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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

Chap 2.3

Gaseous Pollutants
  • Carbon oxides
  • Sulfur compounds
  • Nitrogen compounds
  • Hydrocarbon compounds
  • Photochemical oxidants
carbon oxides
Carbon Oxides
  • Two major carbon oxides
    • Carbon dioxide (CO2)
    • Carbon monoxide (CO)

CO2

  • 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
slide3

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

Is CO2 emission regulated? Should it be?

Figure 2.2

CO2
  • 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)

slide4

Figure 2.3

CO2
  • 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

slide5
CO
  • Colorless, odorless, tasteless gas
  • Produced as a result of incomplete combustion
slide6

Adverse effects on the consumption of OH·?

  • Formation of O3

Overall

CO
  • 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

slide7

Why higher in higher latitudes and altitudes?

CO
  • 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
sulfur compounds
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

SO3

SO2

  • 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
slide9

What is the overall picture?

Data from http://www.uea.ac.uk/~e490/su/sulfur.htm

slide10
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

slide11

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

reduced s compounds
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)
h 2 s
H2S
  • 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
nitrogen compounds
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)
nitrous oxide n 2 o

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))
nitric oxide no

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

nitrogen dioxide no 2
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
slide18

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
slide19

(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
reduced n compounds

Other N Compounds

Example?

  • 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
hydrocarbons
Hydrocarbons
  • 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
hydrocarbons22

http://en.wikipedia.org/wiki/Toluene

http://en.wikipedia.org/wiki/Xylene

Example?

http://en.wikipedia.org/wiki/Benzene

Hydrocarbons
  • 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)
hydrocarbons23
Hydrocarbons
  • 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
hydrocarbon derivatives
Hydrocarbon Derivatives
  • Formed from reactions with O2, N2, S or halogens
  • Derivatives of major atmospheric concern include:
    • Oxyhydrocarbons
    • Halogenated hydrocarbons

Oxyhydrocarbons

  • 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’)
nonmethane hydrocarbons

Why?

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
nmhc sink processes
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
oxidation of hcho
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

photochemical precursors
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
methane ch 4

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
methane
Methane
  • 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
methane31

Why?

Methane
  • 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)
halogenated hydrocarbons
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
volatile halogenated hcs
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

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
halogenated hcs
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.
photochemical oxidants
Photochemical Oxidants
  • Produced in chemical reactions involving:
    • Sunlight
    • Nitrogen oxides
    • Oxygen
    • Hydrocarbons
  • Include
    • Ozone
    • Nitrogen dioxide
    • Peroxyacyl nitrate
    • Odd hydrogen compounds (OH·, HO2·, H2O2)
photochemical oxidants o 3

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

tropospheric o 3 formation
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
tropospheric o 3 formation39

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
tropospheric o 3 concentrations
Tropospheric 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,http://www2.nature.nps.gov/air/webcams/parks/yosecam/yosecam.cfm)
    • Transport of O3 aloft
    • Transport of low reactivity paraffins
ozone sink mechanisms
Ozone Sink Mechanisms
  • 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
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