The Hydroxyl Radical and Its Role in Ozone Formation

Although the HO radical is present in the sunlight-irradiated troposphere at very low concentrations, only about 106 molecules cm−3, it is the most important trace component in our atmosphere. It is a highly reactive transient species and is responsible for initiating the oxidation of the majority of organic compounds in the troposphere. It initiates the chain reactions that produce ozone. All the saturated, H-atom containing molecules react with HO through abstraction of an H atom. In the case of the simplest alkane, methane, reaction (1) leads to the formation of a water molecule and an alkyl (CH3) radical: . . . HO + CH4 → H2O + CH3 (1) . . . The CH3 radical released into the oxygen-rich atmosphere quickly adds O2 to give the methyl peroxy radical in reaction (2), which in NO-containing atmospheres can react to form NO2, and an alkoxy radical, CH3O, in reaction (3). In turn, this radical reacts with O2 to give an HO2 radical and a molecule of formaldehyde in (4). An HO radical can be regenerated as the HO2 molecule oxidizes NO to NO2 in (5), and the chain of events, reactions (1) through (5), leads to ozone generation through the photolysis of the NO2 molecule in reactions (6) and (7): . . . CH3 + O2 → CH3O2 (2) . . . . . . CH3O2 + NO → CH3O + NO2 (3) . . . . . . CH3O + O2 → HO2 + CH2O (4) . . . HO2 + NO → HO + NO2 (5) . . . . . . NO2 + hν → O + NO (6) . . . . . . O + O2 (+ M) → O3 (+ M) (7) . . . Methane is the least reactive of the alkanes with HO. Urban atmospheres contain a complex mixture of the more reactive larger alkanes (RH). The number of different possible geometric isomers and stereoisomers of the alkanes that can be formed by association of C and H atoms is astounding (Calvert et al., 2008). For example, there are more than a thousand structurally different molecules of molecular formula C12H26, more than a million C20H22, more than a billion of formula C25H52, and more than a trillion possible different isomers of molecular formula C31H64.

The Oxides of Nitrogen: Their Relation to Tropospheric Ozone

Reactive (or “odd”) nitrogen is emitted into the atmosphere in a variety of forms, with the most important being NOx (NO and NO2), ammonia (NH3), and nitrous oxide (N2O). Emissions of these species into the atmosphere have been summarized, for example, by the IPCC Fourth Assessment Report (the AR4; IPCC, 2007). Some discussion of NOx emissions and trends has also been presented in Chapter I. Emissions of NOx are mainly the result of anthropogenic activity associated with fossil fuel combustion and industrial activity. For the 1990s, the AR4 estimates total anthropogenic NOx emissions of 33.4 TgN yr−1, with natural emissions (mostly from soil and lightning) accounting for an additional 8.4–13.7 TgN yr−1. Ammonia emissions are comparable in magnitude to those for NOx, with anthropogenic emissions (45.5 TgN yr−1) again exceeding natural emissions (10.6 TgN yr−1). Although the majority of the ammonia produces aerosols or is scavenged by aerosol and is subsequently lost from the atmosphere, some gas phase oxidation does occur, which can in part lead to NOx production. The N2O source strength is about 17.7 TgN yr−1, with natural sources outweighing anthropogenic ones (IPCC, 2007). However, N2O is essentially inert in the troposphere, and thus the vast majority of its photooxidation and concomitant NOx release occurs in the stratosphere. The major NOx − related reactions occurring in the Earth’s troposphere are summarized in Figure III-A-1. As just alluded to, the species NO and NO2 are jointly referred to as NOx and are often treated collectively. This is because, under daytime conditions, these two species are rapidly interconverted, with the interconversion occurring on a much shorter timescale than the loss of either species.

Photodecomposition of Light-Absorbing Oxygenates and Its Influence on Ozone Levels in the Atmosphere

