Reactions in Urban Atmospheres Containing Volatile Organic Compounds

We have just seen that the ozone concentration can be enhanced even in a VOC-free atmosphere containing oxides of nitrogen and methane—anthropogenic and/or natural—or, to a lesser extent, carbon monoxide. These ozone-producing processes are generally well understood now, and they give reasonable simulations in tropospheric computer modeling. In polluted urban areas, however, chemical reactions of VOCs dominate over those involving methane, leading to considerably more complexity because of the number (hundreds) of VOCs and their diverse chemistry. An excellent review by Atkinson42 covers reactions under tropospheric conditions of several classes of organic compounds, such as hydrocarbons (alkanes, alkenes, alkynes, and aromatics and substituted aromatics) and oxygen- and nitrogen-containing compounds, and their degradation products. Although complete coverage of the photochemical smog mechanism is beyond the scope of this book, this section summarizes some of the more important types of these reactions.

One of the major tools used in determining the individual chemical processes taking place is the environmental chamber.43 Also called smog cham

42R. Atkinson, Gas-phase tropospheric chemistry of organic compounds, Monograph no. 2 of The ]ournal of Physical and Chemical Reference Data, J. W. Gallagher, ed. American Chemical Society and the American Institute of Physics for the National Institute of Standards and Technology, Gaithersburg, MD, 1994.

43A coverage in depth of environmental chambers is given in B. J. Finlayson-Pitts and J. N. Pitts Jr., Chemistry of the Upper and Lower Atmosphere: Theory, Experiments, and Applications, Academic Press, San Diego, CA, 2000.

bers, these vessels are often large (from tens to hundreds of cubic meters) receptacles in which reactant mixtures close to actual atmospheric conditions of concentration, pressure, relative humidity and temperature are illuminated with solar spectral distribution light. A typical smog chamber experiment for a mixture of NO, NO2, and hydrocarbon (propene, C3H6) is shown in Figure 5-11. It is seen that the photostationary state projected for a VOC-free atmosphere [equation (5-72)] is destroyed by addition of the hydrocarbon. NO is rapidly oxidized to NO2, O3 is produced after an induction period, and the propene is oxidized to CO, CO2, and a variety of oxygen- and nitrogen-containing organic compounds. This reaction mixture is of course an extreme simplification, containing only one hydrocarbon instead of the hundreds of VOC species emitted into a polluted urban environment. Note, however, that it qualitatively agrees with the actual photochemical smog buildup shown in Figure 5-9. That is, NO is rapidly oxidized to NO2 in the presence of organic compounds (the VOCs emitted along with NO into the atmosphere starting around 6 a .m . from automobile exhausts) and the ozone builds up only after an induction period, during which the concentration of NO is lowered until O3 is no longer destroyed by reaction (5-42).

Irradiation time (min)

FIGURE 5-11 Typical photochemical smog chamber results for a mixture of hydrocarbon, NO, and air. Drawn from data of A. P. Altshuller, S. L. Kopczynski, W. A. Lonneman, T. L. Baker, and R. L. Slater, Environ. Sci. Technol., 1, 899 (1967); and K. J. Demergian, J. A. Kerr, and J. G. Calvert, The Mechanism of Photochemical Smog Formation, in Advances in Environmental Science and Technology, J. N. Pitts Jr., and R. L. Metcalf, eds., 4, p. 1. Wiley, New York, 1974.

Irradiation time (min)

FIGURE 5-11 Typical photochemical smog chamber results for a mixture of hydrocarbon, NO, and air. Drawn from data of A. P. Altshuller, S. L. Kopczynski, W. A. Lonneman, T. L. Baker, and R. L. Slater, Environ. Sci. Technol., 1, 899 (1967); and K. J. Demergian, J. A. Kerr, and J. G. Calvert, The Mechanism of Photochemical Smog Formation, in Advances in Environmental Science and Technology, J. N. Pitts Jr., and R. L. Metcalf, eds., 4, p. 1. Wiley, New York, 1974.

