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 NOx emissions and radical production. In the low NOx state, trace gas inputs to the atmosphere are removed primarily by oxidation reactions. In the high NOx state the oxidative potential of the atmoshere 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 NOx and free radicals in which either reactant can be in excess. ¿American Geophysical Union 1994