Chapter 5 Tropospheric chemistry precipitation

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Chapter 5Tropospheric chemistry - precipitation

• CH350/EV350
• Spring 2008

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Precipitation

• Wet deposition
• Analyte deposition with water based particles
  (rain, snow, sleet, hail, fog)
• Rainout analytes incorporated into cloud
  droplets
• Washout analytes incorporated into
  precipitation as it falls through the air
• Dry deposition (subject of chapter 6)
• Analyte deposition on dry particles or gaseous
  adsorption to earth surface

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Composition of rain

• Rain water is at equilibrium with atmospheric
  gases
• Henrys Law (H) relates the atmospheric pressure
  (P) with the aqueous concentration (C) at
  equilibrium
• CO2 chemistry more complicated
• CO2 378 ppmv
• CO2 in air leads to H ions in solution
• Unpolluted rain pH 5.7 due to dissolved CO2

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Composition of rain affected by chemical and
biological process in region

• Na and Cl- usually derived from ocean
• Ca2 and Mg2 from dust minerals
• NH4 from animal and plant decomposition and
  fertilizers

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Sulfur and Nitrogen species leads to acidity in
precipitation

• Natural and anthropogenic sources

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Nitric acid chemistry

• NOx chemistry leads to HNO3
• NO is converted to NO2 by various methods
• NO2 can generate HNO3 directly or through NO3
  pathways
• HNO3 can be removed by wet or dry deposition or
  by reaction with NH4

.NO ? .NO2 numerous pathways .NO2 .OH M ?
HNO3 M .NO2 O3 ? .NO3 O2 .NO3 RH ? .R
HNO3 .NO3 .NO2 ? N2O5 N2O5 H2O ? 2HNO3 NH3
HNO3 ? NH4NO3
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Sulfur chemistry SO2 production

• Many natural sources of sulfur H2S, CS2, (CH3)2S,
  COS, CH3SH in -2 oxidation state
• Oxidation of sulfur to SO2 (4 oxidation state)
  by various pathways
• COS relatively stable kinetically
• SO2 also emitted by combustion sources burning
  high sulfur fuel
• SO2 emitted by volcanoes
• 1991 Mount Pinatubo released 20 Mtons SO2, 10 of
  global anthropogenic releases

H2S .OH ? H2O .SH CS2 .OH ? COS .SH COS
.OH ? CO2 .SH .SH O2 ? SO .OH .SH O3 ?
SHO. O2 SHO. O2 ? SO .OOH SO O2, O3, NO2
? SO2 other products
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Sulfur chemistry SO2 oxidation

• Homogeneous and heterogeneous (in cloud water
  droplets) oxidation of SO2 to H2SO4 (sulfuric
  acid, strong acid)

SO2 .OH M ? HOSO2 M HOSO2 O2 M ? HOO.
SO3 M SO3 H2O ? H2SO4 SO2(g) ?
SO2(aq) SO2(aq) 2H2O(l) ? HSO3-(aq)
H3O(aq) HSO3-(aq) H2O2(aq) ? HOOSO2-(aq)
H2O HOOSO2-(aq) rearranges to HSO4-
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Global sources of Nitrogen and Sulfur compounds

• Reduced sulfur leads to acid rain, sulfur already
  in 6 oxidation state (SO42-) does not cause acid
  rain

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Why is the pH profile this way?
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Problems with acid rain

• Lowers pH of soils (can cause nutrient leaching)
• Lowers pH of surface water
• Stress on plants, animals, and other organisms
• Dissolving of limestone structures

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• Disease and Insects usually kill a forest
  stressed by acid rain

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• Limestone Acid Rain ? dissolved minerals
• CaCO3(s) H(aq) ? Ca2(aq) HCO3-(aq)

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What is being done?

• Cap and Trade policy
• In 2000 emissions were capped at 9.5 million tons
  (17.3 million tons in 1980)
• Each power plant is allowed a certain quantity of
  emissions.
• If they dont emit their quantity they can sell
  the rights to another company to emit the
  remainder of the emissions.

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Prevent SO2 emissions
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Have we been successful?

• Sulfate emissions
• pH changes?

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1994
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1999
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2004
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