Time constant analysis of tropospheric gas-phase chemistry

Abstract

Time constants are often used to describe and categorize the time dependence of concentrations in chemically reactive systems. Though originally deduced for linear systems (defined as those that have chemical balance equations that are linear in the concentrations), they are also useful for non-linear problems. Here, the concept is reviewed for tropospheric gas-phase chemistry which is substantially non-linear due to the majority of bimolecular reactions introducing quadratic terms into the balance equations. Application to a simplified reaction scheme reveals that, in many cases, the time constants for the locally linearized equations provide a reasonable approximation to the global (as opposed to local in the above sense) temporal behaviour. For a simple CO, NO_x, O₃ and HO_x chemistry, the range of time constants is characterized by one group of small values below 10 min separated by a substantial gap from longer term processes with time constants above 1 day. For NO_x<1 ppb, an assignment of time constants to the relaxation of specific compounds is approximately possible. For NO_x>1 ppb, in particular around the NO_x mixing ratio that maximizes the OH concentration, the analysis indicates linearly unstable motion with an exponential increase on a time scale of weeks which is, however, unimportant under real atmospheric conditions. In this region, an assignment of time constants and compounds is not possible indicating the strong coupling between all compounds.