# Suitability of Low-Cost Sensors for Submicron Aerosol Particle Measurement
## 3 Results and Discussion  
Influence of Different Aerosol Particle Concentrations on the Measurement Capability and Accuracy of the Low-Cost Sensors  
To investigate the measurement capability of the different low-cost sensors with the previously shown test setup (cf. Section 2.2), various concentrations of NaCl (0.25, 0.5, 0.75, and 1.0 g\(\cdot\)L\({}^{-1}\)) in solution were used for the Collison nebulizer. The mixing ratio of B:A is set to 6:1 (particle-free air: NaCl aerosol), with a total volume flow of 30 L\(\cdot\)min\({}^{-1}\). At this flow condition, the relative aerosol humidity is reduced in the diffusion dryer to a constant level below 15% for all measurements at the sample stage. For the following investigations, the temperature is also constant at 25 \({}^{\circ}\)C. During the measurement, the low-cost sensor is installed at the first sample position, and the Palas AQ Guard is placed at position two. The particle mass concentrations (PM2.5) are measured with the low-cost sensors: SDS011 and HPMA compact. In Figure 4, the results for NaCl aerosols produced from NaCl solutions of four different concentrations up to 1.0 g\(\cdot\)L\({}^{-1}\) in the disperser are shown. Each data point results from repeated measurements, with three samples from each sensor and multiple measurements of each (a total of at least nine). The PM10 values are not plotted as they are identical to the PM2.5 values. This observation is consistent with the measurements of the PSD (Figure 3), which does not contain any particles larger than 2.5 \(\upmu\)m.  
Figure 3: Particle size distribution and SEM image of sodium chloride particles used in experiments, nebulizer concentration 0.25 g\(\cdot\)L\({}^{-1}\), humidity of the aerosol at 15%. The measurement performance of the low-cost sensors is comparable. The SDS011 sensor just shows a slightly higher detection capacity at higher aerosol concentrations. The standard deviation increases with higher particle concentrations; however, a decrease in the coefficient of variation (CV) can be observed. At the highest particle concentration, it is approximately 20% for both low-cost sensors. This is due to the difference between the individual models of one sensor type and cannot be attributed to fluctuating aerosol generation. This is confirmed by the comparison to the relative deviation in the parallel measurements with the AQ Guard, which is less than 7%. This is also consistent with the PSD measurements with the SMPS (cf. Section 2.3). In the same experiment, the aerosol is measured with the AQ Guard as well as the low-cost sensors. The resulting particle mass concentrations (PM2.5) are shown in Figure 5 in comparison to the dataset of the low-cost sensors. The detected PM2.5 values of the AQ Guard are much higher, showing a clear deviation from the results of the low-cost sensors. This observation can initially be related to the different particle-size detection ranges of the sensors. The smallest particle size is 0.175 \(\upmu\)m for the AQ Guard, 0.3 \(\upmu\)m for the SDS011 and not known for the HPMA compact. Regarding the PSD of the used NaCl particle system (cf. Figure 3), the number of particles in this size range rises significantly and might explain this deviation. Besides the mass concentration, the AQ Guard detects the number-based particle concentration and its distribution. From  
Figure 4: Comparison of PM2.5 measurements between SDS011 and HPMA compact at identical operating conditions. Figure 5: Comparison of PM2.5 measurements between AQ Guard and low-cost sensors (SDS011 and HPMA compact) at identical operating conditions. this, the size range can be tailored to match that of the low-cost sensors. This new number distribution is then calculated for the mass concentration using the assumptions of constant density and spherical particle shape. Unfortunately, the PM values of the AQ Guard do not match those of the low-cost sensors either.