Determination of atmospheric lifetimes via the measurement of OH radical kinetics.

The chemical composition of the earth’s atmosphere has been an area of general research interest for many decades.1 However, increased attention to a number of environmental issues has fostered a more detailed focus in recent years. These environmental drivers include such topics as (i) the destruction of stratospheric ozone via photochemical cycles involving industrial halocarbons and their reaction products,2 (ii) the alteration of the radiative balance of the earth’s atmosphere due to the buildup of infrared absorbing gases and other constituents in the upper troposphere and lower stratosphere,3 (iii) the production and transport of ozone and other pollutants in the troposphere,4-8 and (iv) the role of the atmospheric sulfur cycle in aerosol and cloud particle formation.9,10 Each topic requires quantitative information on the mechanistic details and time scales for atmospheric chemical transformations. Inherent in such quantification are the atmospheric lifetimes of the applicable source gases and the rates of the initial (generally oxidative) steps in their atmospheric degradation. In this paper, we will examine the concept of an atmospheric lifetime, the role that reactions of tropospheric hydroxyl radicals (OH) play in its quantification, the range of methods used for its estimation, and its applicability in quantifying some environmental impacts of source gas emissions. We will then review the various laboratory techniques used in the experimental determination of OH rate constants, and after examining some of the possible errors associated with each, we will discuss some of the difficulties encountered in producing a recommended data set for atmospheric modeling purposes.