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The Characterization of Atmospheric Particulate Matter

The Characterization of Atmospheric Particulate Matter. Richard F. Niedziela DePaul University 16 May 00. The atmosphere. Have you thought about your atmosphere today? Physical dimensions m atm  5.2  10 18 kg  10 -6 m earth h atm  100 km V atm  1.0  10 11 km 3  10 -1 V earth

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The Characterization of Atmospheric Particulate Matter

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  1. The Characterization ofAtmospheric Particulate Matter Richard F. Niedziela DePaul University 16 May 00

  2. The atmosphere Have you thought about your atmosphere today? • Physical dimensions • matm 5.2  1018 kg  10-6mearth • hatm  100 km • Vatm  1.0  1011 km3  10-1Vearth • Thermal profile • Several different thermal gradients

  3. The atmosphere

  4. The atmosphere • The atmosphere is made out of... • 78% N2 (3.9  1018 kg) • 21% O2 (1.2  1018 kg) • 1% trace gases and suspended matter, or aerosols (0.1  1018 kg)

  5. Aerosols

  6. Aerosols Aerosols are small particles of condensed matter that are found throughout the environment, from the surface of the Earth to the upper reaches of the atmosphere. • Brilliant red sunsets • Blue hazes in forests • Fog

  7. Aerosol characteristics An aerosol is characterized by • Composition • Size • Phase • Shape

  8. Aerosol composition • Organic materials • Long-chained hydrocarbons • Large carboxylic acids • Inorganic materials • Mineral acids • Metals • Organic/inorganic mixtures

  9. Aerosol size Particle diameters range from submicron to tens of microns Atmospheric background aerosols Average atmospheric aerosols Smallest detectable particles Atoms, small molecules Very fine aerosols Cloud droplets Raindrops Drizzle Hail 10-4 10-3 .01 .1 1 10 100 103 104 micron = 1 mm = 10-4 cm = 10-6 m

  10. Aerosol phase • Liquids • Oil droplets from vegetation • Sulfuric acid aerosols • Solids • Suspended crust material • Water ice particles in cirrus clouds • Liquid/solid mixtures

  11. Aerosol shape • Liquids: spherical droplets • Solids: crystals and complex structures • Shape can impact physical, chemical, and optical properties of aerosols

  12. Some actual aerosols Sulfate particle Aluminum particle T. Reichhardt, Environ. Sci. Tech., 29(8), 360A, (1995).

  13. Aerosol sources • Natural sources • Vegetation • Oceans • Volcanoes • Anthropogenic sources • Vehicle and industrial emissions • Agricultural practices

  14. Aerosol production • Mechanical action • Abrasion of plant leaves • Sea spray • Wind • Nucleation and condensation • Cloud formation

  15. Aerosols and the Environment

  16. Aerosols and the Environment • Ozone depletion • Global climate change

  17. The atmosphere thermosphere upper atmosphere mesopause 80 mesosphere 60 altitude (km) middle atmosphere stratopause 40 stratosphere 20 tropopause troposphere lower atmosphere

  18. Ozone • Pungent gas (named after the Greek word ozein, “to smell”) • “Good” vs. “Bad” • Stratosphere • 90% of all ozone • 10 ppmv peak concentration • UV screening • Troposphere • 10 ppbv peak concentration • Disinfectant • Respiratory stress O O O O3

  19. Ozone • Chapman mechanism • Proposed in 1930 • Qualitative prediction of atmospheric ozone profile O2 + h O + O O + O2 + M O3 + M O3 +h O2 + O O3 + O O2 + O2

  20. Ozone depletion There has been a recent overall decrease in the stratospheric ozone concentration. CF2Cl2 + h CF2Cl + Cl Cl + O3 ClO + O2 ClO + O Cl + O2 O3 + O 2 O2 Ozone measured over Payerne, Switzerland

  21. Polar ozone depletion The loss of ozone over the South Pole is more dramatic

  22. Polar ozone depletion theories • Atmospheric motions • Stratospheric air replaced with tropospheric air Discounted due to lack of tropospheric trace gases in the stratosphere

  23. Polar ozone depletion theories • Reactive nitrogen species chemically destroy ozone Discounted due to low concentrations of nitrogen species during depletion events

  24. Polar ozone depletion theories • Chlorine compounds are responsible for the ozone depletion • Produced from CFCs • Persist for up to 100 years

  25. Polar ozone depletion cycle 2ClO + M Cl2O2 + M Cl2O2 + hn ClOO + Cl Cl + O2 + M ClOO + M 2Cl + 2O3 2ClO + 2O2 2O3 + hn 3O2 These reactions are thought to be responsible for 70% of the observed ozone depletion

