EOH 468 Spring 2009 Atmospheric Pollutants and Photochemical Smog Atmospheric pollution Natural Emissions associated with natural processes Anthropogenic Man-caused Natural air pollution Geogenic sources Volcanoes Mineral dusts Ocean salts 1 Natural air pollution Biogenic sources Mold spores and pollen Biological decomposition Volatile emissions from vegetation Natural air pollution Atmospheric sources Electrical storms Stratospheric intrusion Photochemical processes Natural air pollution Significant affects associated with: Volcanoes Forest fires Dust storms Mold and pollen 2 Natural air pollution Low significance in causing human health problems because Exposure levels are low Sources and human populations are often distant Major source emissions are episodic and transient Anthropogenic air pollution Smog Definition-pollutant caused severe visibility reduction Smog London-type Los Angeles-type 3 Smog London-type Has occurred for centuries Coal smoke mixing with fog Gray in color Sulfur odor Relatively rare in western countries Smog Los Angeles-type Photochemically- produced Associated with motor vehicle emissions Brown in color Los Angeles smog Is exacerbated by Large number of sunny days Large population of motor vehicles Topographical limitations on air flow Frequent subsidence inversions 4 Los Angeles-type smog Is also a problem in Houston Denver Mexico City Haze Moderate visibility reduction associated with atmospheric pollutants Common in many American/European cities Broad regional distribution Can be seen by satellites Common pollutant concerns Gases/vapors Particles 5 Gases-phase pollutants Include substances Which exist as gases under ambient conditions That are vapors derived from volatile liquids Particulate pollutants Particles Include both solid and liquid phases of matter Dispersed/suspended in air - aerosols Particles Aerosols Fume Dust Mist Photochemical 6 Pollutant sources Primary pollutants Emitted from sources Secondary pollutants Produced in the atmosphere Pollutant sources Classification Combustion/noncombustion Stationary/mobile Point/area Direct/indirect Gas-phase pollutants Nature of concern Thousands of pollutants Regulated pollutants more limited 7 Gas-phase pollutants Pollutants of concern Carbon oxides Sulfur compounds Nitrogen compounds Hydrocarbons Halogenated hydrocarbons Photochemical oxidants Carbon monoxide Colorless, odorless, tasteless gas Historically considered to be major pollutant Produced as a result of incomplete combustion Produced naturally in the environment Carbon Monoxide Combustion reactions 8 Carbon monoxide concentrations Urban/suburban levels Vary from few ppmv to 60 ppmv Average highs (10-20 ppmv) Higher concentrations in higher altitude cities Carbon monoxide Sink processes Photochemistry Soil uptake Residence time (2 months) Carbon monoxide Photochemical sink processes Oxidation by OH 9 Carbon monoxide photochemical sink processes In the presence of significant NOx Carbon monoxide photochemical sink processes Produce one O3 per CO oxidized Regenerate OH radical Compete with methane for OH Indirectly increase atmospheric methane levels Indirectly increase stratospheric water vapor levels Sulfur Compounds Sulfur Oxides Sulfur trioxide (SO3) Sulfur dioxide (SO2) Reduced sulfur compounds 10 Sulfur oxides Natural sources Volcanoes Oxidation of reduced S compounds Sulfur oxides Anthropogenic sources Combustion of S-containing fuels Smelting of metal ores Sulfur trioxide Produced from SO2 oxidation Rapidly reacts with water Very short atmospheric lifetime 11 Sulfur Dioxide Colorless, sulfurous odor gas Major sulfur oxide in the atmosphere Produced on S oxidation May be converted to SO3 Direct oxidation processes Sulfur dioxide concentrations Historical urban one-hour highs Near non-ferrous metal smelters 100-500 ppbv 1.5-2.3 ppmv 12 Atmospheric reactions Sulfur dioxide oxidized in gas & liquid phase reactions Direct, photochemical, catalytic oxidation Atmospheric reactions Gas phase Photo-oxidation