## Appendix A  
The DMA is a well-established technique employed for particle size classification [45]. Particles are first brought to a controlled charge state in a neutralizer (radioactive source) and subsequently classified according to their electrical mobility. Accordingly, in addition to singly charged particles of the desired particle size, larger multiple charged particles of the same electrical mobility will also be transmitted [46]. If the transmitted particles are neutralized once more and fed to a second DMA, they will be segregated according to their size and charge.  
For example, Figure 11 illustrates experimentally determined modes at the outlet of two DMAs connected in series. The first DMA was set to classify particles of 150 nm, while the second operated in scanning mode [47]. The recovered distributions obtained with the commercial AIM software from TSI were purposely not corrected for multiple charges. That is the signal was inverted assuming that all particles exiting the second DMA are singly charged, in order to illustrate the entire mobility spectrum exiting the second DMA of a tandem setup. The two DMAs operated at sheath and sample flows of 3 and 0.3 L/min, respectively. The second DMA operated at a voltage ramp of 300 s.  
Figure 11: Example tandem DMA size scan. First DMA was set to classify particles of 150 nm, and second DMA operated in scanning mode (300 s scan time). Concentrations were measured with a TSI 3775 CPC operating at full flow mode (0.3 L/min). Both DMAs operated at sheath and sample flow rates of 3 and 0.3, respectively.  
The largest peak (d) in the recovered distribution practically corresponds to singly charged particles at the set size of the first DMA. However, the peaks at smaller size bins (a-c) also correspond to 150 nm particles. All 150 nm particles exiting the first DMA carried a single positive charge, but after being neutralized once more, their charge state was redistributed. When passing through the second DMA, the single charged fraction was classified at the anticipated size bin, but the multiple charged fractions exhibited higher electrical mobility and thus were classified at smaller size bins. In fact, the area of these peaks is proportional to the fraction of single, double, triple and quadruple charged 150 nm particles exiting the second neutralizer [48].  
On the other hand, the peaks at larger size bins (e and f) correspond to larger particles that were classified in the first DMA because they were initially carrying multiple charges (+2 and +3 for 235 and 312 nm, respectively). After passing the neutralizer of the second DMA, their charge level was redistributed. The single charge fraction had lower electrical mobility than singly charged 150 nm particles and was therefore classified at larger size bins. Double charged 235 nm particles and triply charged 312 nm particles were classified at the same bin as single charged 150 nm particles, so in principle, these cannot be discriminated. Therefore, some multiply charged fractions will still be present at 150 nm in this tandem DMA configuration. However, the two-stage neutralization and classifications are guaranteed to reduce the multiple charge fraction interference compared to a single DMA approach.  
In the present work, the two DMAs were set at fixed voltages. When both were set at the same voltage, (predominantly) single charged particles of the desired size are classified (i.e., peak d in Figure 11). Selected tests were also performed with the second DMA set at double the voltage of the first one, thus transmitting double charged particles of the size selected in the first DMA (i.e., peak c in Figure 11).