Inorganic species represent more than 1% of total RSP mass and their sources are identified as crustal elements (silicon, aluminum, iron, calcium, carbonate), sea-salt aerosol (sodium chloride), inorganic secondary species (ammonium nitrate and sulphate), and primary anthropogenic species (elemental carbon). Organic compounds instead represent from 20 up to 60% of RSP mass and include a wide variety of individual species such as harmful compounds or tracers of specific emission sources [17] and biologic material [18]. These measurements are performed on PM\({}_{10}\) or PM\({}_{2.5}\) filters coming from the reference system (gravimetric samplers) so that the chemical speciation is averaged over all particles collected on them. Nevertheless, particles contained in the atmospheric aerosol cover five orders of magnitude, from a few nm up to 100 \(\upmu\)m [16], so complete monitoring coverage of them needs more devices characterized by different physical principles. A wide variety of instruments for aerosol monitoring is present on the market and their technical features could be very different. For example, there are real-time instruments based on laser scattering (optical particle counters, OPC) able to detect particles with optical diameters above 300 nm [19]; gravimetric devices able to detect PM\({}_{1}\), PM\({}_{2.5}\), and PM\({}_{10}\) depending on their inlet impactors; real-time cascade impactors able to detect different particle fractions on different stages from 10 nm up to 10 \(\upmu\)m; and condensing particle counters (CPC) able to detect nanometric particles through their enlargement due to butanol or water condensation followed by optical detection. Real-time instruments such as cascade impactors, and OPC and CPC instruments for aerosol monitoring could be characterized by high precision and sensitivity but they can reach costs up to tens of thousands of euros and require significant resources for their constant maintenance and calibration [20; 21], so a capillary monitoring of aerosol concentrations could only be performed by less precise and sensitive but low-cost (from EUR 10 up to EUR 1000 indicatively), small, easy-to-manage, and portable (their dimension is up to one tenth of the size of the instruments used in air-quality monitoring networks) sensors [19]. Low-cost sensors are based on the optical detection of particles through laser scattering, counting particles with an optical diameter under 1, 2.5, or 10 \(\upmu\)m and indirectly evaluating PM\({}_{1}\), PM\({}_{2.5}\), and PM\({}_{10}\), assuming for all particles a unit density and a spherical shape [22; 23]. The indirect calculation of particle mass from the particle concentration number is often discouraged if not accompanied by reference measurements [19; 24; 25]. The real density and shape of particles as well as their color and refractive index can affect the evaluation of their size distribution [26; 27; 28]. Nevertheless, the availability of diffuse differential concentrations in terms of PM\({}_{2.5}\) or PM\({}_{10}\) over a territorial domain could help a local or regional administrator in understanding eventual critical sites to implement the right policies to abate RSP [21]. Along with the estimation of RSP concentrations, other integrated low-cost sensors in the same monitoring device can evaluate parameters such as temperature, humidity, and carbon dioxide concentrations to additionally estimate the air quality index (EEA, n.d.). Some examples taken from the literature [19; 23] are given by the following devices: OPC-N1 (Alphasense, Essex, UK), the DC1100 Pro (Dylos, Riverside, CA, USA, GP2Y1010 (Sharp, Sakay City, Japan), Kobe, Japan, and AirVisual Pro (IQAir AG, Goldach, Switzerland). New solutions in the field of low-cost RSP sensors are continuously under study, including devices able to detect nanoparticles using different technical principles [29]. Low-cost sensors can be deployed in great quantities and connected in devoted air-quality networks.