Reaction with OH, O3, biradical, O(3P) Reaction with OH (this is major sink process) Atmospheric reactions Reaction with OH 13 Atmospheric reactions Liquid Phase May be further oxidized to H2SO4 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 (2-4 days) Reduced S compounds Include Hydrogen sulfide (H2S) Carbonyl sulfide (COS) Carbon disulfide (CS2) Dimethyl sulfide (CH3)2S 14 Reduced S compounds Carbonyl sulfide Most abundant S species in atmosphere Produced biogenically Background levels (0.5 ppbv) Limited reactivity Atmospheric lifetime ( 44 years) Nitrogen compounds Gas phase Nitrogen (N2) Nitrous oxide (N2O) Nitric oxide (NO) Nitrogen dioxide (NO2) Nitrate radical (NO3) Nitrogen compounds Gas phase Dinitrogen pentoxide (N2O5) Peroxyacyl nitrate (CH3COO2 NO2) Ammonia (NH3) Hydrogen cyanide (HCN) 15 Nitrogen compounds Gas/Liquid phase Nitrous acid (HNO2) Nitric acid (HNO3) Nitrite (NO2-) Nitrate (NO3-) Ammonium (NH4+) Nitrous oxide Colorless, slightly sweet non-toxic gas Concentration increasing with time (0.8 ppbv/yr) Produced biogenically - world’s oceans Anthropogenic sources Soil disturbance Agricultural fertilizers Nitrous oxide Atmospheric lifetime of 150 years Stratosphere is only sink Important in stratospheric chemistry May contribute to Ozone depletion Global warming 16 Nitric oxide Colorless, odorless, relatively nontoxic gas Natural sources include Anaerobic biological processes Combustion processes Photochemical reactions in stratosphere Nitric oxide Anthropogenic sources Fuel combustion Product of high temperature combustion Peak levels associated with transportation cycle Diurnal variations in precursors and photochemical oxidants 17 Nitrogen dioxide Brown colored, relatively toxic gas Absorbs light Promotes atmospheric photochemistry Peak levels occur in mid morning Nitrogen dioxide Production by chemical reactions Direct oxidation Photochemical reactions NOx concentrations Nitric oxide and NO2 concentrations reported as NOx Remote locations : 20-80 pptv Rural locations : 20 pptv -10ppbv Urban/suburban areas : 10 ppbv - 1 ppmv 18 Nitrogen oxides sink processes Chemical reactions convert NO to NO2 to HNO3 Major sink process reaction with OH Nitrogen oxides sink processes Nighttime reactions involving O3 Nitrogen oxides sink processes Other Reactions 19 Nitrogen oxides sink processes Nitric acid tends to remain in gas phase Reactions may occur with NH3 Nitrate aerosols removed by wet/dry deposition Reduced N compounds Hydrogen Cyanide Background levels (160 pptv) Ammonia Produced both bio & geogenically Background levels (0.1-10 ppbv) Atmospheric lifetime (6 day) Neutralizes strong acids Organic nitrate compounds Produced as a result of photochemistry Reactions with HCs and NOx Important atmospheric pollutants Potent eye irritants Phytotoxic 20 Organic nitrate compounds Peroxy compounds Peroxyacyl nitrate (PAN) Peroxypropionyl nitrate (PPN) Peroxybutyl nitrate (PBN) Peroxyacyl nitrate Chemical formation Undergoes thermal degradation Transported to upper troposphere - lifetime of months Carrier of NOx in long range transport Hydrocarbons Comprise large number of chemical substances Basic structure includes only carbon & hydrogen covalently bonded Serves as a base for a number of derivative compounds 21 Hydrocarbons May be straight, chained, branched or cyclic May be Saturated (single bonds, C-C) Unsaturated (double/triple bonds, C = C) Hydrocarbons Unsaturated HCs more reactive May be gas, liquid or solid phase Depends on the number of carbons Nonmethane hydrocarbons Primary focus of air quality regulation Sources Biogenic Anthropogenic 22 Nonmethane hydrocarbons Biogenic sources Trees (isoterpenes, monoterpenes) Grasslands (light paraffins; higher HCs) Soils (ethane) Ocean water (light paraffins, olefins, C9C28 paraffins) Nonmethane hydrocarbons Anthropogenic emission estimates 40% transportation 32% solvent use 38% industrial manufacturing/fuel combustion Photochemical oxidants Produced in chemical reactions