Homogeneous Gas Phase Oxidation Processes in the Troposphere

Abstract

Principal gas phase oxidation reactions governing in situ formation and removal of various C-, N-, S-, and halogen-containing trace gases in the global troposphere are reviewed briefly. Many important gas phase reactions are common to both the troposphere and the stratosphere. However, because of the substantially different chemical compositions and physical conditions between the two spheres, there exist numerous gas phase oxidation processes which can be considered unique to the troposphere. O3 plays the key role as the primary oxidizing agent. It reacts with compounds such as NOx (NO + NO2) and unsaturated hydrocarbons directly, and more importantly, leads to the formation of HO radical upon photodissociation. HO-initiated oxidation provides the major path for transformation of a large variety of tropospheric compounds and determines their chemical lifetimes. In these processes, numerous oxygen containing intermediates including free radicals are produced and subsequently converted to thermodynamically more stable products which are eventually removed from the gas phase by heterogeneous processes. Notably, chemical transformations of various trace gases are closely coupled and mutually interrelated by reactions involving common oxidizing or reducing species. At present, there is considerable uncertainty in the identity and fate of compounds produced in situ in the troposphere. Some of the existing uncertainties concerning the potentially important gas phase oxidation processes are indicated.