2. Reactions of Hydroxyl Radicals with Olefins A. Mechanism of OH-olefin reactions

The mechanism of the OH reactions with ethylene and propylene in the absence of 02 appears now to be reasonably well understood. Hydroxyl radicals add to these two olefins and there is little or no H atom abstraction at room temperature. (A suggestion that there is approximately 8 percent H atom abstraction from ethylene at atmospheric pressure, possibly due to "hot" OH radicals, is given in a separate comment further below). The mechanism of OH-olefin reactions in the presence of O2, a process of crucial importance for the chemistry of photochemical smog, is unfortunately very incompletely understood.

Recommendations:

1) Study of the mechanism of the OH-olefin reactions in the absence of 02 should be extended to olefins other than C2H4 and propylene, especially to the olefins known to be present in the polluted atmosphere.

2) Very high priority should be attached to detailed studies of the OH-olefin reactions in the presence of O2, especially for olefins known to be present in polluted atmosphere.

3) Studies of the OH-olefin reactions under atmospheric conditions in the presence of varying amounts of NO (and possibly of other pollutants, such as S02, etc.) should be carried out with as detailed analysis of the reaction products as possible.

B. Rates of OH-olefin reactions

Good experimental techniques for the determination of OH-olefin reaction rates are now available. However, caution has to be exercised to assure

accurate determination of the very small reactant

concentrations used in some experiments and to establish the extent of the interfering secondary reactions, in particular of the OH-free radical secondary reactions.

2) A set of accurate values of the rate constants of the OH reactions with selected olefins (including ethylene under conditions similar to those in the lower atmosphere would be desirable in order to establish whether the rates are affected by oxygen in the air. The case of ethylene is of special interest in this respect because of a possibility of interception (and consumption by reaction) of the "hot" CH2 CH2OH radicals by 02. Such an interception could result in an appreciable increase in the rate constant of the OH-C2H4 reaction in air relative to the value obtained in laboratory measurements in the absence of 02.

3) An ongoing critical review of the rate constants would be very useful.

Rate constants for the simple olefins are now probably known to within ± 20 percent. The value of the rate constant for C2H4 at 1 atm is probably also accurate within ± 20 percent. Rate constants for higher olefins and cycloolefins are less satisfactory, especially the values obtained by the competitive technique. No values are available for some important naturally occurring olefins such as terpenus and isoprene, although the latter could be roughly estimated from the value of the rate constant for 1,3-butadiene. The range of the literature values of the rate constant for acetylene is large (a factor of about 5-6) and further determinations are required. Recommendations:

1) Further determinations of the rate constants are required for higher olefins, cycloolefins, isoprene, terpenes and acetylene.

1. Introduction

Their

Aldehydes are major products in the oxidation f hydrocarbons and play a rather unique role in he photochemistry of the polluted troposphere. or example, they can contribute to photochemical mog, eye irritation, and odor problems. portance has been recognized for over a decade Leighton, 1961; Altshuller and Cohen, 1963; Itshuller and Bufalini, 1965). While significant rogress has been made in defining the photohemistry, kinetics, and mechanism of aldehyde hotooxidation, much remains to be learned about heir ambient concentrations as a function of ine, season and location. Since aldehydes, oth aliphatic and aromatic, occur as primary and econdary pollutants and are direct precursors of ree radicals in the atmosphere, aldehyde chemistry epresents an important subject area. The

derstanding of this topic is necessary to meet he objective of modeling tropospheric chemical eactions. In this context, the major objective f this paper is to consider the historical nterest in aldehydes; their sources and tmospheric concentrations; the photochemistry, inetics and mechanism of their reactions and inally to delineate current measurement needs nd recommend research priorities based on ssessment of the current status of knowledge of he chemistry of aldehydes in the troposphere.

In addition, the role of other oxygenated ydrocarbons in tropospheric chemistry will be adressed briefly. Although aldehydes are the in oxygenated hydrocarbons generally considered, nd will receive major considerations here, other lasses of oxygenated hydrocarbons merit onsideration and should be assessed in terms of heir involvement in the chemistry of the piluted troposphere. Thus ketones, esters, thers and alcohols will be briefly considered o assess their possible importance in modeling he troposphere. The major areas of uncertainty ill be discussed and research priorities uggested.

This paper is an attempt to survey the current

published literature on aldehydes (and, to a lesser extent, other oxygenated hydrocarbons) as the work relates to modeling the troposphere. It is hoped that the discussion periods will extend the coverage to include unpublished work, preliminary results, and peripheral studies which have a direct bearing on the overall thrust of this paper.

2. Previous Work and Importance of Aldehydes

Initial impetus for the interest in the role of aldehydes in photochemical air pollution stemmed largely from the possibility that they were connected with eye irritation which became a major phenomenon and problem in the Los Angeles basin during the 1940's. However, an early Stanford Research Institute study (SRI 1950) concluded that "concentrations of aldehydes have rarely exceeded 0.2 parts per million by weight and the high concentrations did not coincide with periods of eye irritation. This lack of correlation tends to indicate that aldehydes alone are not responsible for eye irritation." Subsequent work indicated that acrolein was present on highly polluted days and this compound is known to be a potent eye irritant (Los Angeles Air Pollution Control District, 1950; Altshuller and McPherson, 1963; Scott Research Labs, 1969). Acrolein and formaldehyde were shown to be produced upon irradiation of dilute automobile exhaust and olefin-NOx mixtures (Schuck, 1957; Schuck and Doyle, 1959).

