Smog is a synchrony of two words - smoke and fog. Smog can be of two types - industrial or winter smog (e.g. London smog) and photochemical or summer smog (e.g. Los Angeles smog).
Photochemical smog occurs in the troposphere, the lower portion of our
atmosphere. Ground-level ozone, the primary component of photochemical smog, is the most prevalent pollutant that has been known to cause a serious air pollution problem in many developed countries over the past few decades. Photochemical smog is a widespread phenomenon in many population centers of the
World. The components of photochemical smog that are the most damaging to plants and detrimental to human health are the photochemical oxidants. These oxidants include ozone (O_{3}), peroxyacetyl nitrate (PAN), peroxybenzoyl nitrate (PBN), hydrogen peroxide (H_{2}O_{2}), formic acid (HCOOH), and other trace substances. They are collectively termed photochemical oxidants with ozone and
PAN, and are present in the highest concentrations. In addition, the aerosols formed during the chemical reactions cause a marked reduction in visibility with a brownish cast in the atmosphere [13]. PAN in photochemical smog can irritate the eyes, causing them to water and sting.