Impactors are instruments for measuring size distribution in mass, which working principle is gravimetry, with multiple impact stages; in some equipment multiple orifices are found. According to Hinds [29], the most used impactors are the cascade type, which operate based on the inertial classification of particles. As far as the working principle for the Cascade Impactors, Vincent [26] reported that the aerosol sample passes through a sequence of stages. In each stage, an air jet containing the aerosol reaches the impacting plate and particles larger than the cutoff diameter for the stage are collected. Smaller particles follow the gas flow that surrounds the collection plate and are collected in the next stage, in which the orifices are smaller and have conditions for greater air speed. This process continues until smaller particles are removed in the after-filter. Nussbaumer et al. [21] mentioned some of the most used Low Pressure Cascade Impactors (Andersen Impactor, Dekati Low Pressure Impactor (DLPI), and Berner Low Pressure Impactor (BLPI)). The authors described that these impactors collect particles in the range of 30 nm to 10 μm, and could extend the range of measurement to smaller particles, if using appropriate filters. According to Giechaskiel et al. [24] and Vincent [26], the first impactor appeared in 1945, with cylinder nozzles (Casella Mk1) and the most recent version is the known Andersen Mk-II, with a multi-circular jet system. Vincent [26] described that conventional Cascade Impactors operate at atmospheric pressure and do not select particles smaller than 0.4 μm. In this manner, the authors mentioned another family of Cascade Impactors which are based on Micro-Orifice Uniform Deposit Impactor (MOUDI). MOUDI are precision Cascade Impactors that cover a broad range of particle sizes. They include a range of flow rate from 10 to 100 L/min and several combination of impacting stages. There are traditional models with eight stages and models with rotation of impacting plates. These last MOUDI models guarantee a more uniform deposit of particles over the plates, and also reduce bounce related problems as well as evaporation of semi-volatile material. Venkataraman and Rao [42] described that MOUDI has 50% cut-point aerodynamic diameters of 10, 5.6, 3.2, 1.8, 1.0, 0.56, 0.32, 0.18, 0.097, and 0.056 μm on stages 1 to 10, respectively, and collects particles smaller than 0.056 μm on a 37 mm quartz fiber after filter. Another relatively recent impactor concept is the Electrical Low Pressure Impactor (ELPI). The ELPI operation principle is based on charging the aerosol electrically. This equipment is discussed in Section 3.2.8.