## 2 Materials and Methods
### Aerosol Generation and Characterization  
The primary aerosols were generated as described below:  
* Inorganic salt: aqueous solutions of ammonium nitrate (>99%, Acros Organics, Thermo Fisher Scientific, Geel, Belgium) and ammonium sulfate (>99.5%, Acros Organics, Thermo Fisher Scientific, Geel, Belgium) were nebulized with an AGK 2000 atomizer (PALAS, Karlsruhe, Germany). The aerosol had a \(G\mathit{MD}_{\mathrm{mob}}\) (geometric mean mobility diameter, number-based) of 160 nm and a \(GSD_{\mathrm{mob}}\) (geometric standard deviation) of 1.9. The mode of the mass-based aerodynamic size distribution was expected to lie above 300 nm [20];
* Dust particles (ISO A3 test dust, 98% SiO\({}_{2}\), DMT, Longmont, CO, USA) were dispersed with an RBG 1000 rotating brush generator (PALAS, Germany). The aerosol had a \(G\mathit{MD}_{\mathrm{mob}}\) of 300 nm and \(GSD_{\mathrm{mob}}\) of 1.7. The mode of the mass-based aerodynamic size distribution was expected to be in the lower micrometer range [20];
* Soot by propane combustion with a miniCAST 6204 burner (Jing Ltd., Zollikofen, Switzerland), with a \(G\mathit{MD}_{\mathrm{mob}}\) of 120 nm and a \(GSD_{\mathrm{mob}}\) of 1.6 (see [26; 27] and references therein);
* Photochemically aged soot by ozonolysis of \(\alpha\)-pinene (>97%, Sigma-Aldrich, Buchs, Switzerland) in a prototype micro smog chamber ([28; 29]); the size distribution had a \(G\mathit{MD}_{\mathrm{mob}}\) of 180 nm and a \(G\mathit{MD}_{\mathrm{mob}}\) of 1.2. The mobility measurements were performed with an SMPS (SMPS+C with L-DMA and CPC model 5.403, Grimm GmbH, Hochdorf, Germany), which was traceably calibrated. The SMPS mobility size range extended to about 1 \(\upmu\)m. If needed, the aerosols were diluted before injection in the homogenizer by using custom-made adjustable dilution units (ADU) or commercial dilutors (DDS 562, Topas GmbH, Dresden, Germany). Reference gravimetric measurements were carried out according to the EN 12341:2014 standard, i.e., particulate matter was passed through a PM\({}_{2.5}\) or PM\({}_{10}\) impactor and then sampled on filters and weighed by means of a balance. More specifically, the model aerosols were drawn through 47 mm PTFE-coated glass fiber filters (Measurement Technology Laboratories, Bloomington, MN, USA) placed in a metallic filter holder (C806 standard aerosol filter holder, Merck Millipore, Darmstadt, Germany). The aerosol flow was controlled with a needle valve and measured with a calibrated mass flow meter (Natec Sensors GmbH, Garching bei Munchen, Germany) connected to an aerosol pump (VTES, Gardner Denver Thomas GmbH, Mermmingen, Germany) in such a way that the volumetric flow corresponded to 2.3 m\({}^{3}\)/h at ambient conditions. The flow measurement was traceable to national standards. The connecting tube between the isokinetic sampling and the filter holder was made of inert, electrically conducting rubber material, and was kept as short as possible (\(\approx\)5 cm) without bends to minimize deposition losses of particulate matter via kinetic processes,as well as losses due to thermal, chemical, or electrostatic processes. Before and after sampling, the filters were conditioned for 48 h at 20 \({}^{\circ}\)C and 50% relative humidity, and then weighed with a calibrated balance (Model UMT5, Mettler Toledo, Albstadt, Germany), which was traceable to national standards. Temperature, pressure, and relative humidity inside the sampling region were also recorded (ALMEMO FHAL 46x with sinter filter SK6, Ahlborn Mess- and Regelungstechnik GmbH, Holzkirchen, Germany). For quality assurance, the mass concentration of the model aerosols was monitored with a TEOM 1405 (Thermo Scientific, Waltham, MA, USA) operated at a temperature of 30 \({}^{\circ}\)C to minimize losses of semivolatile material. The TEOM used a custom-made isokinetic sampling probe for sampling (see Figure 1b). Moreover, the optical size distribution of the model aerosols was measured using a Fidas Frog monitor (Palas, Germany) or an 11-D dust monitor (Grimm, Germany).