Chuong hoa hoc khi quyen (4) ppt

12.1 Organic Compounds in the AtmosphereDirect effects • Example: Cancer from vinyl chloride Secondary pollutants • Especially photochemical smog Loss of Organic Substances from the Atmo

Chapter 12

ORGANIC AIR POLLUTANTS

Environmental Chemistry, 9th Edition

Stanley E Manahan

Taylor and Francis/CRC Press

2010

12.1 Organic Compounds in the Atmosphere

Direct effects

• Example: Cancer from vinyl chloride

Secondary pollutants

• Especially photochemical smog

Loss of Organic Substances from the Atmosphere

• Precipitation (rainwater) • Dry deposition

• Photochemical reactions • Incorporation into particles

• Tend to undergo photochemical reactions leading to solids that are purged from the atmosphere

• Uptake by plants, especially trees

• Absorbed by lipophilic layer on leaves and needles of trees

12.2 Biogenic Organic Compounds in the Atmosphere

Natural sources most abundant sources of atmospheric organics

• Methane from bacteria and geosphere is the most abundant organic in the

atmosphere

• Anoxic bacteria: 2{CH2O}→ CH4 + CO2

• Flatulent emissions from livestock

Terpenes from vegetation, primarily pine and citrus trees, are second to methane as organics in the atmosphere

• Generally very reactive (in photochemical smog formation)

• Form much of the small particulate matter in atmosphere

• See structural formulas, next slide

Figure 12.1 Common terpenes emitted by trees such as pine and citrus

Oxidation Products of Terpenes

Reaction of Limonene with Ozone

Pinonic Acid from Oxidation ofα-Pinene

Esters in the Atmosphere

• Many kinds of esters, largely from plant sources

Removal of Atmospheric Organic Compounds by Plants

Repositories of persistent organic pollutants

Leaves and needles covered by epicularorganophilic wax that accumulates

atmospheric organics

• Especially in evergreen boreal coniferous forests in the northern temperate zone

• Heavy forestation and large leaf surface per unit forest area

12.3 Pollutant Hydrocarbons

Alkanes are relatively very stable

Alkanes undergo abstraction reactions

• React with O from NO2photodissociation

• React with HO•

• Additional reactions that produce solid aerosols and soluble substances that are removed from the atmosphere

Alkanes, such as 2,2,3-Trimethylbutane

Among top 50 chemicals produced annually

Alkynes are unsaturated hydrocarbons sometimes encountered at very low levels in the atmosphere

Alkenes undergo addition reactions

Example: Propene adds HO• , then reacts with O2 and undergoes further reactions

Further reactions with radicals, NO, and other species to produce aldehydes, additional reactive products

Ozone adds across alkene double bonds

O2

NO3 radical reacts with alkenes

Especially at night when NO3 is relatively long-lasting

Alkenes reacting with hydroxyl radical in presence of N oxides can produce hydroxynitrates

Aromatic (Aryl) Hydrocarbons

• Single ring • Double ring • Polycyclic

Aromatic Hydrocarbons Among Top 50 Chemicals

Reactions of Atmospheric Aromatic Hydrocarbons

• Addition reactions, especially with HO•

Benzene + HO•

Electron delocalized on aromatic ring

Aliphatic side chains on substituted aromatics may undergo abstraction reactions with hydroxyl radical

12.4 Carbonyl Compounds: Aldehydes and Ketones

Carbonyl compounds generated from organic alkoxyl radicals by reactions such as the following:

Atmospheric Aldehydes and Ketones

The carbonyl group is a chromophore absorbing ultraviolet photons, hν

• Produces formyl radical, HCO, as shown for acetaldehyde:

Most common atmospheric carbonyl is formaldehyde typically generated from methoxyl radical

H3CO • + O2→ HOO•+

Because of the C=C bond, alkenylaldehydes are especially reactive in the

atmosphere

12.5 Miscellaneous Oxygen-Containing Compounds

Alkenyl alcohols add HO• or O3 across double bond

Alcohols: Methanol, ethanol, isopropanol, ethylene glycol among top 50 chemicals worldwide

• Potential air pollutants, especially volatile methanol

Alcohols are removed from atmosphere quickly

• Lower alcohols water-soluble

• Higher alcohols not very volatile

Alcohols react by HO• abstraction of H atoms

Alkenyl alcohols

• Cis-3-hexen-1-ol, “leaf alcohol”: CH3CH2CH=CHCH2CH2OH

Ethers

• Not very reactive

• Attacked by hydroxyl radical

Phenol among top 50 chemicals produced

• Can cause localized air pollution problems from industrial use and coal coking

Ethylene oxide

• Colorless, flammable, explosive

• Uses as chemical intermediate, sterilant, fumigant

• Hazardous due to flammability, toxicity

Among top 50 chemicals →

Carboxylic Acids, Figure 12.2

Higher carboxylic acid common in particulate matter

• From photochemical oxidation of hydrocarbons

12.6 Organonitrogen Compounds

More oxidized organonitrogens produced by reaction of photochemical oxidants, NOx, NO3

Hydrocarbon-substituted NH3

Lower amines are volatile and noxious

Some aromatic amines are carcinogens

Volatile acetonitrile (H3CCN) containing the -C≡N group reported as air pollutants from synthetic rubber manufacture

Most common amide in the atmosphere is dimethylformamide

Nitro Compounds

Nitromethane H3CNO2, nitroethane, nitrobenzene have been reportrd as air

pollutants

Highly oxygenated compounds containing the nitro group

• Secondary pollutants from photochemical smog formation

• Intense irritants to eyes and respiratory tract

• Mutagens • Phytotoxins (harm plants)

Heterocyclic nitrogen compounds largely in particles

Special case of carcinogenic nitrosamines (N-nitroso compounds)

12.7 Organohalide Compounds

Most commonly organochlorine compounds

More volatile ones reported as air pollutants:

Aromatic halide compounds

Vinyl chloride (right) is widely used in polyvinylchloride

manufacture and is a known human carcinogen

Chlorofluorocarbons such as dichlorodifluoromethane cause stratospheric ozone depletion (see more detail in Chapter 14)

• Extremely stable

• In stratosphere

• CF2Cl2 + hν→Cl• + CF2Cl2 •

•Cl•+ O3+ hν→ClO• + O2 •

• Additional reactions generate more Cl and destroy more fluorocarbon

• Substitutes for chlorofluorocarbons have been developed

Halons such CBrClF2C3l3 that contain Br strongly deplete stratospheric ozone

• No good substitutes for fire extinguisher fluids

Perfluorocarbons

• CF4 and C2F6 released in aluminum manufacture

• Greenhouse gases that are virtually indestructible in the atmosphere

Marine sources of organohalogens: Many generated by marine microorganisms

12.8 Organosulfur Compounds

Predominantly thiols (R-SH) and thioethers (R-S-R)

Lighter thiols are notable for odors

• Skunk odor due to thiols

Dimethylsulfide generated in large quantities by marine micoorganisms is the largest source of

atmospheric SO2 in marine areas

12.9 Organic Particulate Matter

End product of photochemical smog formation

Organic matter often associated with

• Condensed PAH solid particles

• Elemental C particles

•And N often added to organic particulate matter by the action of reactive species

12.10 Hazardous Air Pollutants Organic Compounds

See organic compounds in EPA list of hazardous organic air pollutants in Table 12.1