Encyclopedia of Environmental Science and Engineering, Volume I and II

ATMOSPHERIC CHEMISTRY 127

the NOx -limited region, there is inadequate NO x present to be

oxidized by all of the peroxy radicals that are being produced

in the oxidation of the VOCs. Adding more NO x in this region

increases ozone production. The base-case point in Figure 5 is

located in the VOC-limited region of the diagram. Increasing

NO x from the base-case point actually leads to a decrease in

the maximum ozone that can be produced.

Nighttime Chemistry

At night, the urban atmospheric chemistry is quite different

than during the day. The ozone present at night may react

with organics, but no new ozone is formed. These ozone reac-

tions with organics are generally slow. Ozone can react with

alkanes, producing hydroxyl radicals. This hydroxyl-radical

production is more important for somewhat larger alkenes.

The significance of this hydroxyl-radical production is limited

by the available ozone. Besides reacting with organics, ozone

can react with NO 2 :

O 3  NO 2 → O 2  NO 3 (28)

forming the nitrate radical (NO 3 ). NO 3 radicals can further

react with NO 2 to form dinitrogen pentoxide (N 2 O 5 ), which

can dissociate to reform NO 3 and NO 2 :

NO 3  NO 2  M → N 2 O 5  M (29)

N 2 O 5 → NO 3  NO 2 (30)

establishing an equilibrium between NO 3 and N 2 O 5. Under

typical urban conditions, the nighttime N 2 O 5 will be 1 to

100 times the NO 3 concentration. These reactions are only

of importance at night, since NO 3 can be photolyzed quite

efficiently during the day.

NO 3 can also react quickly with some organics. A generic

reaction, which represents reactions with alkanes and alde-

hydes, would be

NO 3  RH → HNO 3  R (31)

The reactions of NO 3 with alkenes and aromatics proceed by a

different route, such as adding to the double bond. NO 3 reacts

quite rapidly with natural hydrocarbons, such as isoprene and

α -pinene (Table 4), and cresols (Finlayson-Pitts and Pitts,

2000). Not much is known about the chemistry of N 2 O 5 , other

than it is likely to hydrolyze, forming nitric acid:

N 2 O 5  H 2 O → 2HNO 3 (32)

Summary

The discussion of urban atmospheric chemistry presented

above is greatly simplified. Many more hydrocarbon types

are present in the urban atmosphere, but the examples pre-

sented should provide an idea of the types of reactions that

may be of importance. In summary, urban atmospheric

ozone is formed as a result of the photolysis of NO 2. NO 2 is

formed by the oxidation of the primary pollutant NO, which

accompanies the hydroxyl-radical-initiated chain oxidation

of organics. Hydroxyl radicals can be produced by the pho-

tolysis of various compounds. Ozone formation is clearly a

daytime phenomenon, as is the hydroxyl-radical attack of

organics.

SECONDARY ORGANIC AEROSOLS

With the implementation of air-quality standards for fine (or

respirable) particulate matter in the atmosphere, there has

been increasing interest in the composition and sources of

this fine particulate matter. It has long been recognized that

particles in the atmosphere have both primary (direct emis-

sion) and secondary (formed in the atmosphere) sources.

Among the secondary particulate matter in the atmosphere

are salts of the inorganic acids (mostly nitric and sulfuric

acids) formed in the atmosphere. It has been found that

there is a significant contribution of carbonaceous particu-

late matter, consisting of elemental and organic carbon.

Elemental carbon (EC), also known as black carbon or gra-

phitic carbon, is emitted directly into the atmosphere during

combustion processes. Organic carbon (OC) is both emitted

directly to the atmosphere (primary OC), or formed in the

atmosphere by the condensation of low-volatility products

of the photooxidation of hydrocarbons (secondary OC).

The organic component of ambient particles is a complex

mixture of hundreds of organic compounds, including:

n-alkanes, n-alkanoic acids, n-alkanals, aliphatic dicarbox-

ylic acids, diterpenoid acids and retene, aromatic polycar-

boxylic acids, polycyclic aromatic hydrocarbons, polycyclic

aromatic ketones and quinines, steroids, N-containing com-

pounds, regular steranes, pentacyclic triterpanes, and iso-

and anteiso-alkanes (Seinfeld and Pandis, 1998).

Secondary organic aerosols (SOAs) are formed by the

condensation of low-vapor-pressure oxidation products of

organic gases. The first step in organic-aerosol production

is the formation of the low-vapor-pressure compound in

the gas phase as a result of atmospheric oxidation. The

second step involves the organic compound partitioning

between the gas and particulate phases. The first step is

controlled by the gas-phase chemical kinetics for the oxi-

dation of the original organic compound. The partitioning

is a physicochemical process that may involve interactions

among the various compounds present in both phases. This

partitioning process is discussed extensively by Seinfeld

and Pandis (1998).

Figure 6 (Seinfeld, 2002) illustrates a generalized mecha-

nism for the photooxidation of an n-alkane. The compounds

shown in boxes are relatively stable oxidation products that

might have the potential to partition into the particulate

phase. Previous studies of SOA formation have found that the

aerosol products are often di- or poly-functionally substituted

products, including carbonyl groups, carboxylic acid groups,

hydroxyl groups, and nitrate groups.

A large number of laboratory studies have been done

investigating the formation of SOAs. Kleindienst et al. (2002)

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