either a primary or a secondary pollutant. In the former case, its
origins are numerous, e.g., (1) natural emissions by sand storms,
forest fires, volcano eruptions, and sea sprays and (2) anthropogenic
emissions mostly by combustion from industrial, agricultural,
traffic, and urban sources. In the latter case, particles are sometimes
referred to photochemical smog; this smog is produced in specific
conditions when levels of VOCs, NOx, CO, and O3 are high