Time constant analysis of tropospheric gas-phase chemistry

Time constants are often used to describe and categorize the time dependenc e of concentrations in chemically reactive systems. Though originally deduc ed for linear systems (defined as those that have chemical balance equation s that are linear in the concentrations), they are also useful for non-line ar problems. Here, the concept is reviewed for tropospheric gas-phase chemi stry which is substantially non-linear due to the majority of bimolecular r eactions introducing quadratic terms into the balance equations. Applicatio n to a simplified reaction scheme reveals that, in many cases, the time con stants for the locally linearized equations provide a reasonable approximat ion to the global las opposed to local in the above sense) temporal behavio ur. For a simple CO, NOx, O-3 and HOx chemistry, the range of time constant s is characterized by one group of small values below 10 min separated by a substantial gap from longer term processes with time constants above I day . For NOx ( 1 ppb, an assignment of time constants to the relaxation of spe cific compounds is approximately possible. For NOx > 1 ppb, in particular a round the NOx mixing ratio that maximizes the OH concentration, the analysi s indicates linearly unstable motion with an exponential increase on a time scale of weeks which is, however, unimportant under real atmospheric condi tions. In this region, an assignment of time constants and compounds is not possible indicating the strong coupling between all compounds.