Air Pollution Chemistry

1. Air Pollution Chemistry Introduction

2. Air Pollutants • Carbon monoxide (CO) - Primary • Sulfur dioxide (SO2) - Primary • Nitrogen dioxide (NO2) - Primary and Secondary • Ozone (O3) - Secondary • Particulate matter < 10 um in diameter (PM10) - Primary and Secondary

3. Atmospheric Processes • Emission • Dispersion • Wind • Inversion • Transport • Transformation • Deposition

4. Hydrocarbon Combustion • Ideally Fuel + Air => Products HC+O2 => CO2+H2O • Too Simple

5. Methane (CH4) Combustion • CH4 +O2+Inert (N2 or Ar) • Requires over 170 reactions and over 30 compounds • If add NOx chemistry • Requires over 275 reactions and over 45 compounds

6. Incomplete Combustion • Intermediate Compound Emissions • Formaldehyde and Acetylene • More toxic or reactive • Precursors to secondary pollutants (e.g., ozone and particulate)

7. Oxygenated Compounds • Ethers -- R-O-R CH3-O-C(CH3)3 = MTBE • Multifunctional Carbonyls • More than one oxygenated functional group on same molecule

8. Ambient HC Concentrations

9. Ambient Concentrations of HC Species

10. Outline • Ozone Formation • Hydrocarbon Reactivity • Particulate Matter Formation • Conclusions

11. Nitrogen Cycle Only a few minutes per cycle: O + O2 + M O3 + M (fast) M is the “third body”, primarily N2 and O2 NO2 + hv NO + O (k1) hv is ultraviolet radiation O3 + NO NO2 + O2 (k3) ozone “scavenging” reaction

12. NO2 Photolysis Rates forJune 12 and December 12

13. Photostationary State [O3] = {k1 / k3} . {[NO2] / [NO]} NOx emissions < 90% NO. [O3] = {10} . {1 / 10} [O3] = 1 ppb

14. Fast Radical Transfer Reaction • Oxidizes NO to NO2, allowing O3 to build up. RO2 + NO RO + NO2 • R is any hydrocarbon fragment. 100’s of hydrocarbons 1000’s of reactions • Takes hours to affect PSS.

15. Hydrocarbon Oxidation Cycle The hydroxyl radical begins cycle, OH + HCHO H2O + HCO HCO + O2 HO2 + CO HO2 + NO NO2 + OH and is returned at end of cycle.

16. Sources of Radicals Formaldehyde photolysis HCHO + hv + O2 HO2 + HCO Ozone photolysis O3 + hv + H20 2 OH + O2 Nitrous acid photolysis HONO + hv OH + NO

17. NOx and Radical Sink Reaction OH + NO2 HNO3 (nitric acid) • Limits ozone formation. • Increases acid deposition and particulate matter.

18. Typical Pattern of O3 Formation • Before sunrise • No NO2 photolysis, radicals, or O3 • During the day • Radicals created, convert NO to NO2 • NO2/NO ratio increases, O3 builds up • NO2 decreases via sink reaction, limiting O3 • After sunset • Fresh NO emissions scavenge O3

19. 70% 60% 50% 40% 30% 20% 10% 0% 00-08 08-12 12-16 16-20 20-00 Alkenes Aromatics Alkanes Diurnal Variation in Atmospheric CompositionGlendora, CA -- August 11-21, 1986

20. Control Implications • Role of NOx • Required for ozone formation • Shorter lifetime than hydrocarbons • Emissions inhibit O3 near sources • Emissions limit O3 downwind • Role of hydrocarbons • Greatly enhance the build-up of O3 from NOx

21. Control Implications (Cont’d) • Near NOx source areas • Hydrocarbon control required • Areas affected by transport • NOx control generally required • Large urban areas • Control both NOx and hydrocarbons

22. Ozone Concentration TrendsSouth Coast Air Basin

23. Particulate Matter Formation • NOx, SOx + OH to acids • Also in fog and clouds • NH3, salts + acids to nitrates, sulfates • Organic aerosols • Large alkenes + O3 • Large alkanes + OH • Aromatics + OH

24. PARTICLES IN THE ATMOSPHERE • ORIGINS, EMISSIONS, & CHEMISTRY • PHYSICAL PROPERTIES • DEPOSITION AND TRANSPORT • “ACID RAIN” • HAZE & VISIBILITY • CONCENTRATION DYNAMICS

25. Particles Origins • Primary Particles • Mechanical Processes • Chemical Processes • Weathering • Combustion • Secondary Particles • Precursors & Catalysts • Reaction Rates • Growth and Volatility

26. Primary Particles • Combustion & Process Venting • Open Combustion • “Stack” & Tailpipe Emissions • Urban & Industrial • Cutting, Sand Blasting, Abrasion (Tires, Brakes, etc.) • Natural • Soil, Vegetation, Evaporation, Fire, Volcanoes, etc.

