Oo

02 I

02 I

FIGURE 1.4 Typical sequence of elementary reactions in which OH initiates the oxidation of an alkane in the troposphere.

FIGURE 1.4 Typical sequence of elementary reactions in which OH initiates the oxidation of an alkane in the troposphere.

The chain reactions are eventually terminated by such reactions as

A major source of OH in both clean and polluted air is the photodissociation of 03 by actinic UV radiation in sunlight to produce an electronically excited oxygen atom, O('D),

followed by a very rapid reaction, in competition with deactivation, of the excited oxygen atom with water vapor, which is always present in the atmosphere:

In polluted airsheds, other direct sources also form OH through photodissociation, including nitrous acid:

HONO + hv(A < 370 nm) -> OH + NO, (15) and hydrogen peroxide (H202):

A very important "thermal" source of hydroxyl radicals, as well as NOz, is the reaction of H02 with NO:

This is a major chain propagation step in the overall reaction mechanism for ozone formation in photochemical air pollution. Because HOz is intimately tied to OH through reaction (17) and cycles such as that in Fig. 1.4, when NO is present the sources and sinks of H02 are, in effect, sources or sinks of the OH radical.

Sources of H02 include the reactions of 02 with hydrogen atoms and formyl radicals, both of which are produced, for example, by the photodissociation of gaseous formaldehyde following absorption of solar actinic UV radiation.

Another source of the hydroperoxyl radical is the abstraction of a hydrogen atom from alkoxy radicals by molecular oxygen:

The relative importance of these sources of OH and HOz radicals depends on the species present in the air mass, and hence on location and time of day. Figure 1.5, for example, shows the relative contributions as a function of time of day of three sources of 0H/H02 in an urban air mass. In this case, nitrous acid is predicted to be the major OH source in the early morning hours, HCHO in mid-morning, and 03 later in the day when its concentration has built up significantly (Winer, 1985; Winer and Biermann, 1994).

In summary, NO is now known to be converted to N02 during daylight hours in a reaction sequence initiated by OH attack on organics, and involving HOz and R02 free radicals. These peroxy radicals are the species that actually convert NO to N02 at ambient concentrations where the thermal oxidation of NO by 02 is negligible.

c. Nighttime Chemistry of N02

The late 1970's saw the birth of a new aspect of atmospheric chemistry. Thus, in addition to ozone and photochemical oxidant formed in the daytime photooxi-dation of VOCs, there is an important nighttime chemistry, not only in polluted urban and suburban air environments, but also in relatively remote atmospheres.

In the late 1970's, a group of German researchers developed a novel instrument, the long-path length (e.g., 1-17 km) ultraviolet-visible differential optical absorption spectrometer, DOAS (Piatt et al., 1979). Application of this instrument in field studies in remote

Time (pst)

FIGURE 1.5 Predicted rates of generation of 0H/H02 in a polluted urban atmosphere as a function of time of day for three free radical sources (adapted from Winer, 1985).

Time (pst)

FIGURE 1.5 Predicted rates of generation of 0H/H02 in a polluted urban atmosphere as a function of time of day for three free radical sources (adapted from Winer, 1985).

and polluted ambient air in Europe and the United States provided unequivocal evidence for the presence at night of two important "trace" nitrogenous species: gaseous nitrous acid (Perner and Piatt, 1979) and the gaseous nitrate radical, N03 (Piatt et al., 1980). Both photodissociate rapidly and efficiently in daylight— hence the term "nighttime chemistry." The ambient levels, rates of formation, reactivities, and fates as well as the experimental details of current DOAS systems are discussed in detail in subsequent chapters.

(1) The N03 radical Briefly, the N03 radical is formed in the reaction no2 + o3

and found at night at levels ranging from less than a few ppt (parts per trillion, 109) in remote regions to several hundred ppt in polluted atmospheres. It plays at least two major roles in the troposphere. Thus, it is nighttime sink for certain VOCs through addition as well as H-atom abstraction reactions

Furthermore, it reacts with N02 to form gaseous dini-trogen pentoxide, N205, in the equilibrium

To date, there are no direct tropospheric measurements of N205 at the levels predicted to be in natural or polluted air masses. However, concentrations of N2Os as high as 10-15 ppb have been calculated for the Los Angeles area using simultaneous measurements of ambient N03 and NOz and the equilibrium constant for reaction (24) (e.g., see Atkinson et al., 1986).