Aside from the possible relationship of aldehydes to eye irritation, it was subsequently proposed (Leighton and Perkins, 1956; Leighton, 1961) that aldehydes could act as precursors to radicals which could either directly form oxidant or oxidize NO to NO2. This possibility received support from the results of several experimental studies focused on the photooxidation of aldehydes under laboratory and simulated atmospheric conditions and generally employed formaldehyde and the lower molecular weight aliphatic aldehydes (Haagen-Smit and Fox, 1956; Altshuller and Cohen, 1963; Altshuller, Cohen et al., 1966; Johnston

and Heicklen, 1964; Altshuller, Cohen et al., 1967; Cohen, Purcell et al., 1967; Purcell and Cohen, 1967; Bufalini and Brubaker, 1969).

Recently Dimitriades et al., (1972) and Pitts et al., (1976) carried out experiments in a smog chamber illustrating the effect of initial aldehyde concentrations on oxidant production under simulated atmospheric conditions. Figure 1 shows the significant impact of initial aldehyde concentrations on ozone formation in a nine-hour irradiation of a surrogate hydrocarbon mixture (Pitts et al., 1976). Thus an approximately 100 percent increase in initial formaldehyde concentration from 91 to 185 ppb increases the maximum ozone concentration by approximately 25 percent from about 0.39 to nearly 0.5 ppm in nine hours. Clearly, the rate of formation of 03 is enhanced but it is possible that the 03 maximum value would not be significantly increased if the irradiations were carried out sufficiently long.

Fig. 1.

2

185 ppb

91 ppb

0 ppb

Effect of added HCHO on ozone formation in long-term irradiations of surrogate mixture (from Pitts et al., 1976).

Aldehydes can provide significant sources of radicals such as HO2, OH and RO2 which can influence the rate at which photochemical oxidants are formed under ambient conditions. With the advent of appropriate computer calculation facilities to handle complex kinetic mechanisms, a number of workers demonstrated this effect by carrying out computer simulations of atmospheric chemistry both with and without initial aldehydes (Niki, Daby and Weinstock, 1972; Calvert et al., 1972; Demerjian, Kerr and Calvert, 1974; Dodge and Hecht, 1975; Levy, 1974; Whitten and Dodge, 1976; Graedel, 1976; Carter et al., 1978). Many of these calculations have focused on formaldehyde which photodissociates to produce significant amounts of HO2 radicals under ambient conditions. Thus Demerjian et al., (1974) have shown that this route is the most important source of HO2 radicals in the atmosphere.

Although there is some uncertainty attached to the quantum yields for photodissociation into radicals of HCHO as a function of wavelength (vide infra), aldehydes are well established as important ingredients in photochemical smog formation.

The role of aldehydes as eye irritants and radical precursors has been given above. An additional role for aldehydes is as precursors to the formation of peroxyacyl nitrates. These can be formed by the reaction mechanism

RCHO + OH → RCO + H20

02

RCO + NO2 RCO 3 NO 2

peroxyacyl nitrate

Peroxyacyl nitrate type compounds have been found in many parts of the world e.g., Penkett et al. (1975) in England, van Ham and Nieboer (1972) in Netherlands, in Japan (Akimoto and Kondo, 1975) and in the U.S.A. (Stephens, 1969; Lonneman et al. (1976)).

3. Sources and Ambient Concentrations Sources. There are primary and secondary sources of aldehydes in the atmosphere. The primary sources are related to combustion and result from incomplete combustion in, for example, internal combustion engines, diesel engines and stationary sources, such as incinerators, etc. (Altshuller et al., 1961; Linnell and Scott, 1962; Elliot et al., 1955). Automobiles are a significant source of aldehydes and the latter account for up to one-tenth of the hydrocarbon emissions (Black 1977). Oberdorfer (1964) and Seizinger and Dimitriades (1972) have analyzed the individual aldehydes emanating from pre-controlled automobiles Table 1 shows the percentage of aldehydes from automobile exhaust as determined by several workers (Oberdorfer, 1964; Fracchio et al., 1967; Wodkowski and Weaver, 1970; Wigg et al., 1972). It is evident from these emission sources that formaldehyde is the largest aldehyde component. Similar but more extensive results are shown in table 2 which were obtained by Seizinger and Dimitriades (1972).

It can be seen that in addition to the saturated aliphatic aldehydes, acrolein--a potent eye irritant--is also present. In addition, benzaldehyde and formaldehyde are produced, along with alcohols, ethers and ketones. One would of course expect variations in the relative amounts of these compounds depending on the fuel used, e.g., see table 1.

With the advent of hydrocarbon control measures for automobiles, the aldehyde concentrations have been reduced along with the hydrocarbons. However. different control techniques apparently have varying effects upon the percentage reduction of aldehydes compared with the remaining hydrocarbons. Thus, Black (1977) shows interesting data for emissions from automobiles using thermal reactors, lean burn technology and catalysts of various kinds to reduce hydrocarbons. Table 3 shows a comparison of absolute and relative hydrocarbon class reductions for various automobiles employing different hydrocarbon control systems. It is apparent that cars using the catalyst system, rather than the lean burn system, effect greater reductions of aldehydes.