More on Acid Rain

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More on Acid Rain
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Acid Rain and Acid Deposition

• Acid Rain/ Acid Snow/ Acid Fog have been
  recognized as a problem for some time and thought
  by many to be under control
• Normal (unpolluted) rain has a pH 5.6 due to
  dissolved CO2
• Acid rain refers to precipitation with pH lower
  than 5.0
• Damage to crops, forests, lakes, buildings,
  humans and animals

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Sources of Acid rain

• Acid rain linked to higher than normal levels of
  sulfur oxides and nitrogen oxides (even at
  concentrations a few ppm)
• These oxides are oxidized in the atmosphere and
  return to the ground dissolved in raindrops

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Sources of Acid Rain

• High levels of SOxs and NOxs linked to
  industrialization
• (i) coal burning
• S (as organosulfur or metal sulfides) O2 ? SO2
• (ii) mining - many metals are present as sulfide
  ores NiS, Cu2S, ZnS, PbS...

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Sources of Acid Rain

• Recovery of elemental metal (smelting) involves
  roasting ore in air to give the metal oxide which
  is subsequently reduced to metal
• and for Cu2S and HgS the metal is formed directly
• Sudbury nickel smelter largest point source of
  SO2 in world

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http//www.pollutionissues.com/Re-Sy/Smelting.html
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Sources of Acid Rain

• Natural SO2 sources from oxidizing H2S produced
  by microbial activity in soils and volcanic
  emissions
• NOxs produced in urban areas (photochemical
  smog) but also at sites for coal burning and
  metal smelting (high temperatures can lead to
  oxidation of N2)
• Typically HNO3 emissions about 1/3(HSO3, H2SO4)
  emissions