  26. Homogeneous reactions hn CFCs ClONO2 hn NO2 ClO

  27. Polar stratospheric chemistry • Homogenous chemistry cannot provide all of the ClO needed to deplete ozone • Ozone depletion occurs in the presence of polar stratospheric clouds or PSCs

  28. Polar stratospheric clouds • Type I • Formed near 195 K • Composed of nitric acid and water • Exist in different phases • Type Ia: Solid nitric acid particles • Type Ib: Supercooled liquid droplets (sulfuric acid, nitric acid, water) • Type II • Formed near 185 K • Water ice particles

  29. Heterogeneous reactions • Chlorine is released into the gas phase • Nitrogen is chemically removed • Nitrogen is physically removed ClONO2(s) + HCl(s) Cl2(g) + HNO3(s) PSCs ClONO2(s) + H2O(s) HOCl(g) + HNO3(s) PSCs

  30. Heterogeneous reactions hn HCl CFCs HNO3 ClONO2 Polar Stratospheric Clouds PSCs H2O Cl2 hn HOCl hn Cl Sedimentation Cl

  31. Polar stratospheric chemistry hn hn CFCs HCl ClONO2 ClONO2 HNO3 hn NO2 H2O PSCs ClO Cl2 HOCl ClO + ClO hn Cl2O2 hn ClO hn Cl Sedimentation O3 O2

  32. Polar stratospheric chemistry • Heterogeneous reaction rates are dependent on PSC phase, composition, and size • Need to characterize PSCs to fully investigate depletion process

  33. PSC characterization • Collect infrared spectra of PSCs • Mie scattering theory • Spherical particles • Complex refractive indices for proposed PSC components

  34. Complex refractive indices • n is the real component of the refractive index • determines how fast light moves through material • n = c / v • k is the imaginary component of the refractive index • determines how light is absorbed by material • k = al / 4p • Optical constants

  35. PSC spectra Ice NAD NAT O.B.Toon and M.A. Tolbert, Nature, 375, 218, (1995).

  36. Polar stratospheric clouds • Good fits were not obtained using known optical constants for • Water ice • Nitric acid monohydrate (NAM): HNO3·H2O • Nitric acid dihydrate (NAD): HNO3·2H2O • Nitric acid trihydrate (NAT): HNO3·3H2O

  37. Polar stratospheric clouds • PSCs are not pure water or nitric acid aerosols • Ternary mixtures with sulfuric acid • Determine optical constants for ternary mixtures

  38. Retrieving optical constants • Retrieve optical constants from infrared spectra of model PSC aerosols • Frequency • Temperature • Optical constants for NAD

  39. Aerosol flow cell II

  40. Aerosol flow cell II

  41. Aerosol flow cell II

  42. Retrieving optical constants Collect many scattering spectra representing different particle sizes

  43. Scattering spectra

  44. Retrieving optical constants Collect a non-scattering spectrum to estimate k Collect many scattering spectra representing different particle sizes k(n) = Ka(n)

  45. Non-scattering spectrum

  46. Retrieving optical constants Collect a non-scattering spectrum to estimate k Collect many scattering spectra representing different particle sizes Select a scattering spectrum and guess the particle size k(n) = Ka(n)

  47. Retrieving optical constants Collect a non-scattering spectrum to estimate k Collect many scattering spectra representing different particle sizes Select a scattering spectrum and guess the particle size k(n) = Ka(n) Use Kramers-Kronig relationship to calculate n(n)

  48. Retrieving optical constants Collect a non-scattering spectrum to estimate k Collect many scattering spectra representing different particle sizes Select a scattering spectrum and guess the particle size k(n) = Ka(n) Use Kramers-Kronig relationship to calculate n(n) Use Mie scattering theory to calculate scattering spectrum

  49. Retrieving optical constants Collect a non-scattering spectrum to estimate k Collect many scattering spectra representing different particle sizes Select a scattering spectrum and guess the particle size k(n) = Ka(n) Use Kramers-Kronig relationship to calculate n(n) Use Mie scattering theory to calculate scattering spectrum Compare calculated and experimental spectra

  50. Retrieving optical constants Collect a non-scattering spectrum to estimate k Collect many scattering spectra representing different particle sizes Select a scattering spectrum and guess the particle size k(n) = Ka(n) Use Kramers-Kronig relationship to calculate n(n) Use Mie scattering theory to calculate scattering spectrum Compare calculated and experimental spectra Correct k(n) if necessary

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