## 2 Materials and Methods
However, for this work greased Al foils were used to avoid particles bouncing and to improve the accuracy of particle determination. Thus, no gravimetric data were extracted. The customized kit for outdoor air quality measurements (hereafter called KORA, Acronet, Savona, Italy) is a remote meteorological station that measures PM\({}_{2.5}\) by extractive sensing using a high-flow pump operating at 10 L/min along with air temperature, relative humidity, and atmospheric pressure. Data are registered with a time resolution of 5 s and sent to the remote platform, where they are privately stored and can be downloaded. The personal modular impactor (PMI, SKC, Blandford Forum Dorset, UK) is a portable gravimetric sampler characterized by a single inertial impactor that collects PM\({}_{2.5}\) by an extractive method using a Leland Legacy SKC pump operating at 10 L/min and collecting particles onto a Teflon filter membrane with a pore size of 2 \(\upmu\)m, weighed before and after the sampling to evaluate particulate mass. Sampling was performed on the same filter over all weekdays during the first two weeks and on one filter per day during the last two days, to collect one filter per environment. Filters were weighed with a microbalance (model X53 DU, Mettler Toledo, Milano, Italy) characterized by a sensibility of 0.5 \(\upmu\)g (1 \(\upmu\)g readability between 0-0.8 g and 10 \(\upmu\)g readability up to 3.1 g). The AirVisual Pro is a real time aerosol sampler with time resolution: 10 s based on laser light scattering by particles for the indirect monitoring of PM\({}_{2.5}\) concentrations, integrated with low-cost sensors to monitor CO\({}_{2}\) concentrations and meteorological parameters (air temperature and relative humidity). Within the measuring chamber, a laser beam hits the particles and light is scattered in different directions. A photodetector can count particles according to the light scattering angle through the measurements of single pulses. The device can detect particles with optical diameters ranging from about 0.3 \(\upmu\)m to 2.5 \(\upmu\)m. The device also provides the concentration of PM\({}_{10}\) calculated by an algorithm using the concentration of PM\({}_{2.5}\)[34; 35]. Statistical analysis of the data was performed with the JMP Software (SAS Institute srl, Milano, Italy). Multivariate statistical analysis was performed with the PARAFAC method. Experimental data were arranged in a three-dimensional array, collecting the instruments in the first mode, the PM concentrations in the second mode, and the days in the third mode. The final array was represented by a 9 \(\times\) 401 \(\times\) 10 matrix. The most useful chemometric method to analyze this kind of data is parallel factor analysis (PARAFAC), which is a generalization of PCA to a higher-order array [36]. The calculation was performed using MATLAB 2019a (MATLAB R2019a, The MathWorks, Inc.: Natick, MA, USA) with an N-Way Toolbox [37] and PLS_Toolbox version 8.9 (Eigenvector Research Inc., Manson, WA, USA). Images of particulate matter were collected by field emission scanning electron microscope (FE-SEM, model Leo 1525, Zeiss, Germany), operated in the 1-10 KV range.