## 4 Conclusions  
Controlled laboratory combustion experiments were conducted in a fire test room to mimic freshly emitted smoldering smoke of biomass burning and the relevant field measurements from China. The PNSDs between 5 and 1000 nm, aerosol scattering coefficients, extinction coefficients, and SSA \begin{table}
\begin{tabular}{c c} \hline \hline
**Contents** & **Correlation Coefficient** \\ \hline C & \(-0.71042\) \\ N & 0.89636 \\ S & 0.72526 \\ FC & 0.88980 \\ VM/FC & \(-0.93748\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The correlation coefficients of single scattering albedo (SSA) and the contents of ultimate analysis and proximate analysis. Figure 6: Correlation between SSA and the biomass components: (**a**) VM/FC; (**b**) N.  
were measured. The constituents of biomass samples were determined by the ultimate analysis and proximate analysis. The contents of the major elements in agricultural straw and wood were quite different. The average contents of C, H, and O were all higher in wood than in agricultural straw. The content distributions of C, H, and O were narrower in wood than in agricultural straw. Meanwhile, the contents of N and S in wood were significantly lower than in agricultural straw. The average contents of FC and ash were lower in wood than in agricultural straw, while that of VM was higher in wood than in agricultural straw. The content distributions of VM, FC, and ash were dispersed in a large range. We also revealed the PNSD characteristics of smoldering smoke from biomass burning. For different kinds of biomass samples, the PNSDs of smoldering smoke were quite different from each other. The PNSDs of smoldering smoke from burning of agricultural straw could be fitted with a bimodal lognormal distribution as modes of around 10 nm (nucleation mode) and 60 nm (Aitken mode). The PNSDs of wood sawdust could be fitted with a trimodal lognormal distribution, while the two modes were in nucleation mode, and one was in Aitken mode. We found the correlations between the bulk optical properties (scattering, extinction coefficients, or SSA) and particle sizes of smoldering smoke or the biomass components. The bulk optical properties (scattering and extinction coefficients) of smoldering smoke had strong correlations with particle number concentrations of sizes bigger than 100 nm. The correlation between SSA and FC was strong, while the correlation between SSA and VM (or ash) was weak. The relationship between SSA and N (or S) showed a positive correlation, while that of SSA and C showed a negative correlation. The relationship between SSA and VM/FC (or N) showed a strong linear correlation (r\({}^{2}\) > 0.8). However, the small number of biomass samples and combustion conditions limit the universality of conclusions to be drawn. Further experiments of different biomass samples and combustion conditions should be conducted to demonstrate and quantify the thermochemical, optical, and particle size distribution properties, and their relations to each other of aerosols from various biomass burning. This study could improve the understanding of the relationship between physical properties of smoke and the components of biomass materials under the similar combustion conditions. It would help us to better understand the characteristics of particles from real-world combustion scenarios as well as their impacts on indoor air quality and regional atmospheric environments. Conceptualization, F.W., and X.X. ; methodology, Q.Z. and W.Z. ([PERSON]); resources, Y.Z. and W.Z. ([PERSON]); investigation, F.W. and X.X. ; writing--Original draft preparation, F.W. ; writing--Review and editing, Q.Z., X.X. and W.Z. ([PERSON]); funding acquisition, Q.Z., W.Z. ([PERSON]), Y.Z. and W.Z. ([PERSON]). This research was funded by the National Natural Science Foundation of China (Grant No. 41675024, 41330424, and U1733126), the Natural Science Foundation of Anhui Province (Grant No. 1508085J03), the Youth Innovation Promotion Association CAS (Grant No.