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Chap 2.3. Gaseous Pollutants. Carbon oxides Sulfur compounds Nitrogen compounds Hydrocarbon compounds Photochemical oxidants. Carbon Oxides. Two major carbon oxides Carbon dioxide (CO 2 ) Carbon monoxide (CO). CO 2. Natural atmospheric constituent Sources: Natural

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Gaseous pollutants l.jpg

Chap 2.3

Gaseous Pollutants

  • Carbon oxides

  • Sulfur compounds

  • Nitrogen compounds

  • Hydrocarbon compounds

  • Photochemical oxidants

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Carbon Oxides

  • Two major carbon oxides

    • Carbon dioxide (CO2)

    • Carbon monoxide (CO)


  • Natural atmospheric constituent

  • Sources:

    • Natural

      • Aerobic biological processes, combustion and weathering of carbonates in rock and soil

    • Anthropogenic:

      • Combustion of fossil fuels

      • Land use conversion

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What’s the impact if there is no CO2 in the atmosphere?

Is CO2 emission regulated? Should it be?

Figure 2.2


  • Essential atmospheric gas

  • Present in variable concentrations

  • Not considered to be toxic

  • Environmental concerns are relatively new

  • Changes in atmospheric concentrations

    • Geological time

    • The modern period

      1.5-1.7 ppmv/yr

  • Long atmospheric lifetime

    (~100 years)

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Figure 2.3


  • Major sink processes

    • Oceans

    • Forests

  • Pre-industrial revolution: 98% of exchangeable CO2 were in the oceans and 2% in the atmosphere; for anthropogenic CO2, only 42% dissolves in oceans

More discussion in Atmospheric Effects

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  • Colorless, odorless, tasteless gas

  • Produced as a result of incomplete combustion

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Adverse effects on the consumption of OH·?

  • Formation of O3



  • Sink processes

    • Photochemistry with OH· (hydroxyl radical)

    • Soil uptake

    • Atmospheric lifetime (1 month in the tropics and 4 months in mid-latitudes)

  • Increase CH4 concentration thus enhancing global warming

M: an energy absorbing molecule, e.g. N2 or O2

OH·: hydroperoxyl radical

O(3P): ground-state atomic oxygen

h: a photon of light energy

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Why higher in higher latitudes and altitudes?


  • Background level concentration

    • Vary with latitude, lower in the tropics and higher in the northern middle latitudes

    • Average 110 ppbv

    • Increasing 1%/yr, mostly in the northern middle latitudes

  • Urban/suburban levels

    • Vary from few ppmv to 60 ppmv: mainly associated with transportation emissions

    • Average highs (10-20 ppmv)

    • Higher concentrations in higher altitude cities

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Sulfur Compounds

  • Sulfur Oxides: Sulfur trioxide (SO3), Sulfur dioxide (SO2)

  • Reduced sulfur compounds (COS, CS2, H2S)

Sulfur Oxides

  • Anthropogenic sources

    • Combustion of S-containing fuels

    • Smelting of metal ores

  • Natural sources

    • Volcanoes

    • Oxidation of reduced S compounds



  • Produced from SO2 oxidation

  • Rapidly reacts with water

  • Very short atmospheric lifetime

  • Colorless, sulfurous odor gas

  • Major sulfur oxide in the atmosphere

  • Produced on S oxidation

  • May be converted to SO3

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What is the overall picture?

Data from

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  • Sink processes: SO2 oxidized in gas & liquid phase reactions; can be direct, photochemical or catalytic

  • Gas phase

    • Reaction with OH· (major), O3, HO2·, RO2·, O(3P)

  • Liquid phase

    • It can be further oxidized to H2SO4 by reaction with HNO2, O3, H2O2, RO2· and catalysis by Fe and Mn

H2SO4: sulfuric acid

H2SO3: sulfurous acid

HNO2: nitrous acid

H2O2: hydrogen peroxide

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What is the consequence of the deposition?

