ORGANIC AIR POLLUTANTS
AND PHOTOCHEMICAL SMOG
Organic pollutants effect
direct effects, such as cancer caused by
exposure to vinyl chloride
secondary pollutants, especially
photochemical smog,
Global Distillation and Fractionation of Persistent Organic
Pollutants
persistent organic pollutants (POPs) undergo a cycle of
distillation and fractionation in which they are vaporized into the
atmosphere in warmer regions of the Earth and condense and
are deposited in colder regions. The theory of this phenomenon
holds that the distribution of such pollutants is governed by their
physicochemical properties and the temperature conditions to
which they are exposed. As a result, the least volatile persistent
organic pollutants are deposited near their sources, those of
relatively high volatility are distilled into polar regions, and those
of intermediate volatility are deposited predominantly at mid
latitudes. This phenomeonon has some important implications
regarding the accumulation of persistent organic pollutants in
environmentally fragile polar regions and cold mountainous
areas far from industrial sources.
Reactions and Fates of Organic Compounds
There are two important points related to what happens
to organic compounds in the atmosphere.
1- photochemical reactions initiated by the absorption
of photons of electromagnetic radiation, wavelengths in
the ultraviolet region and visible light. The energy of
these photons is equal hν.
2- the central role played by the highly reactive
hydroxyl radical represented HO. This is a free radical
species meaning that it has an unpaired electron, which
is what the dot in the formula represents. Hydroxyl
radical is involved in virtually all the pathways by which
organic compounds react in the atmosphere and by
which photochemical smog is formed.
ORGANIC COMPOUNDS FROM NATURAL SOURCES
Natural sources are the most important contributors of organics
in the atmosphere, and hydrocarbons generated and released by
human activities consitute only about 1/7 of the total
hydrocarbons in the atmosphere. This ratio is primarily the result
of the huge quantities of methane produced by anaerobic
bacteria in the decomposition of organic matter in water,
sediments, and soil:
2{CH2O} (bacterial action) → CO2(g) + CH4(g)
Methane is a natural constituent of the atmosphere and is
present at a level of about 1.4 parts per million (ppm) in the
troposphere.
Methane in the troposphere contributes to the photochemical
production of carbon monoxide and ozone. The photochemical
oxidation of methane is a major source of water vapor in the
stratosphere.
Atmospheric hydrocarbons produced by living sources are
called biogenic hydrocarbons. Vegetation is the most
important natural source of non-methane biogenic compounds.
Several hundred different hydrocarbons are released to the
atmosphere from vegetation sources. Other natural sources
include microorganisms, forest fires, animal wastes, and
volcanoes.
One of the simplest organic compounds given off by plants is
ethylene, C2H4. This compound is produced by a variety of
plants and released to the atmosphere in its role as a
messenger species regulating plant growth. Because of its
double bond, ethylene is highly reactive with hydroxyl radical,
HO•, and with oxidizing species in the atmosphere. Ethylene
from vegetation sources should be considered as an active
participant in atmospheric chemical processes.
Most of the hydrocarbons emitted by plants are terpenes, which constitute a large
class of organic compounds found in essential oils. Essential oils are obtained
when parts of some types of plants are subjected to steam distillation. Most of the
plants that produce terpenes are conifers (evergreen trees and shrubs such as
pine
One of the most common terpenes emitted by trees is
POLLUTANT HYDROCARBONS
Ethylene and terpenes, are hydrocarbons, organic compounds containing only
hydrogen and carbon. The major classes of hydrocarbons are alkanes (formerly called
paraffins), such as 2,2,3-trimethylbutane;
alkenes (olefins, compounds with double bonds between adjacent carbon atoms),
such as ethylene;
alkynes (compounds with triple bonds), such as acetylene;
and aromatic (aryl) compounds, such as naphthalene:
Because of their widespread use in fuels, hydrocarbons
predominate among organic atmospheric pollutants. Petroleum
products, primarily gasoline, are the source of most of the
anthropogenic (originating through human activities) pollutant
hydrocarbons found in the atmosphere. Hydrocarbons may enter
the atmosphere either directly or as byproducts of the partial
combustion of other hydrocarbons. The latter are particularly
important because they tend to be unsaturated and relatively
reactive. Most hydrocarbon pollutant sources produce about 15%
reactive hydrocarbons, whereas those from incomplete
combustion of gasoline are about 45% reactive. The
hydrocarbons in uncontrolled automobile exhausts are only about
1/3 alkanes, with the remainder divided approximately equally
between more-reactive alkenes and aromatic hydrocarbons, thus
accounting for the relatively high reactivity of automotive exhaust
hydrocarbons.
Alkanes are among the more stable hydrocarbons in the
atmosphere. Straight-chain alkanes with 1 to more than 30
carbon atoms, and branched-chain alkanes with 6 or fewer
carbon atoms, are commonly present in polluted atmospheres.