involving: Sunlight Nitrogen oxides Oxygen Hydrocarbons 23 Photochemical oxidants Include Ozone Nitrogen dioxide Peroxyacyl nitrate Odd hydrogen compounds (OH, HO2, H2O2) Photochemical oxidants Ozone the major photochemical oxidant concern Atmospheric O3 formation Tropospheric O3 formation Requires source of O(3P) Nitric oxide quickly destroys O3 24 Nitrogen dioxide photolysis and O3 formation Tropospheric O3 Nitrogen dioxide photolysis results in little actual net O3 increase Cannot explain much higher levels present in troposphere Tropospheric O3 formation Effect of peroxy compounds Peroxy compounds derived from photochemical oxidation of HCs 25 Tropospheric O3 formation Reactions involving peroxy compounds Photochemical production of O3 Tropospheric O3 formation Rate of O3 formation depends on RO2 available RO2 produced when OH and HOx react with HCs 26 Tropospheric O3 formation Formation of OH Tropospheric O3 formation Concentrations dependent on Intensity of sunlight NO2/NO ratios Reactive HC type and concentration Concentrations of aldehydes/CO Tropospheric O3 concentrations Remote Locations (20-50 ppbv, summer months) Result from Photochemical processes Stratospheric intrusion Populated locations Peak concentrations (50 ppbv - 600 ppbv) 27 Tropospheric O3 levels In urban areas concentrations decline at night In rural areas peak concentrations occur at night Rural O3 Elevated rural levels associated with long-range transport Transport of O3 aloft Transport of low reactivity paraffins Ozone sink mechanisms Major sink processes Photochemical reactions Ozone photodecomposition Nitric oxide reaction with O3 Nitrogen dioxide reaction with O3 Surface deposition 28 Range of particle sizes Particle size Expressed as aerodynamic equivalent diameter Reference to a spherical particle of uniform density that falls at standard velocity Expressed as Stokes number Particle size Small particles (< 1 µm aerodynamic diameter) Behave as gases Subject to Brownian motion Follow fluid streamlines Capable of coagulation Settle out slowly 29 Particle size Larger particles Strongly affected by gravity Settle out more rapidly Rarely coalesce Particle size Determines Atmospheric lifetime Effectiveness of light scattering Deposition in human lung Particle size Relative to numbers present Most particles very small (< 0.1 µm) Relative to particle mass/volume Most volume/mass associated with particles > 0.1 µm 30 Particle size distribution Characterized as multi-modal Particle distribution bi-modal based on particle volume Larger mode – coarse Smaller mode - fine Particle size distribution Tri-modal distribution associated with motor vehicle traffic Fine fraction has 2 modes Nuclei Accumulation Multi-modal particle size distribution 31 Particle size distribution Coarse particles Produced by mechanical processes Size range 2 to 100 µm Particle size distribution Fine particles Produced by high energy processes Have a very high surface area Have a tendency to grow in size Fine particles Nuclei mode Particles smaller than 0.08 µm Produced from nucleation/condensation of molecules Grow in size by agglomeration 32 Fine particles Accumulation mode Typically in the 0.1-1.0 µm size range Produced from agglomeration of nuclei mode particles Particle classes/sources Primary particles Varied sources Secondary Produced from chemical reactions Produced naturally and anthropogenically Chemical composition Chemical composition is variable Affected by Source Atmospheric history May contain dozens of different substances 33 Chemical composition Major PM components Organic carbon Elemental carbon Nitrate Sulfate Crustal elements Chemical composition Organic carbon includes wide variety of substances. Sulfate levels vary regionally. Fine particles contain a variety of trace metals. Atmospheric concentrations Reported as particle mass/unit volume Total suspended particles(TSP) PM 10 – thoracic particles PM 2.5 – respirable particles 34
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