1. Introduction

In planning for the abatement and control of air pollution, it is essential to establish the quantitative chemical relationship between source emission and the resulting air quality. At present, concerted modeling efforts are being made to achieve this goal. Chemical interpretation of smog chamber data [1]1, prediction of "ozone-isopleth" [2], and air-shed modeling [3] of Los Angeles Reactive Pollutant Program (LARPP) data [4] are a few of the notable examples of such endeavor. Clearly, the degree of success of modeling work is governed, in large part, by the availability of reliable kinetic data. Despite recent progress, the knowledge of the chemical reactions taking place in the lower troposphere is far from satisfactory. There exist numerous critical uncertainties in the kinetics and mechanisms of reactions involving a large variety of atmospheric constituents. This paper is intended to assess the needs for improved experimental data for olefin reactions with HO and with 03.

For the purpose of evaluating the existing needs for improved kinetic and mechanistic information for these reactions, their potential role in the lower troposphere is discussed briefly. Further, the current knowledge of these reactions is illustrated by some of the work published within the last few years. This paper is not intended to be an extensive literature review, but rather, to convey the author's thoughts on the future direction and research priorities in the area of tropospheric chemistry. In view of the urgency and long-term interest in establishing firm scientific bases for the abatement of air pollution problems, the more systematic data evaluation efforts, e.g., the "chemical reaction matrix"

Figures in brackets indicate literature references at the end of this paper.

method [5] used for the evaluation of the stratospheric chemistry, should be made in the future.

2. Atmospheric Role of Olefin Reactions with HO and with 03.

Olefins are among the most reactive classes of organic compounds present in the lower troposphere. In particular, their possible role in the formation of photochemical smog has been well-recognized over the last three decades [6]. As a result, olefins have been used extensively as surrogate hydrocarbons in laboratory smog studies, and have played the crucial role in the development of smog chemistry [1]. It now appears that the atmospheric fate of olefins are governed primarily by their reactions with HO and with 03. Conversely, these reactions are responsible for regulating the atmospheric concentrations of HO and 03. The latter aspect is of primary importance, since HO is the major chain carrier of atmospheric reactions and determines the role of other hydrocarbons and organic compounds in the formation of "oxidant", e.g., 03. Whether 03-olefin reactions can lead to the formation of "excess" 03 or alternatively serve as a sink for 03 is another key question to be answered.

It must be stressed that a quantitative evaluation of the atmospheric role of the olefin reactions, or for that matter any other reactions, can be made only on the basis of numerical modeling studies for a given source distribution and strength under a variety of meteorological conditions. Clearly, relative importance of HO and 03 reactions involving various olefins can vary markedly between "fresh" and "aged" air masses because of the chemically and meterologically induced changes in relative and absolute olefin concentrations.

To illustrate the relative importance of various olefinic and other types of hydrocarbons, table l shows the results obtained by Calvert [7] for the relative rates of HO-radical attack on hydrocarbons

Table 1.

b

in Los Angeles air samples [4]. It should be noted in this table, that a variety of olefins, indicated by asterisks, contribute significantly to the removal of HO-radical. The olefins as a whole are responsible for 35 percent of the HO removal by hydrocarbons. Similarly, relative removal rates of hydrocarbons by 03 can be estimated for the data given in table 1. The results are shown in table 2. Only the olefinic hydrocarbons are listed in this table, since 03 reactions with both paraffinic and aromatic hydrocarbons are negligibly slow. Notably, in this particular case, by far the dominant hydrocarbon-ozone reactions involve cyclic olefins, i.e., cyclohexene and 1-methylcyclohexene rather than the straight chain olefins commonly used as surrogates. Calvert [7] also estimated the comparable fractional rates of removal of C3H6 by 03 (3.2 percent h1) and by HO (4.8 percent h1). for conditions given in table 1. These examples amply demonstrate that detailed hydrocarbon analyses of air samples are essential to the understanding of atmospheric chemistry.

Because of the close chemical coupling among 03, NO and NO2 concentrations via photo-stationary relationship [9], the role of olefins in the oxidant formation can be assessed only in terms of their effects on NO chemistry. Specifically, the key question is "how do the atmospheric reactions