (AE), absorbing AOD (AAOD) and chemical components (black carbon (BC) and dust (DU)) using Sun sky radiometer Observation NETwork (SONET) ground-based measurements and POLDER-3/PARASOL satellite products generated by the GRASP algorithm. The available MERRA-2 monthly dataset and PARASOL/GRASP monthly and seasonal products were intercompared over China. MERRA-2 AOD (550 nm) show general good agreement with SONET and PARASOL/GRASP. For example, the correlation coefficients are usually 0.6–0.85 with SONET and 0.75–0.85 with PARASOL/GRASP, the bias is usually −0.293 to +0.008 with SONET. For AE and AAOD, the agreement is still reasonable. MERRA-2 is found to overestimate fine mode AE and to display a general underestimation of aerosol absorption over China. In addition, MERRA-2 BC and DU mass concentrations show spatial and quantitative consistency with PARASOL/GRASP components climatological products. The relatively high columnar BC mass concentration is observed around 1.5–2 mg/m³ over the East China industrial region and high DU mass concentration is around 150 mg/m³ near Taklimakan desert. MERRA-2 shows slightly higher BC and lower DU concentration than PARASOL/GRASP over East China. The evaluations with in situ BC measurements near surface verify the overestimation (MAE = +0.44 µg/m²) of MERRA-2 and underestimation (MAE = −0.38 µg/m²) of PARASOL/GRASP. The analysis demonstrates multi-source datasets, such as ground-based, space-borne remote sensing, in situ measurements, model simulation as well as reanalysis data, complement each other and can be used to refine aerosol characterization.

Using the numerically exact multiple sphere T-matrix (MSTM) method, we explored the effects of brown coatings on absorption enhancement (E_abs) of black carbon (BC) at different wavelengths (λ). In addition, the ratio of the absorption of BC coated by brown carbon (BrC) to an external mixture of BrC and BC (E_{abs_internal}) is also investigated. In this work, thinly coated BC is defined as that with a BC volume fraction over 20 %, and other BC is considered to be thickly coated. E_abs increases with the absorption of coatings, while an opposite trend is observed for E_{abs_internal}. A much wider range of E_abs is observed for BC with brown coatings compared to that with non-absorbing coatings. As the mass ratio of BrC to BC (M_R) is over 13.9, E_abs can exceed 5.4 for BC with brown coatings at λ=0.35 µm under a typical size distribution. Specifically, as M_R increases to approximately 13.9, E_abs values of larger than 3.96 can be observed at 0.532 µm, which is a little higher than the commonly measured E_abs of 1.05–3.5 at this wavelength. Previous studies have focused on the lensing effects of coatings but neglected the blocking effects of absorbing coatings. E_{abs_internal} can be below 1 at an ultraviolet spectral region for BC with brown coatings, which indicates that the absorption of internally mixed BC is less than that of an external mixture of BrC and BC due to the blocking effects of outer coatings, and we named the blocking effect of absorbing coatings the “sunglasses effect”. In addition, the applicability of a core–shell sphere model is also evaluated for BC with brown coatings. The absorption cross section (C_abs) of thickly coated BC is underestimated by the core–shell sphere model for all wavelengths while the underestimation becomes negligible as the imaginary part of the refractive index of brown carbon (k_BrC) becomes very large. The lensing effect and the sunglasses effect are clearly defined. Moreover, the effects of composition ratios and the size distribution are explored at different wavelengths. Our findings can improve the understanding of the absorption enhancement of BC with brown coatings.

In this work, an assembly of coating structures is generated for coated black carbon (BC) based on two different coating methods with limited tunable parameters. By defining typical parameters, the proposed coating structures show great agreement with the detailed morphologies of coated BC particles in the atmosphere. Our models can accurately reproduce the measured linear backscattering depolarization ratio (LDR). We noticed that BC with spherical coatings (Configuration E) has difficulty estimating large LDRs, while the ability to reproduce a large LDR is improved by making the coating structure slightly nonspherical (Configuration C). Our models also show a better performance in reproducing the laboratory‐measured absorption enhancement (E_abs), mass absorption/scattering cross sections, and absorption Ångström exponent than the simplified models. In addition, sizable uncertainties in the optical properties of coated BC with different coating configurations are found. The uncertainties in the LDR, E_abs, mass absorption cross section, and absorption Ångström exponent caused by the coating configurations can reach approximately 220%, 35%, 90%, and 20%, respectively. Therefore, complex coating structures should be carefully considered. In this work, various coating structures are represented by limited tunable parameters, which is beneficial to the parameterization of the optical properties of BC with a complex coating structure. Our models can also provide tools for exploring the agreement between calculations and measurements.

Absorption cross section (C_abs), scattering cross section (C_sca) and asymmetry parameter (ASY) of soot particles in different atmospheric aging status were investigated under fixed equivalent volume radius (R_V) using the numerically exact multiple-sphere T-matrix method. The radiative properties of soot particles would be largely diverse in different aging status even R_V is fixed. However, there are many insensitive parameters under different aging status. The C_abs and ASY is insensitive to monomers number (N_s) when N_s is larger than a threshold value. For bare and thinly coated soot aggregates, C_abs is insensitive to fractal dimension (D_f) when the R_V is small, where the relative errors of C_abs for different D_f are within 2.5%. However, the effects of D_f is obvious for large soot due to the shielding effects of large monomers, and the relative errors for different D_f can reach to 18% for bare soot. For thinly coated soot, the changes of ASY with soot volume fraction (f_soot) is small due to the little changes of the fractal structure when the R_V is fixed. In addition, for thickly coated soot, ASY is insensitive to N_s due to the unchanged overall spherical structure. Our results give a further understanding of the influences of morphology on radiative properties. It may be helpful for model selection and model simplification.