## 2 Materials and Methods
### Experimental Methods  
Experiments were conducted in a fire test room with a size of 10 m in length, 7 m in width and 4 m in height. The fire test room is located in State Key Laboratory of Fire Science at the University of Science and Technology of China [27]. The room was used to mimic smoldering biomass combustion scenarios in the field by using measurements with appropriate dilution of the combustion emissions and fuel arrangements to approximate and steady combustion conditions, similar to for chamber experiments of biomass burning [28; 29; 30]. The fire test room was sealed during combustion and sampling with the exhaust closed. After the biomass samples were burnt out, the smoke from the whole smoldering fire was well mixed with air in the room, sampled at the corner of 1.92 m height, and distributed through a manifold to a high sampling frequency size spectrometer (flow rate of 7.5 L/min; DMS500, Cambustion Ltd., Cambridge, UK) and a cavity-enhanced aerosol albedometer with wavelength of 532 nm (flow rate of 1.3 L/min; Anhui Institute of Optics and Fine Mechanics, Hefei, China). The sample flow rate was 4.4 L/min. The dilution gas was N\({}_{2}\) with a flow rate of 4.4 L/min. The PNSD between 5 and 1000 nm was measured every 3 s with DMS500 [31; 32] and the calibration of DMS was derived from spherical particles with the inversion matrix of unconstrained lognormal parameterization. The cavity-enhanced aerosol single-scattering albedometer was based on a light-emitting-diode (LED) incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) system combined with an integrating sphere (IS) for simultaneous in situ measurements of aerosol extinction and scattering coefficients in an exact same sample volume [33; 34; 35]. The detection limits for the scattering and extinction channels with 9 s integration time were 0.54 and 0.15 Mm\({}^{-1}\), respectively. The aerosol light absorption can be quantitatively expressed using single scattering albedo (SSA) which is defined by the ratio of aerosol light scattering to aerosol light extinction (the sum of absorption and scattering). For non-absorbing aerosols, SSA is 1. The total uncertainty in the measurements of SSA was estimated to be less than 5% [35]. The experiments of soya, wheat, corn, rice, and Pi were repeated three times. The experiments of Po and Pi were repeated twice. The results showed a good repeatability with a maximum uncertainty of SSA of less than 4% for Po. A schematic diagram of the fire test room and the experimental setup is shown in Figure 1.  
The biomass samples were put in the room with similar temperature and humidity for more than 24 h before experiments. In the experiments, the samples of agricultural straw were cut to pieces of 4-6 cm in length, and the samples of wood sawdust were sieved by screen mesh of 300 \(\upmu\)m. About 5 g of biomass sample was put in a container made of aluminum foil with size of 10 cm \(\times\) 10 cm in the center of the room. All the biomass samples were burned on a heater (OMEGALUX) with a size of 30 cm \(\times\) 30 cm. The combustion state of biomass samples was controlled at smoldering by the voltage of the heater (about 100 V, with the thermal radiation power of 4.5 kW/m\({}^{2}\)) in every experiment. Before every experiment, the particle size distribution and optical properties of aerosol in the fire test room were measured for about 5 min as the background data. During every experiment, the door and windows of the room were closed to reduce the influence from outside. After about 10-25 min, the samples burnt out and smoke from the whole smoldering fire was well mixed in the fire test room and monitored under approximately steady-state for up to several minutes. If the optical properties of sampled smoke were approximately steady for more than 5 min, which meant the fluctuations of the scattering and extinction coefficients did not exceed 5% at last, then the experiment was finished. After experiments, the windows and exhaust were opened to clean air in the room.