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Gaseous Pollutants

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

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  1. Chap 2.3 Gaseous Pollutants • Carbon oxides • Sulfur compounds • Nitrogen compounds • Hydrocarbon compounds • Photochemical oxidants

  2. 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

  3. 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)

  4. 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

  5. CO • Colorless, odorless, tasteless gas • Produced as a result of incomplete combustion

  6. 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

  7. 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

  8. 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

  9. What is the overall picture? Data from http://www.uea.ac.uk/~e490/su/sulfur.htm

  10. 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

  11. 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

  12. 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)

  13. 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

  14. 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)

  15. 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))

  16. 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

  17. 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

  18. 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

  19. (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

  20. 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

  21. 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

  22. 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)

  23. 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

  24. 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’)

  25. 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

  26. 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

  27. 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

  28. 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

  29. 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

  30. 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

  31. 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)

  32. 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

  33. 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)

  34. 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

  35. 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.

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

  37. 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

  38. 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

  39. 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

  40. 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

  41. Tropospheric O3 levels

  42. 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

  43. Quick Reflection

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