Photochemistry provides the important driving force that initiates chemistry in the atmosphere. We saw in Chapter II how light absorbed by ozone generates the important HO radical, and, in Chapter III, we reviewed how light absorption by NO2 leads to ozone formation. In this chapter, we discuss the photochemistry of the light-absorbing oxygenates: their photochemical lifetimes and the nature of the modes of photodecomposition they undergo. Of course, light of sufficient energy per quantum must be absorbed by a molecule if its photodecomposition is to occur. The hydrocarbons do not absorb tropospheric sunlight, as seen in Figure VIII-A-1. The light gray and dark gray lines, respectively, show the distribution of actinic flux present in the troposphere and upper stratosphere for overhead Sun. It can be seen that the larger alkanes, alkenes, and aromatic hydrocarbons absorb at somewhat longer wavelengths than the first member of the family, but none can be electronically excited by tropospheric radiation. Among the hydrocarbons, only the polycyclic aromatics absorb appreciable tropospheric sunlight, and their π → π* excitation does not result in decomposition but likely generates O2(1Δg) molecules by energy transfer; these molecules are usually quenched by collision to ground state O2(3Σg−) molecules (see Calvert et al., 2000). As atmospheric oxidation of the hydrocarbons occurs, initiated largely by HO radicals, a multitude of oxygenated organic species are generated. The absorption region for the oxygenates is generally shifted to longer wavelengths, although the alcohols, ethers, acids, and esters still show no overlap of the regions of tropospheric actinic flux. For the families of compounds shown, the only significant absorbers of tropospheric sunlight are the aldehydes (e.g., CH2O) and the ketones (e.g., CH3C(O)CH3). Formic acid and methyl formate, as well as the larger members of the acid and ester families, absorb sunlight available only at the higher altitudes of the stratosphere, where they are expected to photodecompose. However, these species are not expected to be present in the stratosphere because they are removed in the troposphere largely via HO reactions. In this chapter, we focus on the rates and pathways for photodecomposition of the aldehydes and ketones with less detailed considerations of the other less prevalent light-absorbing trace compounds.

The Impact of Inorganic Trace Gases on Ozone in the Atmosphere

A major focus of the previous six chapters has been on the chemistry and interactions of the HOx, NOx, and volatile organic compound (VOC) families. Details of the reactions of O3 NO3, and HO that act to initiate VOC oxidation have been presented, as has the ensuing chemistry involving organic peroxy and alkoxy radicals and their interactions with NOx. In this chapter, we complete our discussion of thermal chemical reactions that impact tropospheric ozone. The chapter begins with a discussion of the budgets of two simple (inorganic) carbon-containing species not yet discussed, carbon dioxide (CO2) and carbon monoxide (CO). Although CO2 is not directly involved in ozone-related tropospheric chemistry, it is of course the species most critical to discussions of global climate change, and thus a very brief overview of its concentrations, sources, and sinks is presented. CO is a ubiquitous global pollutant, and its reaction with HO is an essential part of the tropospheric background chemistry. This is followed by a presentation of the tropospheric chemistry of halogen species, beginning with a discussion of inorganic halogen cycles that impact (in particular) the ozone chemistry of the marine boundary layer (MBL) and concluding with a detailed presentation of the reactions of Cl atoms and Br atoms with VOC species. The chapter concludes with an overview of tropospheric sulfur chemistry. The reactions leading to the oxidation of inorganic (SO2 and SO3) as well as organic sulfur compounds (e.g., DMS, CH3SCH3) are detailed, and a brief discussion of the effects of the oxidation of sulfur species on aerosol production in the troposphere and stratosphere is also given. The abundance of CO2 in the atmosphere has obviously received a great deal of attention in recent decades due to the influence of this gas on Earth’s climate system. Indeed, changes in the atmospheric CO2 concentration represent the single largest contributor to changes in radiative forcing since preindustrial times (c. 1750). The atmospheric burden of CO2 is controlled by the processes that make up the global carbon cycle—the exchanges of carbon (mostly in the form of CO2) between various “reservoirs,” including the atmosphere, land (vegetation and soil), the surface ocean, the intermediate and deep ocean, sediment on the ocean floor, and the fossil fuel reservoir (IPCC, 2007).

