## 4 Conclusions  
This manuscript describes a novel method for the traceable calibration of low-cost PM\({}_{2.5}\) and PM\({}_{10}\) sensors with ambient-like aerosols generated in the laboratory. The setup consisted of multiple aerosol generators for producing soot, aged soot (i.e., coated  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Model Aerosol** & \(\mathbf{T}\)/\({}^{\circ}\)**C** & **RH/\%** & **DUT** & \(\mathbf{C_{\text{DUT}}\pm 2\text{ }s/\mu\text{g}\cdot\text{m}^{-3}}\) & \(\mathbf{C_{\text{ref.}}\pm U/\mu\text{g}\cdot\text{m}^{-3}}\) & \(\mathbf{E_{\text{DUT}}\pm U}\) \\ \hline \multirow{8}{*}{2} & 21 & 50 & \multirow{2}{*}{AirVisual Outdoor} & 22.6 \(\pm\) 1.4 & 22.6 \(\pm\) 1.8 & 1.00 \(\pm\) 0.10 \\ \cline{2-6}  & 21 & 50 & & 59.3 \(\pm\) 1.8 & 64.5 \(\pm\) 5.0 & 0.92 \(\pm\) 0.08 \\ \cline{2-6}  & 21 & 50 & & 160 \(\pm\) 18 & 156 \(\pm\) 10 & 1.03 \(\pm\) 0.14 \\ \cline{2-6}  & 21 & 50 & & 15.5 \(\pm\) 2.0 & 22.6 \(\pm\) 1.8 & 0.69 \(\pm\) 0.10 \\ \cline{2-6}  & 21 & 50 & & 43.8 \(\pm\) 2.2 & 64.5 \(\pm\) 5.0 & 0.68 \(\pm\) 0.07 \\ \cline{2-6}  & 21 & 50 & & 2.40 \(\pm\) 0.20 & 154 \(\pm\) 10 & \multicolumn{1}{c}{\textasci{-}0} \\ \hline \hline \end{tabular}
\end{table}
Table 2: PM\({}_{10}\) mass concentrations reported by the DUT and the reference gravimetric method—\(C_{\text{DUT}}\) and \(C_{\text{ref.}}\), respectively—and measurement efficiency of the DUT, \(E_{\text{DUT}}\). \(U\) denotes expanded uncertainties (\(k\) = 2; 95% confidence interval); \(s\) is the standard deviation of the mean (i.e., \(\frac{\sigma}{\sqrt{n}}\), with \(n\) being the number of measurements).  
Figure 6: PM\({}_{10}\) 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.  
with secondary organic matter), inorganic species, and dust particles. The aerosols were mixed in an aerosol flow tube homogenizer and sampled through a custom-made system equipped with PM\({}_{2.5}\) and PM\({}_{10}\) impactors for the gravimetric reference method. Up to eight low-cost sensors could be calibrated simultaneously. As a proof of concept, the AirVisual Outdoor and the SDS011 sensors were calibrated against the gravimetric method in the mass concentration range \(\sim\)20-150 \(\upmu\)g/m\({}^{3}\). The AirVisual Outdoor sensor agreed very well with the reference method, showing measurement efficiencies typically in the range of 0.9-1.1, whereas the SDS011 exhibited a rather erratic behavior, with large deviations from the reference method. We believe that the calibration procedure outlined in this study can contribute to the performance characterization and standardization of low-cost sensors for air quality control.  
[PERSON] performed the experiments. [PERSON] analyzed and visualized the data. [PERSON] and [PERSON] wrote the manuscript. [PERSON] and [PERSON] designed the new sampling system. All authors have read and agreed to the published version of the manuscript.  
The work received funding from Innosuisse (grant number: 36779.1 IP-ENG).  
Not applicable.  
Data Availability StatementData can be shared upon request.  
We thank [PERSON] (Sector Technology, METAS) for valuable technical support, and [PERSON] (Digitel, Switzerland) for providing us with the drawings of the PM impactors. We also thank [PERSON] (Laboratory for Particles and Aerosols, METAS) for performing the EC/OC analysis.  
The authors declare that they have no conflict of interest.