Laboratory measurements of chemical parameters are central to our
understanding of the transformations occurring in the atmosphere.
Laboratory kinetics and photochemistry provide direct input for
chemical models of the atmosphere, and constitute an important
basis for many of the field measurement techniques in use today.
Atmospheric chemical kinetics is basically the measurement of the rate of reaction between two chemical components of the atmosphere. In the laboratory we attempt to isolate a single reaction or a small subset of reactions for study. Ideally, at least one of the reacting species should be detected directly, along with unique products formed from the different reaction channels. This is not often achieved in practice, because of the limited number of species that can be detected quantitatively (and easily!).
Atmospheric photochemistry deals with the processes occurring after absorption of a photon (ultraviolet, visible or infrared) in the atmosphere. Absorption of light in the visible and ultraviolet regions is usually associated with a change in the electronic configuration of a molecule. The energies involved can be enough to break chemical bonds, and ultraviolet radiation is largely responsible for initiating chemical changes in the atmosphere. Parameters which are measured in the laboratory include the absorption cross section, which is a measure of the intensity of the absorption, and the quantum yield, which describes the probability of producing various new products upon absorption of a photon. Absorption of infrared radiation corresponds to changes in the vibrational and rotational excitation of a molecule, and is not usually accompanied by chemical change. However, the absorbed radiation leads to heating of the atmosphere (greenhouse effect). Also, the same infrared absorption cross sections are needed to provide quantitative measurements of atmospheric species (see sections on satellite measurement and tunable diode laser absorption).
The topics of kinetics and photochemistry are not separate. The kinetics measurements often require a knowledge of the spectroscopic parameters of the chemical species being studied, while the cross sections of some of the more transient species can only be determined in real-time kinetics experiments. As the demand to understand the chemistry of the minor constituents of the atmosphere has become more stringent, the methods have become increasingly sophisticated, and the methods of generating and detecting the species have become more and more ingenious. We will describe briefly in this section how laboratory measurements are made, showing wherever possible the application to field measurements.