![[Equation]](/mopitt/mdd_93/eqn/eqn01.gif)
Much of the chemistry in the troposphere is induced by the chemical activity of the OH radical. It is important because it acts much as a detergent, reacting rapidly with hydrocarbons, both natural and anthropogenic, preventing their buildup in the troposphere. Since many hydrocarbons are radiatively active gases, OH may be considered to play a role, albeit indirect, in determining the climate. OH reacts rapidly with oxides of sulphur and nitrogen with the ensuing production of soluble species that are rained out or lost from the atmosphere by dry deposition. It also plays a major role in the production and loss of tropospheric O3, an important radiatively active gas whose concentration may have been changing over the last century (Volz and Kley, 1988).
The source of OH is reaction of O(¹D) atoms with H2O; the O(¹D) being produced by photolysis of O3. An outline of the main features of the reaction scheme for OH is illustrated in Figure 2.1 and the detailed equations are summarized in Appendix B. The broad lines linking OH and HO2 indicate that the time constants for reaction are rapid, being less than 2 minutes. Thus OH and HO2, often referred to as HOx, form a closely knit system where each constituent cycles many times before being destroyed. The major agents in the recycling process are CO, O3, and NO. The main loss of HOx is their self reaction, although reaction with CH4, HNO3, CH3CCL3 etc., also plays a role. Reaction with CH4 may also result in the generation of HOx radicals. Formation of H2O2 and HNO3, if followed by rainout or dry deposition, also leads to the loss of HOx.
In the background atmosphere the combination of H2O, O3, and UV sunlight (290-310nm) normally provides the starting ingredients. However, in polluted airmasses and perhaps also in boundary layer airmasses over hydrocarbon source regions, aldehydes (RCHO) may also provide important sources of HOx.
To first order approximation we may write the OH density as:
where A,B, and C are constants can that be obtained from relatively well known laboratory or atmospheric parameters. J represents the photodissociation frequency for production of O(¹D) from O3.
Since OH has such a low concentration it is extremely difficult to measure, even with the full complement of support available to ground based experiments. The prospect of obtaining the necessary global distribution from a satellite seems extremely unlikely. Thus, in order to provide the needed OH database it will be necessary to measure the various quantities including those in the above equation to enable OH densities to be modelled. Clearly one of the important ingredients is CO as it provides the major "loss" for OH in the troposphere. We have set the word loss in quotes since really the reaction of CO with OH is not a final loss of OH, rather just a recycling of HOx. Thus, CO is a vital chemical element of the troposphere.