Acid Rain

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Acid Rain

• RUSSELL R. DICKERSON

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Sandstone portal Figure on Herten Castle in Ruhr
district of Germany. Sculpted 1702 photographed
in 1908.
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Same sandstone portal figure photographed in 1969.
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• ACID DEPOSITION
• See Turco, Ch. 9
• Finlayson-Pitts Pitts, Part 6
• Seinfeld Pandis Ch. 20
• Introduction
• The industrial revolution greatly increased the
  amount of acid in the atmosphere. These acids
  accelerate the rate of weathering of both
  natural and man-made materials. (Figures of
  sandstone Madonna). Today we will discuss what
  acid deposition is, what its effects are, the
  chemistry and physics of formation and transport,
  and who is responsible for it.

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• History
• As early as 1852, R. A. Smith analyzed rain that
  near the industrial city of Manchester, England
  and found that urban aerosol particles tend to be
  composed primarily of sulfuric acid, but as the
  air is transported away from sources over more
  rural areas, the acid is neutralized by
  absorption of ammonia.
• urban ? suburban ? rural
• H2SO4 NH3 ? (NH4)HSO4 (NH3) ? (NH4)2SO4
• sulfuric acid ? ammonium bisulfate ? ammonium
  sulfate
• Throughout the early part of the twentieth
  century, European scientists documented the
  sources and effects of atmospheric acids. It was
  not until 1958 that acidity of precipitation in
  the US was characterized (Junge and Werby, 1958)

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• Definitions
• wet deposition
• dry deposition
• occult deposition
• Effects
• Soils
• Soils (Figure) have colloidal molecules (clay
  particles) that have a layer of negative charge.
  They hold positively charged cations such as
  , Al³?, K?, Mg²?, and Ca²?. Some of these
  cations are essential plant nutrients (K?, Mg²?,
  Ca²?), while Al³? is toxic. Hydrogen ions from
  acid deposition replace these cations on the
  outer layer of colloidal molecules. The metal
  ions are then dissolved and leached into solution
  and can be washed away from the soil and into
  surface or ground water. Soil fertilitiy is
  reduced and aluminum ions can replace calcium in
  the fishs gills. Acidity can also come from
  oxidation (weathering) of sulfur-rich mine
  tailings, and from nitrification of ammonium.

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• A soils capacity to exchange base ions for those
  in solution (cation exchange capacity) is a
  measure of potential fertility. The impact of
  acids on soil fertility depends on the structure
  and composition of the clays in the soil.
  Maryland has serveral geological regimes with
  varying sensitivities to acid deposition
  (Figure).
• Forests can be especially sensitive to nutrient
  loss. In Europe in 1993 abou ta quarter of the
  trees have died or are more than 25 defoliated.
  This Waldsterben or forest death has been
  attributed at, least in part, to environmental
  degradation from a combination of acid
  deposition, ground-level ozone, and excess
  nutrification, primarily nitrogen. In the US,
  loss of forests has been so dramatic, although
  several species including ash and oak are
  sensitive to acidification of soils.

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• Lakes and Streams
• The sensitivity surface waters depends critically
  on their neutralizing or buffering capacity.
  Alkaline materials such as CaCO3, and MgCO3 can
  neutralize acids.
• The surface of the American Midwest is
  predominantly limestone (CaCO3), and lakes and
  streams have high neutralizing capacity. In the
  East granite dominates soils and surface waters
  lacking buffering capacity, are highly sensitive
  to acidification.