Removal processes

  • Aerosol formation by nucleation/condensation

  • Sulfuric acid reacts with ammonia: forms sulfate salts

  • SO2 + aerosols removed by wet & dry deposition processes

  • SO2 atmospheric lifetime (1-7 days)

SO2 concentration

  • Background levels: ~20 pptv over marine surface to 16- pptv over clean areas of US

  • Historical urban 1-hour highs: 1-500 ppbv

  • Highest 1 hr near non-ferrous metal smelters: 1.5-2.3 ppmv

More discussion in Welfare Effects

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Reduced S compounds

  • (CH3)2S (Dimethyl sulfide)

    • Released from oceans in large quantities

    • Short atmospheric lifetime (0.6 days) by rapid conversion to SO2

  • COS (Carbonyl sulfide)

    • Most abundant S species in atmosphere

    • Produced biogenically

    • Background levels (0.5 ppbv)

    • Limited reactivity

    • Atmospheric lifetime ( 44 years)

  • Mercaptans

    • Source of malodors: “Rotting cabbage”

  • CS2 (Carbon disulfide)

    • Produced biogenically

    • Photochemically reactive

    • Global concentrations range (15-190 ppbv)

    • Atmospheric lifetime (12 days)

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  • Major environmental and health concern (toxic): characteristic malodor (rotten egg odor, threshold of 500 pptv)

  • Sources:

    • Natural: primarily by biological decomposition

    • Anthropogenic sources: Oil & gas extraction, Petroleum refining, Coke ovens, Kraft paper mills

  • Short atmospheric lifetime (4.4 days): Oxidized to SO2

  • Background concentrations( 30-100 pptv); concentrations in industrial and surrounding ambient environments can be above the odor threshold

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Nitrogen Compounds

  • Gas/Liquid phase

    • Nitrous acid (HNO2)

    • Nitric acid (HNO3)

    • Nitrite (NO2-)

    • Nitrate (NO3-)

    • Ammonium (NH4+)

  • NOx: NO and NO2

  • NOy: NOx and their atmospheric oxidation products

  • Gas phase

    • Nitrogen (N2)

    • Nitrous oxide (N2O)

    • Nitric oxide (NO)

    • Nitrogen dioxide (NO2)

    • Nitrate radical (NO3)

    • Dinitrogen pentoxide (N2O5)

    • Peroxyacyl nitrate (CH3COO2NO2; PAN)

    • Ammonia (NH3)

    • Hydrogen cyanide (HCN)

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So, why do we care about its increase in the atmosphere?

Nitrous Oxide (N2O)

  • Colorless, slightly sweet non-toxic gas

  • Also called “laughing gas” because human exposure to elevated concentrations produces a kind of hysteria

  • Atmospheric concentration increasing: (0.8 ppbv/yr)

  • Sources:

    • Natural: by nitrification and denitrification processes biogenically

    • Anthropogenic sources: Soil disturbance, Agricultural fertilizers

  • No known sink in the troposphere: atmospheric lifetime of 150 years

  • Stratosphere is only sink: photolysis and subsequent oxidation by singlet oxygen (O(1D))

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So, why do we care about NO emission?

Nitric oxide (NO)

  • Colorless, odorless, relatively non-toxic gas

  • Natural sources:

    • Anaerobic biological processes

    • Biomass burning processes, lightning

    • Oxidation of NH3

    • Photochemical reactions in stratosphere and transport from there into the troposphere

  • Anthropogenic sources

    • Fuel combustion (transportation, coal-fired power plants, boilers, incinerators, home space heating)

    • Product of high temperature combustion; concentration depends on temperature and cooling rate

More details about NO formation in Reaction/Kinetics

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Nitrogen Dioxide (NO2)

  • Brown colored, relatively toxic gas with a pungent and irritating odor

  • Absorbs light and promotes atmospheric photochemistry

  • Peak levels occur in mid morning

  • Production by chemical reactions

    • Direct oxidation

    • Photochemical reactions

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Weekly pattern?

Seasonal pattern?

NOx concentrations

  • Remote locations: 20-80 pptv

  • Rural locations: 20 pptv -10ppbv

  • Urban/suburban areas: 10 ppbv - 1 ppmv

  • Diurnal variation

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(Reverse reaction under sunlight)

(removed by dry & wet deposition)

NOx Sink Processes

  • Chemical reactions convert

    NO to NO2 to HNO3

  • Major sink process reaction with OH·

  • Nighttime reactions involving O3

  • Reactions with organic compounds

  • Neutralized by ammonia to form salts

  • HNO3 serves as a reservoir and carrier for NOx

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Other N Compounds


  • HCN (Hydrogen cyanide)

  • Organic nitrate compounds: Peroxyacyl nitrate (PAN), Peroxyproprionyl nitrate (PPN), Peroxybutyl nitrate (PBN) – potent eye irritants

Reduced N Compounds

  • NH3 (Ammonia)

    • Sources: anaerobic decomposition of organic matter, animals and their wastes, biomass burning, soil humus formation, fertilizer application, coal combustion, industrial emissions

    • Background levels (0.1-10 ppbv)