As we will see in this section, the organic alkyl (R"), alkoxy (RO"), and alkylperoxy (RO2") radicals (in addition to "OH and HO2") are important intermediates in the tropospheric degradation of VOCs and the formation of photochemical smog" Each is involved in many chain-propagating steps in which NO is converted to NO2 before being terminated by reactions such as (5-75) or (5-103), shown later" We have seen that the methyl ("CH3) radical is produced in the chain-initiating H-atom abstraction by the "OH radical [reaction (5-79)] in the methane chain; similarly, this is the major mode of formation of alkyl and substituted alkyl radicals from several classes of compounds:

With alkenes, however, the major reaction of OH is addition to the carboncarbon double bond to give ^-hydroxyalkyl radicals,

with only a small fraction of the "OH radicals involved in H-atom abstraction" Radicals are also produced by photolysis of some VOCs" In Section 5"3"2, for example, we saw that the photolysis of formaldehyde below 330 nm generates H and HCO radicals [reaction (5-86)]" In a similar manner, higher aldehydes such as acetaldehyde, propanal, or butanal, which are formed in the tropospheric degradation of many VOCs, photodissociate into an R" radical and the formyl radical:

Smog chamber studies of hydrocarbon-NO mixtures have shown that addition of aldehydes does indeed enhance NO oxidation and production of secondary pollutants"

Most of the alkyl and substituted alkyl radicals very rapidly combine with O2 to produce alkylperoxy radicals:

For allyl, benzyl, substituted benzyl, or alkyls with more carbons than ethyl, reaction (5-101) is second order under atmospheric conditions (see footnote 40)" The p-hydroxyalkyl radicals formed by the addition of "OH to the alkene double bond [reaction (5-999)] also rapidly add to O2 forming ^-hydroxy-alkylperoxy radicals, RHC(O2)C(OH)HR'"

All the alkylperoxy (RO2") radicals react with NO with approximately the same rate constant" Both CH3O2" and C2H5O2" primarily oxidize NO to NO2, forming an alkoxy radical:

However, for the larger alkylperoxy radicals, addition to form organic nitrates also occurs,

with reaction (5-103) becoming more important with increasing number of carbon atoms in R.

We thus see that the H02. and R02. radicals generated in an oxygen environment by the .OH radical in the degradation of many VOCs are at least qualitatively the major contributors to the enhanced N0-to-N02 oxidation—and therefore to buildup of 03—in photochemical smog via reactions like (5-102) and chain reactions similar to the methane chain, (5-83).

Other radicals generated from VOCs react differently with O2 other than by the addition reaction (5-101). For example, we saw in reaction (5-88) that the formyl (HCO) radical from the photolysis of formaldehyde reacts with O2 to give HO2. and CO. The a-hydroxy radicals also react with O2 via H-atom abstraction to give the HO2. radical, as for example the simplest a-hydroxy radical, .CH2OH:

This is the only process leading to the loss of .CH2OH under atmospheric conditions. The vinyl radical .C2H3 initially adds to O2 across the. double bond to form a C2H3O2 adduct, which decomposes to H2CO and HCO giving the overall reaction

The three major reactions of the alkoxy or substituted alkoxy (RO.) radical from (5-102) in the troposphere are reaction with O2, unimolecular decomposition, and (for four or more carbon atoms) unimolecular isomerization. However, all these reactions eventually lead to formation of HO2. and/or the oxidation of NO to NO2 by reactions similar to (5-102).44

Aliphatic aldehydes and ketones are products of the atmospheric degradation of a wide variety of VOCs, primarily from reactions of alkoxy and alkylperoxy radicals. The .OH radical reacts with carbonyl compounds via H-atom abstraction; with the aldehydes, abstraction is mostly from the primary carbon to give the acyl radical and water:

Acyl radicals are also produced by H-atom abstraction from aldehydes by the NO3. radical, giving HNO3, and by other organic radicals, RS

44For example, with the smallest alkoxy radical, methoxy (H3CO' ), H3CO' + O2 ) H2CO + HO2-.

