Biological particle materials have significant wide-band extinction performance, and the monomer shapes of bioparticles are complex with some irregular non-spherical shapes. However, the differences in extinction characteristics of bioparticle aggregates with different monomer shapes are still uncertain and have been ignored in previous research. Thus, we build bioparticle aggregation models with different monomer shapes to calculate the extinction coefficients in the 3-5 μm and 8-14 μm wavebands and analyze the differences in extinction characteristics caused by monomer shapes.

Five typical monomer shapes are constructed by employing multi-sphere models in terms of the scanning electron microscopy (SEM) images, and the complex refractive indices (CRIs) of three biomaterials are calculated according to Kramers-Kronig relations based on specular reflectance in the 2.5-25.0 μm waveband. A novel simulation code or non-spherical particle aggregation (NSPA) model is applied to build realistic spatial structure models of bioparticle aggregates with different monomer shapes. To eliminate the influence of spatial structure density, we select the bioparticle aggregates with the same porosity of 0.840 to obtain the extinction characteristics. The discrete dipole approximation (DDA) method is adopted to calculate average mass extinction coefficient αext, average mass absorption coefficient αabs, and average mass scattering coefficient αsca in the 3-5 μm and 8-14 μm wavebands respectively. Then the differences in extinction characteristics of the bioparticle aggregates with different monomer shapes can be analyzed.

According to the calculation results, the influence of the monomer size, monomer number, CRI, and aspect ratio (AR) on the absorption and scattering effects of bioparticle aggregates with different monomer shapes is investigated. The results show that the closer size of bioparticle aggregates to the wavelength of incident light leads to stronger scattering of incident light by bioparticle aggregates. For bioparticle materials with monomer particle sizes ranging from 0.5 to 3.0 μm, the extinction ability in the 3-5 μm waveband is significantly stronger than that in the 8-14 μm waveband (Figs. 6 and 7). When the monomer diameter is 2.0 μm, the αext of bioparticle aggregates with different monomer shapes are about 0.820-0.850 m2/g in the 3-5 μm waveband (Fig. 8) and about 0.430-0.470 m2/g in the 8-14 μm waveband (Fig. 9). Within a certain range, an increase in monomer size enhances scattering effects (Fig. 6), but the trend of absorption and scattering effects is usually opposite. In the 3-5 μm waveband, the relative deviation of αext, αabs and αsca can reach about -6%, -1.3%, and -14% respectively (Figs. 6 and 8). In the 8-14 μm waveband, the relative deviation of αext, αabs and αsca can reach about -3.3%, -1.2%, and -14% respectively (Figs. 7 and 9). There are indeed differences in the optical properties of bioparticle aggregates with different monomer shapes. For the bioparticle aggregates with the monomer shape of pancake, when the monomer number is 15, the relative deviation of αext and αsca can reach -2.8% and -6.1%, but under the monomer number of 60, the relative deviation has reduced by more than 50% (Fig. 8). As the monomer number increases, the specific surface area differences of the overall spatial structure among bioparticle aggregates with different monomer shapes become smaller, and the scattering differences of bioparticle aggregates with different monomer shapes are relatively weakened. The extinction abilities of biological particle aggregates are more sensitive to CRI (Figs. 10 and 11). Therefore, the actual relative deviation in the extinction characteristics of bioparticle aggregates is not directly determined by the degree to which the monomer shape deviates from the spherical shape, but by the specific result of the combined effect of absorption and scattering. As the AR of ellipsoid rises, the absorption changes slightly while the scattering ability declines significantly (Figs. 12 and 13). Additionally, the results in the case of similar monomer shapes also demonstrate that the scattering differences caused by monomer shapes are indeed related to the degree to which the monomer shape deviates from the spherical shape.

We construct bioparticle aggregates with five typical monomer shapes and calculate and compare the extinction characteristic parameters of them under the influence of multiple factors. The results indicate that there are great differences in the extinction characteristics of bioparticle aggregates with different monomer shapes, which is mainly caused by the differences in light scattering. Meanwhile, generally the more deviation of the monomer shape from the sphere causes greater differences in extinction characteristic of bioparticle aggregates, but the specific magnitude of the relative deviation of the extinction characteristic parameters is the coupling result of various factors such as monomer size, monomer number, and CRI. Our study is of significance for accurately evaluating and optimizing the extinction performance of biological particle materials.