    • Sink processes: reaction with acids, absorption by water and soil surface

    • Atmospheric lifetime (10 days)

    • Very important neutralizer for strong acids

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  • Comprise a large number of chemical substances

  • Basic structure includes only carbon & hydrogen covalently bonded

  • Serves as a base for a number of derivative compounds

  • May be straight, chained, branched or cyclic

  • May be

    • Saturated (single bonds, C-C)

    • Unsaturated (double/triple bonds, C = C)

  • Unsaturated HCs more reactive

  • May be gas, liquid or solid phase, depending on the number of carbons: gases 1-4 C; volatile liquids 5-12 C; semivolatile liquids or solids > 12 C

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  • Types

    • Aliphatic

      • Paraffins/Alkanes - single bond

      • Olefins/Alkenes - have 1 double bond

      • Alkynes – have 1 triple bond

    • Aromatic

      • Have at least one benzene ring

        • Benzene

        • Toluene

        • Xylene

    • Lifetime

      • Paraffins – days

      • Olefins – hours

      • Alkyenes – weeks

      • Benzene (12 days), toluene (2 days), m-xylene (7 hr)

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  • Polycyclic aromatic HCs (PAHs)

    • Multiple benzene rings

    • Solids under ambient conditions

    • Produced in combustion processes

    • Components of atmospheric aerosol

    • Potent carcinogens

  • Classification by volatility

    • VVOC (Very Volatile Organic Compounds): BP up to 50-100 oC

    • VOC (Volatile Organic Compounds): BP 50-100 to 240-260 oC

    • SVOC (Semi-Volatile Organic Compounds): BP 240-260 to 380-400 oC

    • SOC (Solid Organic Compounds): above 400 oC

  • NMHCs: Non-Methane HydroCarbons; Methane is excluded because of its low reactivity in the atmosphere

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Hydrocarbon Derivatives

  • Formed from reactions with O2, N2, S or halogens

  • Derivatives of major atmospheric concern include:

    • Oxyhydrocarbons

    • Halogenated hydrocarbons


  • Direct emissions from industrial/commercial use: adhesives, solvents

  • By-products of combustion

  • Produced from photochemical reactions

  • Include

    • Aldehydes (C=O)

    • Acids (-COOH)

    • Alcohols (-OH)

    • Ketones (CO)

    • Ethers (C-O-C)

    • Esters (R-CO-OR’)

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Nonmethane Hydrocarbons

  • Primary focus of air quality regulation

  • Biogenic sources

    • Trees (isoterpenes, monoterpenes)

    • Grasslands (light paraffins; higher HCs)

    • Soils (ethane)

    • Ocean water (light paraffins, olefins, C9-C28 paraffins)

    • Order of magnitude higher than anthropogenic

    • Question of their significance

  • Anthropogenic emission estimates

    • 40% transportation

    • 32% solvent use

    • 38% industrial manufacturing/fuel combustion

  • Identification is challenging; concentration of individual NMHC is not commonly measured

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NMHC Sink Processes

  • Oxidation by OH· or O3

    • Produce alkylperoxyradicals (ROO·)

    • ROO· is converted to alkoxy radical (RO·) by reacting with NO

    • RO· reacts with O2 to produce aldehyde

    • Longer chained NMHCs result in ketones

  • Ethane reaction

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Oxidation of HCHO

  • Acetaldehyde more reactive than ethane

  • Acetaldehyde oxidized to HCHO through a series of reactions with OH·

  • HCHO can decompose by ultraviolet (UV) light in the range of 330-350 nm and produce CO

2nd pathway

1st pathway produces OH· for oxidizing other NMHC

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Photochemical Precursors

  • CO (above) can be eventually converted to CO2

  • Aldehydes/ketones removed by wet/dry deposition

  • Longer chained HCs may produce condensible products

  • These oxidation products (e.g. ROO·, RO·, HO2· and CO) serve as major reactants in forming smog; they also serve to produce elevated tropospheric O3

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So, why do we care about CH4?

