Low and high NOx tropospheric photochemistry

Emission inputs to the troposphere are processed in two fundamentally different ways, leading to two atmospheric states with greatly different properties. We define these states in terms of rates of NO x emissions and radical production. In the low NO x state, trace gas inputs to the atmosphere are removed primarily by oxidation reactions. In the high NO x state the oxidative potential of the atmosphere is greatly diminished and primary pollutants build up, as alternate removal processes are slow. We show here that this dual behavior is due to a titrationlike reaction between NO x and free radicals in which either reactant can be in excess.