Acyl radicals combine with O2 to form peroxyacyl radicals RC(O)O2"

which further contributes to NO oxidation, hence to photochemical smog, by the rapid reaction

followed by formation of another alkylperoxyl radical from the RC(O)O" (acyloxy) radical:

The peroxyacyl radical also combines with NO2 in a three-body reaction to produce peroxyacyl nitrate, RC(O)OONO2:

The simplest and most abundant peroxyacyl nitrate is peroxyacetyl nitrate (PAN), CH3C(O)OONO2. PAN is an important component of photochemical smog. For example, it is highly phytotoxic: one of the most toxic compounds known for vegetation. It is also a potent lachrymator, and is perhaps the major eye irritant in photochemical smog. It is also a greenhouse gas in that it absorbs radiation in the tropospheric IR "window." It is unstable, decomposing thermally by the reverse of reaction (5-111) to the peroxyacetyl radical and NO2; yet it is sufficiently stable in the absence of NO to aid in the transport of nitrogen oxides to initially unpolluted "downwind" areas. In fact, it has been detected even in remote parts of the troposphere. The next most abundant peroxyacyl nitrate is peroxypropionyl nitrate (PPN), C2H5C(O)OONO2; although it may be more phytotoxic, its concentration is only about 10-20% that of PAN.

In addition to its key role in the NO-to-NO2 oxidation, the hydroxyl radical is also the primary oxidant and remover of VOCs and other pollutants in the atmosphere. Briefly, its major reactions with VOCs, in addition to those already given are addition, across the carbon-carbon triple bond in alkynes and to the aromatic ring in monocyclic aromatics and in naphthalene and methyl- and dimethyl-substituted naphthalenes, and H-atom abstraction, from ethers, a, ^-unsaturated carbonyls, and alkyl nitrates, and from both the C—H and O—H bonds in alcohols.

The hydroxyl radical can also initiate the oxidation of reduced forms of sulfur such as hydrogen sulfide to 'SH radicals:

Further oxidation of 'SH leads to sulfur dioxide, SO2. The hydroxyl radical is also involved in the continuing oxidation of SO2 by the addition reaction to form the HOSO2. radical, reaction (5-52), followed by oxidation to sulfuric acid, H2SO4, and sulfate aerosol droplets in the presence of water.45 Large amounts of sulfur oxides are released into the troposphere over industrialized areas, and they become the major acidic component of acid rain (see Section 11.4).

While ozone is the major contaminant product and its concentration is taken as the measure of the intensity of photochemical smog, it also contributes to decomposition reaction of the VOCs. For example, the addition of O3 to the double bond in alkenes is a major pathway in their atmospheric decomposition. An energy-rich ozonide is formed, which rapidly decomposes by breaking the C—C single bond and either of the two O—O single bonds to form a carbonyl and a biradical RR'COO\ This energy-rich biradical either decomposes, forming a variety of products including the .OH radical, or reacts with aldehydes, H2O, NO2, and so on.

We have seen that the nitrate radical is formed by the reaction of NO2 with ozone (5-92), that it is in equilibrium with NO2 and N2O5 (5-94), that it reacts with NO to produce NO2 (5-93), and that it is photolyzed to NO + O2 (5-96) or to NO2 + O (5-97). It also reacts with alkanes via H-atom abstraction, similar to .OH reactions:

However, although the NOy concentration is greater at night than during the day because of its daytime photolysis, it is nevertheless much less important at nighttime than the .OH radical is during the daytime as a source of alkane decomposition. NOy readily reacts with alkenes by addition to the carboncarbon double bond, followed by rapid addition to O2. The resulting nitratoalkylperoxy radicals can react with HO2. and RO2. radicals, and can also reversibly add NO2 to give the unstable nitratoperoxynitrates, which may serve as temporary "sinks" for the peroxy radicals.

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