Intercomparison of the GOS approach, superposition T-matrix method, and laboratory measurements for black carbon optical properties during aging

Abstract

We perform a comprehensive intercomparison of the geometric-optics surface-wave (GOS) approach, the superposition T-matrix method, and laboratory measurements for optical properties of fresh and coated/aged black carbon (BC) particles with complex structures. GOS and T-matrix calculations capture the measured optical (i.e., extinction, absorption, and scattering) cross sections of fresh BC aggregates, with 5-20% differences depending on particle size. We find that the T-matrix results tend to be lower than the measurements, due to uncertainty in theoretical approximations of realistic BC structures, particle property measurements, and numerical computations in the method. On the contrary, the GOS results are higher than the measurements (hence the T-matrix results) for BC radii < 100 nm, because of computational uncertainty for small particles, while the discrepancy substantially reduces to 10% for radii > 100 nm. We find good agreement (differences < 5%) between the two methods in asymmetry factors for various BC sizes and aggregating structures. For aged BC particles coated with sulfuric acid, GOS and T-matrix results closely match laboratory measurements of optical cross sections. Sensitivity calculations show that differences between the two methods in optical cross sections vary with coating structures for radii < 100 nm, while differences decrease to ~10% for radii > 100 nm. We find small deviations (≤ 10%) in asymmetry factors computed from the two methods for most BC coating structures and sizes, but several complex structures have 10-30% differences. This

Optical Properties of Biomass Burning Aerosols: Comparison of Experimental Measurements and T-Matrix Calculations

“…They attributed the discrepancy to the morphology, i.e., the greater dependence on the fractal dimension than on the size of the primary spherules. While our results are consistent with their results that the T-matrix calculations underestimated the SSA values, the discrepancy is even larger if we use the refractive index of 1.95 + 0.79i used by He et al [65,66], which was suggested by Bond et al [2] for soot produced from the combustion of hydrocarbons and diesel. In fact, we have seen that the refractive index suggested by Bond, when used for biomass burning aerosols, further underestimated the SSA values.…”

“…He et al [65,66] showed that the two methods they used (Geometric-optics surface waves (GOS) and T-matrix) capture the measured optical properties, but with a 5-20% difference, and the highest difference being the calculations based on T-matrix. They attributed the discrepancy to the morphology, i.e., the greater dependence on the fractal dimension than on the size of the primary spherules.…”

“…An accurate estimate of the BC radiative effects requires the incorporation of the aging process and coating structure [65,66] and consideration of particle mass [67]. This current work was focused on freshly emitted biomass burning aerosols.…”

“…An accurate estimate of the BC radiative effects requires the incorporation of the aging process and coating structure [65,66] …”

Size-resolved mixing state of black carbon in the Canadian high Arctic and implications for simulated direct radiative effect

“…The scattering component of the shell acts as a lens to focus more photons onto the core, thus increasing absorption (Borhen and Huffman, 1983;Bond et al, 2006). However, the degree of absorption enhancement is a strong function of the structure and geometry of the mixed particle as well as the core diameter and shell thickness (He et al, 2015(He et al, , 2016. Theoretical calculations (known as "core-shell Mie theory") and laboratory studies have estimated enhancements in absorption for a given BC mass of a factor of 1.3 to 2 (Schnaiter et al, 2003(Schnaiter et al, , 2005Zhang et al, 2008).…”