## 2 Materials and Methods  
The schematic diagram of the experimental devices is shown in Figure 1. In this experiment, the feeding system comprised a PALAS monodisperse aerosol generator which generates monodisperse particles called diisooctyl sebacates (DEHs). The particle size of monodisperse particles can be changed by adjusting the temperature of the PALAS generator according to the Sinclair-LaMer principle. The relationship between particle size and temperature is as shown in Table 1. Here, 0.5-um (PM\({}_{1}\)), 2-um (PM\({}_{2.5}\)) and 4-um (PM\({}_{5}\)) DEH particles were selected as the particle sources. The agglomeration system consisted of an agglomeration chamber, acoustic wave system, and pulsed discharge system. The 6 cm \(\times\) 6 cm \(\times\) 30 cm agglomeration chamber was made up of polymethyl methacrylate, inside which four stainless-steel wire electrodes were placed every 7 cm. In addition, two stainless-steel plate electrodes were contained. To drive the YF-513 compression driver in the acoustic wave system, an SFG-1013 signal generator and QSC RMX2450 power amplifier were used. The frequency band in this system was between 800 Hz and 2400 Hz, and SPLs were selected from 133-143 dB to prevent aggregates from breaking up. These acoustic parameters were proven to have high efficiency in the former studies [16; 22]. The pulsed discharge system comprised a high-voltage pulse power supply, Tektronix TCP0150 current probe, Tektronix DPO4034 digital oscilloscope, and P150-GL HV probe. Sampling measurement was performed using an electrical low-pressure impactor (ELPI) with an air intake of approximately 10 L-min\({}^{-1}\). The air velocity in the agglomeration chamber was approximately 7.5 cm-s-1, and the stay time was approximately 4 s. At the outlet of agglomeration chamber, DEH particles were isokinetically sampled and measured by ELPI. The schematic diagram of ELPI is shown in Figure 1. The particles sampled flowed through a precharging system, where particles were saturated with positive charges. Subsequently, the charged particles entered a low-pressure cascade impactor, which was divided into 13 stages. There were holes in the impact layer of each stage, and the hole size decreased successively from top to bottom. When particles passed through these impact holes, the gas flow line changed, and the larger particles continued to fall before hitting the collecting layer due to inertia, while the smaller particles followed the airflow to the lower impact layer. Each stage had a conducting needle connected to an electrometer. The number and mass concentration of particles on the impactor were calculated by measuring the real-time change of particle charge on each layer. The experiment was carried out at 20 \({}^{\circ}\)C. The initial particle size distribution (PSD) and particle number concentration (PNC) measured by the ELPI under the three conditions are shown in Figure 2.  
In Figure 2, \(\mathrm{D_{P}}\) is the particle size, and N is the PNC with different sizes. It can be observed that the particle size distribution basically satisfies the monodisperse condition. The evaporation capacity of DEHs varies with temperature change, and the particle number concentrations under the three  
Figure 1: Schematic diagram of experimental set-up and electrical low-pressure impactor (ELPI).  
temperature conditions differ. According to the research results of [PERSON] et al. [21], the concentration does not affect the selection of particles for optimal parameters. The particles generated under these three conditions can, therefore, be used for experiments on monodisperse aerosol agglomeration in single and coupling fields.  
In the experiment, the monodisperse aerosol generator is firstly turned on. The ELPI is thereafter turned on for particle size measurement. By changing the acoustic wave frequencies, SPLs, and pulse input voltages, the monodisperse aerosol particle size distribution after agglomeration under different conditions can be measured.