Air pollutants are primarily classified as primary and secondary pollutants. Primary pollutants (e.g., CO, SO2, NOx, etc.) exist in the same form in which they were emitted from their original identifiable sources. Secondary pollutants (O3, HNO3, H2SO4, PAN, etc.) are formed after chemical reactions in the environment. CO, CO2, CH4, SO2, H2S, NOx, O3, volatile organic compounds (VOCs), and peroxyacyl nitrates (PANs) are the notable air pollutants, each with specific chemical characteristics exerting specific effect on organisms and ecological processes. Photochemical smog refers to a state of atmospheric pollution prevailing due to chemical reactions between some air pollutants in the presence of sunlight. Exposed to ultraviolet radiation, NO2 undergoes a series of complex chemical reactions in the atmosphere with hydrocarbons (HCs), resulting in the production of photochemical smog components: a mixture of O3, HNO3, aldehydes, PANs, and VOCs. Particulate matter (PM) comprises nongaseous materials—solid particles and tiny liquid droplets—in the atmosphere. Particle size determines whether PM is settable or suspended. From the viewpoint of the behavior of the pollutant, SPM is of three types: aerosols, dust, and mist.