27. Combustion & Process Venting • Combustion • Carbon - Organic & Elemental (Fuels) • “Ash” - “Inert Contaminants” (Metals, Soil, etc.) • Vapors • Coking & Cooking (Organics, sulfur, etc.) • Metals (Foundries, Welding, etc.) • Non-Metallic (Fluxes, Salts, etc.)

28. “Fugitive” Material • Soil • Roads • Material Handling and Storage • Blasting & Grinding • Drift

29. Secondary Particles • Oxidation Pathways • Gas Phase Conversion • Aqueous Conversion • Catalysis • Particle Growth and Decay • Nucleation • Condensation and Accumulation • Vaporization

30. Sulfate Aerosol Formation • Gas Phase • Oxidation(OH, O2) of H2S & SO2 • O3 Catalysis • Aqueous Conversion • Oxidation (OH) & Catalysis (Mn)

31. Natural Sulfate Aerosols • Sulfuric Acid H2SO4 • Organo - Sulfate • MgSO4 - Sea Salt (6%) • CaSO4 - Gypsum • Ammonium Compounds • Ammonium Sulfate (NH4)2SO4 • Ammonium Bisulfate (NH4)HSO4

32. Anthropogenic Sulfate Aerosols • Sulfuric Acid H2SO4 • Organo - Sulfate • CaSO4 - Scrubbers, Fugitives • Zn, Fe, Cu SO4 - Smelting, etc. • Ammonium Compounds • Ammonium Sulfate (NH4)2SO4 • Ammonium Bisulfate (NH4)HSO4

33. Nitrate Aerosols • Reversible Reactions • T, RH, Pp (NH3, HNO3) • Generally Winter - Cold, Wet • Ammonia Forcing (Chino example) • Reactions (e.g. Sea Salt) • Growth on Pre-existing Particles

34. Secondary Hydrocarbons • Phase Determinants • Concentration • Temperature • O3 Catalysis • Episodic in Ozone Season

35. Aerosol Characteristics

36. PM Size Distributions

37. Hygroscopic Aerosols • Growth and Humidity • Hysteresis 2 Drying Relative Diameter 1 Wetting 40 50 60 70 80 90 RH%

38. 1 m m P a r t i c l e 5 m P a r t i c l e m Differential DepositionIdealized - Dry, Steady State Nitric 1.0 0.8 Acid 0.6 Remaining Fraction 0.4 Vapor 0.2 0.0 0 24 48 72 96 120 Hours

39. Evolving Size Distributions • Nucleation • Coagulation • Deposition

40. Size - Origin Relationships • Primary • Mechanical Particles • Weathering Particles • Combustion Particles • Secondary • Gas-Phase Nucleation • Dry Particle Growth • Aqueous Conversion

41. Clouds, Fog & Rain • Stimulated Condensation • Selective Rain-out • Cloud Scavenging • Inertial Scavenging • Evaporation & Residual Aerosol • Grand Canyon Example

42. Aerosol History • Aerosol - Emission Relationships are Non-Linear • Air Mass History Determines Aerosol Properties

43. Deposition & Transport

44. 1 m m P a r t i c l e 5 m P a r t i c l e m Deposition • Deposition “Velocity” • Size & Chemistry Dependent • Meteorology • Particle “Half-Life” Nitric 1.0 0.8 Acid 0.6 Remaining Fraction 0.4 Vapor 0.2 0.0 0 24 48 72 96 120 Hours

45. Winter Mixing, Deposition & Lifetimes

46. Transport • Source Areas & Receptors • Prevailing Winds • Mixing Height & Topography • Distance, Time, and Aerosol Evolution • Secondary Formation / Dissociation • Dispersion • Deposition

47. ACID DEPOSITION - “ACID RAIN”

48. Acid Formation • Sulfuric Acid • Nitric Acid • Organic Acids

49. Acid Transport & Deposition • Acid Rain • Local or Upwind Emissions • Cloud Scavenging and Precipitation • Acid Fog • Dry Deposition • Upwind Emissions • Wet or Dry Oxidation • Concentration and Residence

50. California Acid Deposition Monitoring Program 100 km Precipitation Sites Alpine Wet Sites Dry Deposition Sites