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Canadian SO2 Emissions from Acid Rain Sources
(1980-2001)
U.S. SO2 Emissions from Electric Power Generation
(1980-2003)
http//www.ec.gc.ca/cleanair-airpur/CAOL/canus/rep
ort/2004CanUs/section1a_e.html
Note the differences in scale and sources
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U.S. NOx Emissions from Electric Power Generation
(1990-2003)
http//www.ec.gc.ca/cleanair-airpur/CAOL/canus/rep
ort/2004CanUs/section1a_e.html
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http//www.ec.gc.ca/cleanair-airpur/CAOL/canus/rep
ort/2004CanUs/section1a_e.html
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Annual Standard Visual Range (2002)
http//www.ec.gc.ca/cleanair-airpur/CAOL/canus/rep
ort/2004CanUs/section1a_e.html
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Annual volume-weighted values of pH for
precipitation at W6. Hubbard Brook has longest
continuous record of precipitation chemistry in
North America. pH values are low at Hubbard Brook
due to inputs of strong acids associated with
sulfur dioxide and nitrogen oxide emissions.
Recent increases in pH are largely due to
decreases in sulfur dioxide emissions in the
eastern U.S.
http//www.hubbardbrook.org/overview/research_acti
vities.htm
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Comparison of global production of anthracite and
bituminous hard coal, brown coal, and oil.
http//capita.wustl.edu/CAPITA/CapitaReports/GlobS
Emissions/GlobS1850_1990.htm
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Comparison of anthracite and bituminous hard coal
production of USSR, China, and the US.
http//capita.wustl.edu/CAPITA/CapitaReports/GlobS
Emissions/GlobS1850_1990.htm
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Comparison of global sulfur emission estimates.
http//capita.wustl.edu/CAPITA/CapitaReports/GlobS
Emissions/GlobS1850_1990.htm
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Comparison of estimates of European sulfur
emissions.
http//capita.wustl.edu/CAPITA/CapitaReports/GlobS
Emissions/GlobS1850_1990.htm
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Estimated sulfur emissions for Europe, North
America, and Asia.
http//capita.wustl.edu/CAPITA/CapitaReports/GlobS
Emissions/GlobS1850_1990.htm
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Change in major sulfur emitters in comparison to
remainder of the world.
http//capita.wustl.edu/CAPITA/CapitaReports/GlobS
Emissions/GlobS1850_1990.htm
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Comparison between SO2 emissions for Croatia,
Czech Republic, Germany, France, Hungary, Italy,
Poland, Slovakia, Slovenia and Switzerland and
concentration trends for sulfate. The sulfate
concentrations (thin line) are not observations,
but smoothed by TSA for the Haunsberg site the
sulfur dioxide emissions are indicated by the
thick line.
http//www.rsc.org/ej/EM/2000/b004170j/
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http//www.inchem.org/documents/ehc/ehc/ehc008.htm
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http//www.inchem.org/documents/ehc/ehc/ehc008.htm
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http//www.inchem.org/documents/ehc/ehc/ehc008.htm
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Sulfur dioxide (SO2) and particulates are
considered by many environmental experts in China
to be the ambient air pollutants of gravest
concern. In 1995, SO2 emissions in China were
estimated at 20.8 million tons, and particulate
emissions totaled 23.3 million tons (see figure).
By comparison, SO2 emissions in the United States
were estimated at 18.6 million tons in 1995a and
particulate emissions totaled 3.3 million tons.
Coal is estimated to be the source of
approximately 90 percent of Chinas SO2 emissions
and 70 percent of its particulate emissions.
Emissions of Sulfur Dioxide and Particulates in
China, 1985-1995
http//www.eia.doe.gov/oiaf/archive/ieo99/boxtext.
html
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Although Chinas SO2 emissions are similar to
those in the United States, the direct use of
coal in China in the industrial and residential
sectors within or in close proximity to urban
areas has created serious health problems in its
major cities. As recently as 1996, five cities in
China (Beijing, Shenyang, Xian, Shanghai, and
Guangzhou) were ranked among the worlds ten
worst for air pollution 8, p. VIII-3.
Mortality data for China indicate that 17
percent of the deaths in Chinas urban areas are
due to acute and chronic respiratory illnesses
8, p. VIII-3. This is substantially higher than
the 7-percent share estimated for urban areas in
the United States. According to a World Bank
study completed in 1997, an estimated 178,000
people in Chinas major cities suffer early
deaths each year because of ambient pollution
levels in excess of Chinas standards 9, p. 19.
In China, SO2 emissions originate from a wide
range of industrial sources, including power
plants, which account for approximately one-third
of Chinas total sulfur emissions, and the
production of chemicals, building materials, and
metals, which, taken together, account for an
additional 20 percent of total sulfur emissions
8, p. VIII-2.b The consumption of coal for
residential heating and cooking also accounts for
about 20 percent of total SO2 emissions. The
widespread dispersion of point sources in China
makes it more complicated than in other countries
to reduce emissions through stack emission
controls. In most countries, a relatively small
number of centralized power plants account for
the majority of total SO2 emissions. At present,
very little of Chinas manufacturing or
generating plants are fitted with FGD equipment.
http//www.eia.doe.gov/oiaf/archive/ieo99/boxtext.
html
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Particulate emissions originate primarily from
the same sources that emit sulfur dioxide. The
key industrial source of particulates is the
building materials sector, accounting for almost
25 percent of total emissions, followed by power
plants (approximately 20 percent) and ferrous
metals (approximately 8 percent). The
nonindustrial sectors, primarily residential,
account for more than 25 percent of Chinas total
emissions of particulates. To date most efforts
at reducing emissions in China have focused on
the control of particulates, primarily those
associated with such noncombustion processes as
grinding, crushing, and sorting. Between 1986 and
1995, the average concentration of particulates
in Chinas top ten populous cities declined from
636 micrograms per cubic meter (µg/m3) to 300
µg/m3 (see figure). During the same period, the
share of total particulate emissions originating
from noncombustion sources fell from 50 percent
to 30 percent. Improvements in ambient SO2
levels have been relatively steady but less
impressive than those for particulates. Between
1986 and 1995, the average annual SO2
concentration for the ten most populous cities in
China declined from 149 µg/m3 to 91 µg/m3. The
most recent levels, however, continue to exceed
the 50 µg/m3 guideline for sulfur dioxide set
forth by the World Health Organization. The
Chinese government recognizes the need to reduce
pollution further, particularly in urban areas.
http//www.eia.doe.gov/oiaf/archive/ieo99/boxtext.
html
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Ambient Air Quality for Chinas Ten Most Populous
Cities, 1985-1995
http//www.eia.doe.gov/oiaf/archive/ieo99/boxtext.
html
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Canada's carbon dioxide emissions and Kyoto
target, 1983-2003. (Source EIA, International
Energy Annual 2003)
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Per capita energy consumption, 2001. (Source
EIA)
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Oxidation of SO2

• Three separate routes
• homogeneous gas phase
• homogeneous aqueous phase
• heterogeneous on particles

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Oxidation of SO2

• Homogeneous Gas Phase Oxidation
• Direct oxidation is not a major mechanism
• SO2 ½ O2 ? SO3 (?Gº -71 kJ/mol)
• Favoured thermodynamically but kinetically very
  slow (requires high temperatures and catalysts)

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Oxidation of SO2
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Oxidation of SO2
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Oxidation of SO2
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Oxidation of SO2
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Oxidation of SO2
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Oxidation and Deposition of Sulfur Oxides

• SO2, SO3 have t1/2s of several days
• acid precipitation can occur over distances up to
  4000 km regional problem
• e.g. Average wind speed 20 km/hr
• air mass can travel 3500 km in a week

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Oxidation and Deposition of Sulfur Oxides

• Oxidation and deposition rates depend on
  atmospheric conditions
• typical oxidation rates 1-10 /hr but depends
  on relative humidity, degree of particulate
  matter etc.
• rate of oxidation in dry air 0.5 /hr
• rate of oxidation in wet air as large as 30 /hr
  (aqueous oxidation)

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Oxidation and Deposition of Sulfur Oxides

• Simple model for deposition
• SO2 followed by
• (ks assumed to be 1st order rate constants)

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