+ O2 + M -> RO2{.} + M (2) RO2{.} + NO -> NO2 + RO{.} (3) HO2{.} + NO -> NO2 + OH{.} Nitrogen dioxide (NO2) and nitric oxide (NO) further react with ozone (O3) in a series of chemical reactions: (4) NO2 + hv -> O(^3P) + NO, (5) O(^3P) + O2 + M-> O3 + M(heat) (6) O3 + NO -> NO2 + O2 This series of equations is referred to as the photostationary state (PSS). However, because of the presence of Reaction 2 and 3, NOx and ozone are not in a perfectly steady state. By replacing Reaction 6 with Reaction 2 and Reaction 3, the O3 molecule is no longer destroyed. Therefore, the concentration of ozone keeps increasing throughout the day. This mechanism can escalate the formation of ozone in smog. Other reactions such as the photooxidation of formaldehyde (HCHO), a common secondary pollutant, can also contribute to the increased concentration of ozone and NO2. Photochemical smog is more prevalent during summer days since incident solar radiation fluxes are high, which favors the formation of ozone (reactions 4 and 5). The presence of a temperature inversion layer is another important factor. That is because it prevents the vertical convective mixing of the air and thus allows the pollutants, including ozone, to accumulate near the ground level, which again favors the formation of photochemical smog. There are certain reactions that can limit the formation of O3 in smog. The main limiting reaction in polluted areas is: (7) NO2 + OH{.} + M -> HNO3 + M This reaction removes NO2 which limits the amount of O3 that can be produced from its photolysis (reaction 4). HNO3 is a sticky compound that can easily be removed onto surfaces (dry deposition) or dissolved in water and be rained out (wet deposition). Both ways are common in the atmosphere and can efficiently remove radicals and nitrogen dioxide. Natural causes Volcanoes An erupting volcano can emit high levels of sulfur dioxide along with a large quantity of particulates matter; two key components to the creation of smog. However, the smog created as a result of a volcanic eruption is often known as vog to distinguish it as a natural occurrence. The chemical reactions that form smog following a volcanic eruption are different than the reactions that form photochemical smog. The term smog encompasses the effect when a large number of