Figure 2.5

Methane (CH4)

  • Most abundant HC in atmosphere

  • Low reactivity with OH

  • Little significance in urban/suburban photochemistry; hence, levels subtracted from total HC concentration

  • Can affect downwind of urban sources

  • Thermal absorber - global warming concern

  • Concentrations average ~ 1.75 ppmv

  • Significant increases over time since industrial revolution

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  • Natural Sources

    • Anaerobic decomposition in swamps, lakes and sewage wastes

    • Rice paddies

    • Ruminant/termite digestion

  • Anthropogenic Sources

    • Coal/lignite mining

    • Oil/gas extraction

    • Petroleum refining

    • Transmission line leakage

    • Automobile exhaust

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  • Sink processes

    • In the troposphere, reaction with OH·

    • Produces HCHO, CO & ultimately CO2

    • Competes with CO for OH·

    • Photodecomposition in stratosphere

      • Produces H2O

      • Major source of water in stratosphere

  • Levels in atmosphere increase with increasing CO

  • Atmospheric lifetime (~10 years)

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Halogenated Hydrocarbons

  • Contain one or more atoms of halogen (Cl, Br, or F); include a variety of compounds

    • Chlorinated HCs

    • Brominated HCs

    • Chlorofluoro HCs

  • Remarkable persistence (i.e. low reactivity)

  • Include both natural/anthropogenic sources; both volatile and semi-volatile compounds

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Volatile Halogenated HCs

  • Methyl Chloride (CH3Cl)

  • Methyl Bromide (CH3Br)

  • Methyl Chloroform (CH3CCl3)

  • Trichloroethylene(CH2CCl3)

  • Perchloroethylene(C2Cl4)

  • Carbon tetrachloride (CCl4)

Semi-volatile Halogenated HCs

  • Chlorinated pesticides (DDT, Dieldrin, Aldrin)

  • Polychlorinated biphenyls (PCBs)

  • Polybrominated biphenyls (PBBs)

Chlorofluoro hcs cfcs l.jpg

So, why do we care about them?

Chlorofluoro HCs (CFCs)

  • Trichlorofluoromethane (CFCl3): CFC-11

  • Dichlorodifluoromethane (CF2Cl2): CFC-12

  • Trichlorotrifluoroethane (C2Cl3F3): CFC-13

  • Characterized by

    • Low reactivity

    • Low mammalian toxicity

    • Strong thermal absorption properties

    • Good solvent properties

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Halogenated HCs

  • Most halogenated HCs have tropospheric sinks

  • CFCs have no tropospheric sinks.

  • Atmospheric Lifetimes

    CH3Cl, CH3Br ~ 1 year

    CH3CCl3 ~ 6.3 years

    CCl4 ~ 40 years

    CFCl3 ~ 75 years

    CF2Cl2 ~ 111-170 years

  • Concentrations vary spatially, with highest in source regions over the northern hemisphere.

  • Concentrations in both the troposphere and stratosphere have been increasing until the early 1990s.

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Photochemical Oxidants

  • Produced in chemical reactions involving:

    • Sunlight

    • Nitrogen oxides

    • Oxygen

    • Hydrocarbons

  • Include

    • Ozone

    • Nitrogen dioxide

    • Peroxyacyl nitrate

    • Odd hydrogen compounds (OH·, HO2·, H2O2)

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Is O3 level high or low at a highway tollbooth?

This doesn’t explain the high level O3 in smog! What’s wrong?

Figure 2.6

Photochemical oxidants: O3

  • Ozone the major photochemical oxidant of concern

  • Atmospheric O3 formation

  • Requires source of O(3P): through photolysis of NO2 at wavelengths of 280-430 nm

  • Nitric oxide quickly

    destroys O3

  • Steady-state concentration of

    20 ppb under solar noon

    conditions in mid-latitudes

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Tropospheric O3 Formation

  • Elevated O3 levels occur as a result of reactions that convert NO to NO2 without consuming O3!

  • Role of peroxy compounds (ROO·) derived from photochemical oxidation of HCs

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In summary, what are the important parameters in determining O3 level?

Tropospheric O3 formation

  • Rate of O3 formation depends on ROO· availability

  • ROO· produced when OH· and HOx react with HCs

  • OH· is formed by photo-dissociation of O3, aldehydes and HNO2

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Tropospheric O O3 Concentrations

  • Remote Locations (20-50 ppbv, summer months)

    • Photochemical processes

    • Stratospheric intrusion

  • Populated locations

    • Peak concentrations (50 ppbv - 600 ppbv)

  • In urban areas concentrations decline at night

  • In rural areas peak concentrations occur at night

  • Elevated rural levels associated with long-range transport (Yosemite NP,

    • Transport of O3 aloft

    • Transport of low reactivity paraffins

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Tropospheric O O3 levels

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Ozone Sink Mechanisms O

  • Photo-dissociation

  • Reaction with NO in polluted area

  • Reaction with NO2 at night time

  • Surface destruction: reaction with plants, bare land, ice/snow and man-made structures