## 2 Materials and Methods  
### Experimental Setup  
Part of the experimental facility has been described in a previous publication [20]. Briefly, the setup consists of multiple aerosol generators, a 2.1-meter-long flow tube homogenizer placed vertically, a set of custom-made isokinetic sampling probes, and a system for temperature and humidity control. A graphical illustration of the flow tube homogenizer is shown in Figure 1a.  
Soot, aged soot, ammonium nitrate, ammonium sulfate, and dust particles were aerosolized (see Section 2.2 for a detailed description of the generators) and mixed in the homogenizer by three turbulent air jets. The inorganic salt and dust particles passed through two separate radioactive \({}^{85}\)Kr sources (TSI Inc., Shoreview, MN, USA) to reduce the charges of the particles and render the aerosols (as an ensemble) neutral before entering the homogenizer. Experiments with soot and aged soot aerosols have revealed that these are already neutral and do not require any further treatment. Salt particles were dried by passing them through a bed of silica gel beads (2-5 mm, Dry & Safe GmbH, Oensingen, Switzerland). Tests of hygroscopic growth of pure ammonium sulfate particles showed that particles were dried below the efflorescence point before entering the homogenizer.  
Figure 1: (**a**) CAD illustration of the homogenizer (see [20] for more information). (**b**) Enlarged view of the aerosol sampling system. An impactor selects PM\({}_{2.5}\) or PM\({}_{10}\) size fractions for the gravimetric reference method. The low-cost PM sensors are mounted on a rack surrounding the impactor. Aerosols can be also sampled through additional isokinetic sampling probes for further aerosol characterization. For simplicity, only one such probe is displayed. ADU: adjustable dilution unit (custom-made); VOC: volatile organic compound.  
In this study, we designed an entirely new sampling system that enables PM\({}_{2.5}\) and PM\({}_{10}\) calibration of multiple low-cost PM sensors simultaneously based on the selection of different particle size fractions by standardized impactors. A computer-aided design (CAD, Inventor Professional 2019, Autodesk, San Rafael, CA, USA) of the sampling system is displayed in Figure 1b. The central component is the PM\({}_{2.5}\) or PM\({}_{10}\) impactor (Digitel, Zurich, Switzerland). A metallic rack, which can accommodate several PM sensors (the exact number depends on the sensor's dimensions), is placed around the impactor at the height of the impactor's aerosol inlet. The sensors are placed upright, so that the aerosol inlet is on the top, right next to the impactor. The impactor is connected to the filter holder of the gravimetric reference method through a straight metallic tube. Up to eight additional isokinetic sampling probes for the aerosol monitoring instruments--e.g., TEOM (tapered element oscillating microbalance) and SMPS (scanning mobility particle size)--and/or PM monitors under test can be attached to the bottom metallic plate of the homogenizer, depending on the experimental needs. The aerosol spatial homogeneity at the sampling region is within 2.6% (\(k=2\); 95% confidence interval) in particle number concentration [20].