## 2 Materials and Methods  
The present research foresaw the integration of more atmospheric aerosol monitoring devices to detect particle number and mass due to the activation of specific particle sources. The tests were conducted in the northeast of Italy (the city of Brescia), inside the Po Valley, characterized by high pollution levels [32]. The places selected for sampling differed based on environmental conditions and/or location. Three locations were considered: an indoor warehouse (laboratory of civil and construction engineering), a garage, and an outdoor area adjacent to the warehouse (Figure 1). With reference to the environmental conditions in the warehouse, the monitoring period comprised both the presence of building and construction activities (referred to as \"indoor busy\") and the absence of building work (referred to as \"indoor quiet\"). The outdoor area was characterized by the occasional presence of smokers in the surroundings. Finally, the garage events were only related to cars passing by. The sampling period covered 12 workdays in March 2018. The calendar of measurements is reported in the Supplementary Materials (Figure S1). Labels assigned to each day identify the specific monitoring site and environmental conditions. Aerosol concentrations were monitored by running different instruments in parallel. In particular, the integrated monitoring system comprised two low-cost instruments characterized by the optical detection of particles, a gravimetric sampler, and a cascade impactor. In the following paragraphs we provide details about the technical features of all instruments. The Electrical Low-Pressure Impactor (ELPI+, Dekati, Tampere, Finland) provides the real-time particle number size distribution in a wide dimensional range (6 nm to 10 \(\upmu\)m) with a sampling frequency up to 10 Hz. For the measurements foreseen by this research the sampling frequency was set to 1 Hz. It also indirectly estimates PM\({}_{10}\)-PM\({}_{2.5}\)-PM\({}_{1}\) with the usual approximations of particle unit density and sphericity. The measurement principle is based on the unipolar charging of particles through a discharge achieved by a positive high voltage of approximately 3.5 kV. To achieve stable charging conditions, the discharge current is kept at a constant value of 1 \(\upmu\)A. The following size classification according to the particles' aerodynamic diameter is given by the impact of particles on 14 different impactor stages [29] characterized by different cut-off efficiencies reported in Table 1. \begin{table}
\begin{tabular}{c c} \hline \hline
**Stage** & **D50\% (\(\upmu\)m)** \\ \hline F & 0.006 \\
2 & 0.016 \\
3 & 0.0309 \\
4 & 0.0547 \\
5 & 0.0951 \\
6 & 0.155 \\
7 & 0.256 \\
8 & 0.382 \\
9 & 0.603 \\
10 & 0.948 \\
11 & 1.63 \\
12 & 2.47 \\
13 & 3.66 \\
14 & 5.37 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Cut-off particle size (D50%) of each impactor stage. Figure 1: Pictures representing the sampling environments: (**a**) indoor, (**b**) outdoor, and (**c**) garage. Particles with aerodynamic diameters lower than the D50% value are ideally collected in the following stages in the air flow direction: from stage 14 to filter stage F, whereas particles with aerodynamic diameters between the D50% values of two consecutive stages are collected in the stage with the lower D50%. The 14 impactor stages of the instrument are electrically insulated and connected to sensitive electrometers. Currents measured by each electrometer are directly proportional to the particle number concentrations with calibration factors provided by the manufacturer [33]. The downstream pressure is measured and can be set to the manufacturer-specified value of 40 mbar by adjusting a control valve, located between the filter stage and the connection to the external vacuum pump, with a flow rate of 10 L/min. The EPLI+ can be used for gravimetric measurements by calculating the weight difference of the filters placed on each impactor stage.