## 2 Methods  
### Particle Aggregate  
In this work, particle aggregates are composed of single-sized spherical particles, as shown in Figure 1. The reason for choosing spherical particles is that large numbers of real particle aggregates can be regarded as composed of spherical particles, and the radiometric properties of spherical particlesare relatively easy to calculate, so this work can be focused on the influence of chemical component distribution in particle aggregates. The particle aggregates were generated by a diffusion-limited aggregate (DLA) [36; 37] method by which particles are initially randomly distributed in space and then randomly walk until they become stuck to each other. The particle radius \(R\) was set to 2-5 \(\upmu\)m; this particle radius range is common for a number of real particles, such as comet dust [11; 38; 39] or coal smoke particles [40; 41]. The particle number in the aggregates was set to 500. Increasing the particle number was found not to significantly change the radiometric properties of the aggregate, while it exponentially increases the computation time; moreover, the influence of chemical component distribution can be sufficiently revealed with the aggregates of 500 particles. The particle aggregates have two chemical components, namely the major component and the minor component. For the detailed DLA process, the seed particle was placed at the origin point. After that, the initial position of the particle was randomly generated on a spherical surface with a radius 100 times that of the particle. The random walking process was applied to the particle until it hit the seed particle; the walking step length was set to 0.1 times the radius of the particle. After hitting, a minor adjustment was made based on the two particles' radii to make sure the two particles contacted each other. This process was repeated until an aggregate structure was formed. Using similar processes, different aggregate structures were generated. To investigate the influence of chemical component distribution, two component distribution modes were introduced as Mode A and Mode B. In Mode A, a particle is composed either of a major component or a minor component; the minor component is concentrated in a few particles, as it is in many natural agglomerations and particles [42; 43]. In Mode B, all particles are the same, and the major and minor components are evenly mixed in each particle. Figure 1a,d show a particle aggregate in Mode A and Mode B; the two modes have different distributions of the minor component. Figure 1b,e show a second aggregate of Mode A and Mode B; Figure 1c,f show a third. Figure 1: Particle aggregates composed by spatially randomly distributed spherical particles of uniform size. Mode A: chemical component concentrated in a few particles; Mode B: chemical components evenly distributed in each particle. (**a**) and (**d**) are of the same structure, as are (**b**) and (**e**) as well as (**c**) and (**f**). The volume fraction of the minor component, \(f\), is calculated by dividing the volume of the minor component by the volume of both components in the aggregate. In this study, \(f=1-25\%\), a relatively wide range of the volume fractions of the minor component. In Mode A, the particles of the minor component may have various positions in the aggregate, which may affect the radiometric properties of the aggregate. When the minor component particles are randomly positioned, it is said that the minor component particles have a random position pattern. For a particle aggregate, three random position patterns of the minor component particles are considered, as shown in Figure 2d-f; the radiometric properties of the aggregate with the three patterns are calculated, and the average radiometric properties are used as the radiometric properties of the particle aggregate with the random position pattern, thus minimizing the impact of certain patterns on the calculation.