In the troposphere N205 is an important nighttime source of nitric acid through its rapid hydrolysis on wet surfaces and aerosol particles:

surfaces, aerosol particles

Furthermore, N205 plays an important role in key stratospheric heterogeneous processes (see Chapter 12).

(2) Gas-phase nitrous acid Gaseous HONO plays an important role in the chemistry of irradiated mixtures of VOC and NOx in air, whether in smog chambers or in ambient atmospheres. Thus, it strongly absorbs actinic UV radiation and, at sunrise (Fig. 1.5), decomposes into OH radicals and NO with a high quantum efficiency:

Use of the long-path length DOAS technique has confirmed its presence in polluted ambient as well as relatively clean continental air masses at levels ranging from ~ 15 ppb at night in a highly polluted air mass down to sub-ppb levels in remote regions (see Chapter 11). It is found at much higher levels in indoor air environments having combustion sources such as gas or propane stoves (see Chapter 15).

Direct sources of ambient HONO, established unequivocally through use of the long-path length DOAS technique, include primary emissions, e.g., from light-duty motor vehicles having high levels of NOx in exhaust gases (Pitts et al., 1984). As discussed in Chapter 15, emissions from indoor combustion sources, e.g., gas-fired kitchen stoves and gas or propane heaters, can also produce high levels of HONO in poorly ventilated indoor air environments.

Interestingly, heterogeneous processes appear to be involved in HONO formation, certainly in smog chambers and indoor air environments and most likely on a variety of surfaces outdoors. It is produced from gaseous N02 and adsorbed water in a heterogeneous reaction on surfaces (see Chapter 7):

4. Acid Deposition a. Historical

The recognition of acid deposition, commonly called "acid rain," also has a long history. In England and Sweden, the presence of sulfur compounds and acids in polluted air and rain was recognized as early as the eighteenth century. Indeed, in 1692, Robert Boyle referred to "nitrous or salino-sulphurous spirits" in the air in his book A General History of the Air [see the excellent historical perspectives given by Brimblecombe (1978) and Cowling (1982)].

Remarkably, in 1872, a century before it became an international issue, a treatise on acid rain was published in England by Robert Angus Smith. Twenty years earlier, he had analyzed rain near Manchester and noted three types of areas as one moved from the city to the surrounding countryside:

"that with carbonate of ammonia in the fields at a distance, that with sulfate of ammonia in the suburbs and that with sulphuric acid or acid sulphate, in the town."

In his 1872 book Air and Rain: The Beginnings of a Chemical Climatology, Smith coined the term acid rain and described many of the factors affecting it, such as coal combustion and the amount and frequency of precipitation. He also suggested experimental protocols to be followed in sample collection and analysis and described acid rain damage to plants and materials.

b. Overview of Acidic Rain and Fogs

Acid rain arises from the oxidation of S02 and N02 in the troposphere to form sulfuric and nitric acids, as well as other species, which are subsequently deposited at the earth's surface, either in precipitation (wet deposition) or in dry form (dry deposition). The contribution of organic acids has also been recognized recently (see Chapter 8). These oxidation and deposition processes can occur over relatively short distances from the primary pollutant sources or at distances of a 1000 km or more. Thus both short-range and long-range transport must be considered.

The gas-phase oxidation of both S02 and N02 is initiated by reaction with hydroxyl radicals:

OH + S02 ™ HOSOz -» H2S04, (28) OH + NOz HN03. (10)

In the case of S02, oxidation in the aqueous phase, present in the atmosphere in the form of aerosol particles, clouds, and fogs, is also important. Thus S02 from the gas phase dissolves in these water droplets and may be oxidized within the droplet by such species as H202, 03, 02, and free radicals. Oxidation of S02 on the surfaces of solids either present in the air or suspended in the water droplets is also possible. On the other hand, it is believed that HN03 is formed primarily by reaction (10) in the gas phase and subsequently dissolves in droplets.

These oxidation processes can lead to highly acidic fogs. For example, pH values as low as 1.69 have been measured in coastal regions of southern California (Jacob and Hoffmann, 1983). These high acidities, accompanied by high concentrations of other anions and cations, are likely due to evaporation of water from the fog droplets, leaving very high concentrations of ions in a strongly acidic liquid phase. Such acid fogs, whether in London or Los Angeles, are a major health concern because the droplets are sufficiently small to be efficiently inhaled (Hoffmann, 1984).

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