ORGANIC AIR POLLUTANTS AND PHOTOCHEMICAL SMOG

Organic pollutants effect

direct effects, such as cancer caused by exposure to vinyl chloridesecondary 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: