## 3 Results and Discussion  
### PM2.5 Calibration  
An AirVisual Outdoor sensor unit (comprising two PM sensors) was mounted on the rack surrounding the PM impactor (see Figure 1b) throughout all experiments with model aerosols 1 and 2 (see Figure 2). Due to limited availability, the SDS011 PM sensor (InovaFitness, China) was only mounted during the experiments with model aerosol 2. The AirVisual Outdoor is a photometer recording the light scattered from all particles present in the measurement cell, whereas the SDS011 is based on light scattering from individual particles. Note that both devices report PM2.5 and PM10 measurements as 1 min averages.  
The AirVisual Outdoor sensor was calibrated against the gravimetric reference method with the two different model aerosols presented in Section 2.2. As an example, a PM2.5 mass concentration measurement is shown in Figure 3. The green line corresponds to the mass concentration according to the gravimetric reference method, while the shaded green area designates the expanded uncertainty (\(k=2\); 95% confidence interval) thereof. A very good agreement between the AirVisual Outdoor sensor (purple dots) and the gravimetric reference method was observed. The sensor reported an average PM2.5 mass  
Figure 2: PM composition (%) of the two model aerosols.  
concentration \(C_{\text{DUT}}\) of 83.4 \(\upmu\)g/m\({}^{3}\), thus underestimating the PM\({}_{2.5}\) mass concentration by \(\sim\)6% compared to the reference measurement (\(C_{\text{ref.}}=\) 88.5 \(\upmu\)g/m\({}^{3}\)). The calibration was repeated at various mass concentrations, as shown in Figure 4a and summarized in Table 1. The AirVisual Outdoor sensor performed very well with both model aerosols, in most cases exhibiting a measurement efficiency (\(E_{\text{DUT}}=C_{\text{DUT}}/C_{\text{ref}}\)) of 0.9-1.1. The only exception was the measurement of model aerosol 1 at high relative humidity, where the measurement efficiency was around 1.3, presumably due to hygroscopic growth of the ammonium nitrate and ammonium sulfate particles.  
Figure 4: PM\({}_{2.5}\) mass concentrations reported by (**a**) the AirVisual Outdoor and (**b**) the SDS011 versus the gravimetric reference mass concentration. The green line denotes a perfect agreement, while the shaded green area designates a maximum 10% relative expanded uncertainty (\(k=2\); 95% confidence interval) of the gravimetric reference method.  
Figure 3: PM\({}_{2.5}\) mass concentrations reported by the AirVisual Outdoor and SDS011 as a function of time for model aerosol 2. The green line refers to the average mass concentration over the whole measurement period according to the gravimetric reference method, while the shaded green area designates the expanded uncertainty (\(k=2\); 95% confidence interval) thereof.  
In contrast, the SDS011 sensor showed a rather erratic behavior, reporting a sudden decrease in the PM\({}_{2.5}\) mass concentration during measurement (Figure 3). The measurement efficiency ranged between 0.4 and 2.6 (Figure 4b, Table 1). [PERSON] at al. compared 17 SDS011 sensors to a Palas PROMO 2000 reference in a laboratory setting using ambient air, and reported an average PM\({}_{2.5}\) measurement efficiency \(E_{\text{DUT}}\) of 0.66 (\(E_{\text{DUT}}\) ranging from 0.48 to 0.85) [31]. The measurement efficiency decreased even further to an average of 0.49 (0.41-0.63) with pure ammonium sulfate as the test aerosol. [PERSON] et al. also reported underestimation of PM\({}_{2.5}\) mass concentration in their field campaign, particularly for higher pollution levels (\(E_{\text{DUT}}\) ranging from 0.7 to 0.9) [32].