Nitrogen Oxides and the Photostationary State

The photolysis of NO2 to NO and the subsequent regeneration of NO2 via reaction of NO with ozone is sufficiently fast, in the moderately polluted environment, for these species to be in dynamic equilibrium, viz 

Therefore, at suitable concentrations, ambient NO, NO2 and O3 can be said to be in a photochemical steady-state or photostationary state (PSS), provided that they are isolated from local sources of NO and that sunlight intensity is relatively constant, therefore 

The reactions (2.2, 2.20 and 2.24) constitute a cycle with no net chemistry. The PSS expression is sometimes expressed as a ratio, viz 

If ozone is the sole oxidant for NO to NO2 then 0 = 1. This situation often pertains in urban areas where NO levels are high and other potential oxidants of NO to NO2 such as peroxy radicals are suppressed. In the presence of peroxy radicals, Equation (2.25) has to be modified as 

The extent of the influence of NO in any given atmospheric situation depends on its sources, reservoir species and sinks. Therefore, an important atmospheric quantity is the lifetime of NO . If nitric acid formation is considered to be the main loss process for NO i.e. NO2), then the lifetime of NO ( no ) can be expressed as the time constant for reaction (2.23), the NO2 to HNO3 conversion. 

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