This article will review the properties and behavior of nanoparticles suspended in air and measurement technologies for monitoring, characterizing, and controlling them, to reduce their adverse effects on the environment and human health. A huge variety of particles is found in the atmosphere and built environment. Many environments include unwanted gases and particles that can originate from primary emissions or from secondary transformations in the atmosphere. In addition there is significant processing within the atmosphere including condensation and evaporation, multiple cycles of activation into a droplet followed by dehydration, coagulation, photolysis, and chemical reaction. Research has shown that aerosol particles in the indoor and outdoor environments have significant detrimental impacts on health. The word aerosol was introduced in ca. 1920, in analogy to “hydrosol,” a liquid colloidal suspension of solid particles. Aerosols are particulate matter, either liquid or solid or a combination, suspended in a gaseous medium. “Aerosol particle” refers to the suspended particles themselves and can be divided into two groups, primary and secondary, depending on their origin. The former is generated by a source at the surface, for example, abrasion or combustion. The latter is often produced from gas-to-particle conversion or agglomeration of primary aerosols. Primary and secondary aerosol particles are characterized by their shape, size, and chemical composition. A spherical shape is often assumed, to simplify calculations. Aerosol particles are commonly classified by their “aerodynamic diameter,” that is, the diameter of a spherical particle with the same aerodynamic behavior. These classifications divide particulate matter (PM) into coarse (PM10 ),

fine (PM2.5 ), and ultrafine (PM0.1 ) fractions. The subscripted number is a cutoff size, for example, PM 2.5 is the total mass density of all aerosol particles with an aerodynamic diameter less than 2.5 mm. Airborne nano-sized particles, i.e., nanoparticles, belong to the ultrafine class of particles; they are often solid phase and have a dimension less than 100 nm. Nanoparticles are commonly emitted during combustion and are formed through gas-to-particle conversion.