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Does the photochemistry of the troposphere admit more than one steady state?


Nitrogen oxides are released to the troposphere primarily as NO and NO2, relatively insoluble gases which are not effectively scavenged by clouds and precipitation1. Nitrogen oxides are removed from the troposphere primarily as HNO3, the highly soluble product of reactions with O3 and various free radical species2. The NOx(= NO + NO2) removal rate thus depends on the oxidizing potential of the troposphere, which in turn is sensitive to the NOx concentration3. Here we demonstrate that in certain conditions the NOx removal rate is a non-monotonic function of NOx concentration. A consequence is that NOx emission rates within a critical range can support two or three different steady-state NOx concentrations. The ‘nonstandard’ steady states are characterized by low concentrations of ozone and radicals, and by high accumulations of the trace gases these normally scavenge. The emission rates required to support such alternative steady states appear to be somewhat in excess of present levels.

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White, W., Dietz, D. Does the photochemistry of the troposphere admit more than one steady state?. Nature 309, 242–244 (1984).

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