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Salt Waters The Ocean is strongly pH buffered,
has a high sulfate content naturally, and is
relatively insensitive to acid rain. Over much
of the ocean, however, nitrate is the limiting
nutrient for biological activity. Addition of
nitrogen from the atmosphere can increase primary
productivity, and the consequences reverberate
throughout the ecosystem. The Chesapeake Bay is
particularly sensitive to excess nutrients.
Additional nitrate or ammonium from atmospheric
deposition to the Bay or its watershed
contributes to the problem of excess
eutrophication. Nitrate allows thick beds of
aquatic layers of the waters. Loss of higher
animals follows.
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• Materials
• The Taj Mahal, the friezes on the Parthenon, the
  Madonna in Herten, Germany, and the Lincoln
  Memorial are made of marble. Marble, a
  particular crystalline form of calcite (CaCO3),
  and sandstone (grains of sand held together by
  calcite), are subject to attack by sulfuric acid.
• CaSO4 is gypsum, which is 100 times more soluble
  than CaCO3. Many priceless historic structures
  have been lost to acid deposition.
• On a more pragmatic note, the rate of corrosion
  of galvanized (zinc coated) steel is 0.62 um/yr
  in the Adirondacks, 1.01 in Washington, DC, and
  1.47 in Stubenville, OH.
• Health
• Covered already in Lectures 1 and 9.

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• Origins
• Primarily power generation and ore smelting (pie
  charts).
• For example nickel is mined as nickel sulfide,
  NiS. In smelting, it is heated in air (Sudbury,
  Canada).
• The molecular weight of nickel is 57 g/mole, so
  smelting produces more than a ton of SO2 for each
  ton of nickel produced.
• d) Formation and Composition
• Gas Phase production of nitric acid
• OH NO2 M ? HNO3 M
• Aqueous phase production of nitric acid
• NO2 O3 ? NO3 O2
• NO3 NO2 M N2O5 M
• N2O5 H2O(l) ? 2HNO3(aq)

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• This process is important only at night, and when
  air temperatures are low because the formation of
  N2O5 is reversible, and the equilibrium
  coefficient is highly temperature dependent.
  Also, NO3 is rapidly photolyzed by visible
  radiation.
• NO3 hv ? NO2 O
• Gas Phase production of sulfuric acid
• OH SO2 M ? HOSO2 M
• Aqueous phase production of sulfuric acid
• SO2 H2O2 ? H2SO4
• Lifetime and Transport
• Wet vs dry deposition

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• Problem Solving
• European community requirements 2.0 g/m3 and 0.4
  g/m3 if coal is 0.5 to 7 S by weight, can such
  goals be met by buying low sulfur coal??
• Calculate the pH of a solution of 10 µM
  and 10 µm
• From the mean pH of 4.1, and the assumption of
  equal molar nitrate and sulfate, estimate the
  molarity of nitric and sulfuric acids in College
  Park.
• Estimate the rate of dry deposition for a give
  PBL height, and concentration of H2SO4 and HNO3.
• If rainout plus washout is 50 efficient, what
  would have to be precipitation frequency for
  there to be equivalent amounts of wet and dry
  acid deposition?

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Average Precipitation pH Over the United States
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CHEMICAL COMPOSITION OF PRECIPITATION
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PATTERNS OF SO2 AND NOx EMISSIONS IN EASTERN U.S.
NOx transportation, power plants
SO2 coal-fired power plants
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GLOBAL SULFUR BUDGET(flux terms in Tg S yr-1)
SO42- t 3.9d
SO2 t 1.3d
cloud
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OH
H2SO4(g)
8
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4
OH
NO3
(CH3)2S
64
dep 6 dry 44 wet
10
dep 27 dry 20 wet
(DMS) t 1.0d
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Phytoplankton
Volcanoes
Combustion Smelters
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GLOBAL SULFUR EMISSION TO THE ATMOSPHERE1990
annual mean
Chin et al. 2000
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AREAS (IN BLACK) WITH LOW ACID-NEUTRALIZING
CAPACITY
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TRENDS IN U.S. EMISSIONS OF NOx AND SO2
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TRENDS IN SULFATE AND NITRATE WET DEPOSITION
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DEPLETION OF BASE CATIONS FROM ACID RAIN(Hubbard
Brook Experimental Forest, New Hampshire)
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QUESTIONS ON ACID RAIN
1. A dinosaur extinction theory suggests that
massive heating of the Earth's atmosphere in the
path of a falling asteroid would have led to
extremely acid rain, killing the land vegetation
and hence the dinosaurs' food supply. What acid
would have been involved?   2. Nitrate deposited
by acid rain can fertilize the biosphere. Explain
why.