## 5 Results
The total number concentration that was obtained by integration of a size distribution function was 3.69 \(\times\) 10\({}^{6}\) particles per cm\({}^{3}\) of air volume upstream but decreased to 3.45 \(\times\) 10\({}^{6}\) particles per cm\({}^{3}\) of air volume (6.5% decrease) downstream. The observed 6.5% decrease agreed with an 8% decrease (numerical calculation, see black dotted line). To investigate wall losses, the DPM was utilized to determine the trajectories of submicron particles and the corresponding wall losses. A total of 200 particles were introduced, of which 3 particles were captured by the wall, resulting in a wall loss of 1.5%. Details are presented in Section S2 of the Supplementary Materials. Figure 4: Current–voltage characteristics. Therefore, the decrease in the number concentration of submicron particles shown in Figure 5b is believed to be a result of particle agglomeration induced by the vortices generated when the flow passed through the vortex generator. In other words, submicron particles are swept and scavenged by larger particles (micron particles). The results of Figure 5b can be simplified using the relation between upstream and downstream number concentrations of submicron particles (Figure 5c) as follows:  
\[N_{Down}=(1-\eta_{VG})N_{Up} \tag{27}\]  
where \(N_{Down}\) is the downstream submicron particle concentration (3.45 \(\times\) 10\({}^{6}\) particles per cm\({}^{3}\) of air volume), \(N_{Up}\) is the upstream submicron particle concentration (3.69 \(\times\) 10\({}^{6}\) par  
Figure 5: (**a**) Overview: without an electric field applied to the agglomerator, (**b**) particle size distributions upstream and downstream of the agglomerator, (**c**) vortex generation as an agglomeration factor. ticles per cm\({}^{3}\) of air volume), and \(\eta_{VG}\) is the submicron particle reduction efficiency in passing through the vortex generator (=0.065). Size distributions of submicron particles were also obtained both upstream and downstream of the agglomerator when the AC electric field was applied to the agglomerator (see Figure 6a). The results in Figure 6b show that the total number concentration downstream decreased to 1.87 \(\times\) 10\({}^{6}\) particles per cm\({}^{3}\) of air volume (measured) from the upstream concentration of 3.69 \(\times\) 10\({}^{6}\) particles per cm\({}^{3}\) of air volume when the AC electric field was applied. The observed 50% decrease agreed with a 46% decrease (numerical calculation, see black dotted line). Therefore, the decrease in the number concentration is believed to be a result of particle agglomeration induced by the electric field when the flow passed through the vortex generator. When an electric field was applied, a relatively greater number of particles were reduced. Figure 6: (**a**) Overview: with an electric field applied to the agglomerator, (**b**) particle size distributions upstream and downstream of the agglomerator, (**c**) vortex generation combined with AC electric field as an agglomeration factor. The results of Figure 6b can be simplified using the relation between upstream and downstream number concentrations (Figure 6c) as follows:  
\[N_{Down}=(1-\eta_{VG+E})N_{Up} \tag{28}\]  
where \(N_{Down}\) is the downstream particle concentration (1.87 \(\times\) 10\({}^{6}\) particles per cm\({}^{3}\) of air volume) and \(N_{Up}\) is the upstream particle concentration (3.69 \(\times\) 10\({}^{6}\) particles per cm\({}^{3}\) of air volume). \(\eta_{VG+E}\) is the particle reduction efficiency in passing through the vortex generator in which an electric field is applied (=0.5).