Mechanisms of Reactions of the RO Radicals

The atmospheric chemistry of alkoxy radicals determines the first-generation oxidation products of organic compounds in the atmosphere. There are three competing fates for alkoxy radicals: reaction with molecular oxygen (O2), isomerization, and decomposition (Atkinson and Arey, 2003b; Devolder, 2003; Orlando et al., 2003b; Calvert et al., 2008). Reaction with O2 preserves the carbon chain of the parent alkane and results in the production of a carbonyl compound and HO2. Unimolecular decomposition usually results in the formation of an alkyl radical and a carbonyl compound with a shortening of the carbon chain. Unimolecular isomerization usually leads to multifunctional oxidation products (e.g., 1,4-hydroxycarbonyls and 1,4-hydroxynitrates) and a preservation of the carbon chain. These potentially competing pathways are illustrated in Figure VI-A-1 for the 2-pentoxy radical: Absolute rate coefficients for these processes have been obtained for only a few of the smaller alkoxy radicals. For example, rate coefficients have been firmly established only over a range of temperatures for reaction of a subset of the C1–C6 alkoxy radicals with O2; dissociation rate coefficients have only been directly measured for ethoxy, 2-propoxy, 2-butoxy, and tert-butoxy radicals (Balla et al., 1985; Blitz et al., 1999; Caralp et al., 1999; Devolder et al., 1999; Fittschen et al., 1999, 2000; Falgayrac et al., 2004); and no direct measurement of isomerization rates have been reported to date. A large portion of the database describing the atmospheric behavior of alkoxy radicals has been built up primarily from two sources: (1) environmental chamber experiments, where end-product distributions observed under atmospheric conditions have been used to infer relative rates of competing alkoxy radical reactions (e.g., Carter et al., 1976; Cox et al., 1981; Niki et al., 1981a; Eberhard et al., 1995; Aschmann et al., 1997; Orlando et al., 2000a; Cassanelli et al., 2006); and (2) from theoretical methodologies that lend themselves well to the study of unimolecular processes (e.g., Somnitz and Zellner, 2000a, 2000b, 2000c; Mereau et al., 2000a, 2000b; Fittschen et al., 2000; Lin and Ho, 2002; Mereau et al., 2003; Davis and Francisco, 2011). An overview of these three classes of competing alkoxy radical reactions (reaction with O2, unimolecular decomposition, and isomerization) is given in this section.

Mechanisms of Ozone Reactions in the Troposphere

In Chapter I, we identified the origin of stratospheric ozone and its role in limiting the short wavelengths of sunlight reaching the Earth. We also saw the importance of trace impurities of NOx and hydrocarbons in the development of tropospheric ozone. In this chapter, we review and evaluate the chemical reactions of ozone that create the important hydroxyl (HO) radical. It is the photodecomposition of tropospheric ozone that is the major source of the important HO radical, and it is the HO radical that initiates the destruction of most of the reactive trace gases that are emitted into the atmosphere. Ozone also serves as a major reactant for removal of the alkenes and other reactive unsaturated compounds, and, in this chapter, we review and evaluate the rate coefficients and mechanisms of these reactions and the expected products that result from them. The reactions that generate oxygen atoms in their first excited electronic state, O(1D) atoms, and ultimately HO radicals within the atmosphere are initiated through ozone photodecomposition: . . . O3 (X1A1) + hν → O(1D) + O2(a1Δg) (I) . . . . . . → O(1D) + O2(X3Σ–g) (II) . . . A fraction of the O(1D) atoms formed in the reactions (I) and (II) react with water molecules to generate HO radicals in reaction (1) and a larger fraction are deactivated by collisions with N2 and O2 molecules to form ground state O(3P) atoms in reaction (2): . . . O(1D) + H2O → HO + HO (1) . . . . . . O(1D) + M (N2, O2) → O(3P) + M (N2, O2) (2) . . . The competition between H2O and other air molecules (N2, O2) for reaction with O(1D) atoms results in HO generation being dependent on relative humidity. Rate coefficients for reaction of O(1D) with H2O, N2, and O2 at 298 K (in units of 10−10 cm3 molecule−1 s−1) recommended by the International Union of Pure and Applied Chemistry (IUPAC) panel are 2.14, 0.31, and 0.40, respectively (Atkinson et al., 2004). To better understand the factors that control HO formation, we will review ozone photochemistry, its cross sections, quantum yields of its major photodecomposition modes, and its photolysis frequencies under varied atmospheric conditions.

Ozone in the Atmosphere

The importance of ozone to life on Earth and to atmospheric chemistry cannot be overstated. Nucleic acids and other macromolecules essential to life absorb strongly in the ultraviolet (UV) and are damaged by UV radiation with wavelengths of less than approximately 300 nm. For proper functioning, such biological macromolecules need to be shielded from the full intensity of solar radiation. Molecular oxygen (O2) absorbs strongly and blocks solar radiation with wavelengths below 230–240 nm from reaching the Earth’s surface. However, oxygen is transparent at wavelengths above approximately 245 nm. Fortunately, absorption of UV radiation of wavelengths of less than 242 nm by molecular oxygen (O2) yields oxygen atoms that add to O2 to form ozone which has a very strong absorption band at 200–300 nm. Even though it is present in only trace amounts in the atmosphere, absorption by ozone effectively blocks harsh solar UV radiation from reaching the Earth’s surface. There is no other molecule in the atmosphere that provides protection from solar UV radiation in the 250–300 nm region. The development of the ozone layer is intimately connected to the development of life on Earth. Oxygen levels in the prebiotic atmosphere were less than 5 ×10−9 of the current level. Photosynthesis after the appearance of life on the planet more than 3.5 billion years ago led to increased oxygen levels in the atmosphere. By approximately 600 million years ago, the O2 concentration had exceeded 10% of the current level, and the corresponding layer of ozone was sufficient to offer an effective UV shield for the migration of life onto land (Wayne, 1991). Life on Earth as we know it would not have developed without the protection offered by the ozone layer, and, equally, the ozone layer would not have developed without life on Earth. In addition to its obviously important physical role in shielding biota from the damaging effects of harsh UV radiation, ozone plays an essential chemical role as a photolytic source for HO radicals.