Because of their high vapor pressures, alkanes with 6 or fewer
carbon atoms are normally present as gases, alkanes with 20
or more carbon atoms are present as aerosols or sorbed to
atmospheric particles, and alkanes with 6 to 20 carbon atoms
per molecule may be present either as vapor or particles,
depending upon conditions.
Alkenes enter the atmosphere from a variety of processes,
including emissions from internal combustion engines and
turbines, foundry operations, and petroleum refining. Several
alkenes, including the ones shown below, are among the top 50
chemicals produced each year, with annual worldwide
production of several billion kg:
Aromatic hydrocarbons can be divided into the two major
classes of those that have only one benzene ring and those
with multiple rings known as polycyclic aromatic
hydrocarbons, PAH. Aromatic hydrocarbons with two rings,
such as naphthalene, are intermediate in their behavior. The
first six compounds shown in thisfigure are among the top 50
chemicals manufactured each year, so they are commonly
encountered from pollution sources.
Polycyclic aromatic hydrocarbons are present as aerosols in
the atmosphere because of their extremely low vapor
pressures. These compounds are the most stable form of
hydrocarbons having low hydrogen-to-carbon ratios and are
formed by the combustion of hydrocarbons under oxygen-
deficient conditions. The partial combustion of coal, which
has a hydrogen-to-carbon ratio less than 1, is a major source
of PAH compounds.
NONHYDROCARBON ORGANIC COMPOUNDS IN THE
ATMOSPHERE
Carbonyl Compounds
Carbonyl compounds, consisting of aldehydes and ketones
that have a carbonyl moiety, C=O, are often the first species
formed, other than unstable reaction intermediates, in the
photochemical oxidation of atmospheric hydrocarbons. The
aldehydes have the carbonyl group on an end carbon and the
ketones have it on a
carbon atom that is not at the end of a hydrocarbon chain:
Formaldehyde is produced in the atmosphere as a product of the reaction of
atmospheric hydrocarbons beginning with their reactions with hydroxyl radical, HO.
The structures of some important aldehydes and ketones are shown below:
Miscellaneous Oxygen-Containing Compounds
In addition to aldehydes, ketones, and esters, other oxygen-containing com-pounds
in the atmosphere include aliphatic alcohols, phenols, ethers, and carbox-ylic
acids. These compounds have the general formulas given below, where R and R'
represent hydrocarbon moieties (groups), and Ar stands specifically for an aromatic
moiety, such as the phenyl group (benzene less an H atom):
Organohalides
Organohalides consisting of halogen-substituted hydrocarbon
molecules, each of which contains at least one atom of F, Cl,
Br, or I, may be saturated (alkyl halides), unsaturated (alkenyl
halides), or aromatic (aromatic halides).
Chlorofluorocarbons and Stratospheric Ozone Depletion
Chlorofluorocarbons (CFCs), such as
dichlorodifluoromethane, commonly called Freons, are volatile
1- and 2-carbon compounds that contain Cl and F bonded to
carbon. These compounds are notably stable and nontoxic.
They were widely used in recent decades in the fabrication of
flexible and rigid foams and as fluids for refrigeration and air
conditioning. A related class of compounds, the halons, such
as CBrClF2 (Halon-1211), are used in fire extinguisher
systems particularly on aircraft
Cl2CF2 + hv→ Cl. + ClCF2.
. .
Cl + O3 → ClO + O2
Nitric oxide, NO, is also present. The ClO species may react
with either O or NO, regenerating Cl atoms and resulting in
chain reactions that cause the net destruction
of ozone:
The substitutes for CFCs hydrogen-containing
chlorofluorocarbons (HCFCs) and hydrogen-containing
fluorocarbons (HFCs). These include CH2FCF3 (HFC-134a,
1,1,1,2-tetrafluoroethane, a substitute for CFC-12 in automobile
air conditioners and refrigeration equipment),
Organosulfur Compounds
Substitution of alkyl or aryl hydrocarbon groups such as phenyl
and methyl for H on hydrogen sulfide, H2S, leads to a number
of different organosulfur thiols (mercaptans, R–SH) and
sulfides,
also called thioethers (R–S–R).
Although not highly significant as atmospheric contaminants on a
large scale, organic sulfur compounds can cause local air pollution
problems because of their bad odors. Major sources of organosulfur
compounds in the atmosphere include microbial degradation, wood
pulping, volatile matter evolved from plants, animal wastes, packing-
house and rendering-plant wastes, starch manufacture, sewage
treat-ment, and petroleum refining
Organonitrogen Compounds
Organic nitrogen compounds that may be found as atmospheric
contaminants can be classified as amines, amides, nitriles,
nitro compounds, or heterocyclic nitrogen compounds.
Structures of common examples of each of these five classes
of compounds reported as atmospheric contaminants are
These organonitrogen compounds can come from anthropogenic
pollution sources. Significant amounts of anthropogenic
atmospheric nitrogen may also come from reactions of inorganic
nitrogen with reactive organic species.