Chemical Mechanisms for Air Quality Modeling and Their Applications

A chemical mechanism is a critical component of an air quality model. Tropospheric gas phase chemical mechanisms for air quality modeling are designed to simulate the production of ozone, acids, and aerosol precursors. Therefore, their focus is on the oxidation chemistry of ozone, nitrogen oxides, sulfur compounds, and organic compounds. Figure IX-A-1 is an overview of the most important cycles of radicals that must be represented in a chemical mechanism for air quality modeling. The processes shown schematically on one level may appear to be relatively simple, but, in reality, the chemical mechanism is extremely complicated due to the very large number of organic compounds present in the atmosphere. Atmospheric chemistry mechanisms are based on laboratory data and tested against environmental experiments and field measurements (Stockwell et al., 2012). Usually, the mechanism is considered to consist of chemical species and their reactions and rate coefficients, along with the photochemical data (used to calculate photolysis frequencies). An atmospheric chemical mechanism employed in an air quality model could be considered to include the rules for aggregating emissions and initial concentrations into species (Middleton et al., 1990). There are many thousands of volatile organic compounds (VOCs) emitted into the atmosphere, and each has its own decomposition mechanism that determines the effect of the VOC on ozone production. It is critical for a chemical mechanism to characterize the chemistry of the VOCs and their differences in chemical reactivity as accurately as possible. Middleton et al. (1990) pointed out that air quality models have only a limited number of species compared to emission inventories. An emissions aggregation scheme is the process of mapping a detailed emissions inventory into the limited number of species used in an air quality model. The scheme is an important component of any model chemical mechanism. Middleton et al. published their aggregation process for the mechanism used in the Regional Acid Deposition Model (RADM2, Stockwell et al., 1990), but, too often, the emissions aggregation scheme for a given chemical mechanism is in the gray literature and difficult to access.

Mechanisms of Reactions of HO2 and RO2 Radicals

The peroxy radicals are an important link in the reaction chain that develops ozone in the atmosphere through their reactions with NO. This chapter explores the kinetics and mechanisms of these RO2 reactions. In Chapters III and IV, the kinetics and mechanisms of the reactions of organic compounds with the major atmospheric oxidants [HO, NO3, and O3] were discussed. The organic radicals formed in these reactions add O2 to form organic peroxy radicals (RO2). The rate coefficient for these reaction is typically of the order of (10−12–10−11) cm3 molecule−1 s−1 under tropospheric conditions. One atmosphere (1 atm) of air contains 5 × 1018 molecule cm−3 of O2, and the lifetime of organic radicals with respect to addition of O2 to give peroxy radicals is 10–100 nanoseconds. Addition of O2 is essentially the sole atmospheric fate of the organic radicals formed during the oxidation of organic compounds. As examples, consider the HO-initiated oxidation of ethane and acetone (M is a third body, such as N2, which collisionally deactivates the nascent peroxy radical): . . . HO + CH3CH3 → CH3CH2 + H2O . . . . . . CH3CH2 + O2 + M → CH3CH2O2 + M . . . . . . HO + CH3C(O)CH3 → CH3C(O)CH2 + H2O . . . . . . CH3C(O)CH2 + O2 + M → CH3C(O)CH2O2 + M . . . Because of the rapidity and exclusivity of the O2 addition to alkyl radicals, the organic peroxy radicals (CH3CH2O2 and CH3C(O)CH2O2) can be thought of as the primary products of the initial oxidation step. HO2 radicals are formed in reactions of O2 with alkoxy radicals (e.g., CH3O) and by the association reaction of H atoms with O2: . . . CH3O + O2 → CH2O + HO2 . . . . . . H + O2 + M → HO2 + M . . . Peroxy radicals (HO2 and RO2) have a rich atmospheric chemistry and undergo reactions with NO, NO2, HO2, and other peroxy radicals (R′O2). Unimolecular isomerization is also an important fate for larger organic peroxy radicals where the peroxy radical can abstract a hydrogen atom from another part of the organic moiety (the peroxy radical bites its own tail). Reactions of peroxy radicals with NO3 radicals at night, and ClO and BrO radicals in maritime environments, can also be of importance on local scales.