16.5 PHOTOCHEMICAL SMOG
The most important effects of organic pollutants on the
atmosphere result from the formation of secondary
pollutants characteristic of photochemical smog. Smog
has a long history. In 1542, exploring what is now southern
California, Juan Rodriguez Cabrillo named San Pedro Bay
“The Bay of Smokes” because of the heavy haze that
covered the area. Complaints of eye irritation from
anthropogen-ically polluted air in Los Angeles were
recorded as far back as 1868. Characterized by reduced
visibility, eye irritation, cracking of rubber, and deterioration
of materials, smog became a serious nuisance in the Los
Angeles area during the 1940s. It is now recognized as a
major air pollution problem in many areas of the world.
The three ingredients required to generate photochemical
smog are:
ultraviolet light,
hydrocarbons,
and nitrogen oxides.
Indicarors for photochemical smog formations
The formation of oxidants in the air, particularly ozone
Smoggy conditions are manifested by moderate to severe eye
irritation
Decrease in visibility below 3 miles
Serious levels of photochemical smog can be assumed to be
present when the oxidant level exceeds 0.15 ppm for more
than 1 hour.
SMOG-FORMING REACTIONS OF ORGANIC COMPOUNDS
IN THE ATMOSPHERE
Hydrocarbons are eliminated from the atmosphere by a number
of chemical and photochemical reactions. These reactions are
responsible for the formation of many noxious secondary
pollutant products and intermediates from relatively innocuous
hydrocarbon precursors. These pollutant products and
intermediates make up photo-chemical smog.
Hydrocarbons and most other organic compounds in the
atmosphere are thermodynamically unstable toward oxidation
and tend to be oxidized through a series of steps. The oxidation
process terminates with formation of CO2, solid organic
particulate matter that settles from the atmosphere, or water-
soluble products (for example, acids, aldehydes), which are
removed by rain. Inorganic species such as ozone or nitric acid
are byproducts of these reactions.
MECHANISMS OF SMOG FORMATION
The primary oxidants in atmosphere is ozone, other ones are
H2O2, organic peroxides (R-OO-R*), organic hydroperoxides
(R-OO-H), peroxyacetyl nitrates(PAN)
Peroxyacetyl nitrate and related compounds containing the
-C(O)OONO2 moi-ety, such as peroxybenzoyl nitrate (PBN),
OCOO NO2 a powerful eye irritant and lachrymator, are produced
photochemically in atmos-pheres containing alkenes and NOx,
and the presence of compounds of this type are important
indicators of the presence of photochemical smog. PAN,
especially, is a notorious organic oxidant. In addition to PAN and
PBN, some other specific organic oxidants that may be important
in polluted atmospheres are peroxypropionyl nitrate (PPN);
peracetic acid, CH3(CO)OOH; acetylperoxide,
CH3(CO)OO(CO)CH3
The latter kind of reaction is the most common chain-terminating process
in smog because NO2 is a stable free radical present at high
concentrations. Chains may terminate also by reaction of free radicals
with NO or by reaction of two R• radicals, although the latter is
uncommon because of the relatively low concentrations of radicals
compared to molecular species. Chain termination by radical sorption on
a particle surface is also possible and may contribute to aerosol particle
growth.
As noted at the beginning of this chapter, the key reactive intermediate
species in smog formation is the very reactive HO• radical consisting of a
hydrogen and oxygen atom bonded together and with an unpaired
electron on the oxygen. During the smog-forming process, hydroxyl radical
is readily produced by the reaction of an oxygen atom with a hydrocarbon,
as shown in Reaction 16.8.5. Hydroxyl radical can also be formed by the
reaction of excited (“energized”) atomic oxygen with water,
Among the inorganic species with which the hydroxyl radical reacts are
oxides of nitrogen,
The last reaction is significant in that it is responsible for the disappearance of
much atmospheric CO and because it produces the hydroperoxyl radical
HOO•. One of the major inorganic reactions of the hydroperoxyl radical is the
oxidation of NO:
HOO• + NO → HO• + NO2 (16.8.15)
16.10 EFFECTS OF SMOG
The harmful effects of smog occur mainly in the areas of
(1) human health and comfort,
(2) damage to materials,
(3) effects on the atmosphere,
(4) toxicity to plants.
The exact degree to which exposure to smog affects human
health is not known, although substantial adverse effects are
suspected. Pungent-smelling, smog-produced ozone is known
to be toxic. Ozone at 0.15 ppm causes coughing, wheezing,
bronchial constriction, and irritation to the respiratory mucous
system in healthy, exercising individuals. In addition to ozone,
oxidant peroxyacyl nitrates and aldehydes found in smog are
eye irritants.
Materials are adversely affected by some smog components.
Rubber has a high affinity for ozone and is cracked and aged
by it. Indeed, the cracking of rubber used to be employed as a
test for the presence of ozone. Ozone attacks natural rubber
and similar materials by oxidizing and breaking double bonds
in the polymer according to the following reaction: