This document discusses reactions in the atmosphere involving oxides of sulfur, nitrogen, and carbon monoxide. It focuses on sulfur dioxide entering the atmosphere from natural and pollution sources, and its conversion to sulfuric acid through oxidation which causes acid rain. Nitrogen oxides are formed through high-temperature combustion, and nitrogen dioxide plays a key role in photochemical smog formation through generating ozone and hydroxyl radicals. Tropospheric ozone levels are influenced by a balance of production from NO2 photolysis and consumption reacting with NO.

1. GREEN TECHNOLOGY (Environmental Science)
SUB Code: CH104
Dr. Aniruddha Paul
NATIONAL INSTITUTE OF TECHNOLOGY PATNA
DEPARTMENT of CHEMISTRY
1
02/23/2016

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UNIT 5: Atmosphere
Reactions in Atmosphere
Atmospheric chemistry of Oxides of Sulfur, Nitrogen and CO

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UNIT 5: Atmosphere
Reactions in Atmosphere
Atmospheric chemistry of Oxides of Sulfur, Nitrogen and CO
Oxides of sulfur
Sulfur dioxide (SO2) enters the atmosphere as the result of the following:
 Natural sources
 Direct emissions from volcanoes
 Atmospheric oxidation of H2S emitted to the atmosphere by bacteria and from geothermal
sources (volcanoes, hot springs, geysers)
 Atmospheric oxidation of dimethyl sulfide, (CH3)2S, emitted to the atmosphere from marine
organisms

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Oxides of sulfur
 Pollution: Pollution sources are
 The combustion of organic sulfur and iron pyrite, FeS2, in fossil fuels.
• Sulfur is present in most fuels:
• 1–2%, by weight in coal
• 2–3% in heavy fuel oils
• Lesser amount in natural gases and others.
 The pollutant sources are of most concern because of their contribution to local and
regional air pollution problems.

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Oxides of sulfur
 Pollution: Pollution sources are
 The combustion of organic sulfur and iron pyrite, FeS2, in fossil fuels.
• Sulfur is present in most fuels:
• 1–2%, by weight in coal
• 2–3% in heavy fuel oils
• Lesser amount in natural gases and others.
 Pollution is the largest source of Sulfur dioxide (SO2) emission in atmosphere,
because 97% of the sulfur emitted in combustion is as SO2.
Pyrite is oxidized to produce SO2:
4FeS2(s) + 11O2(g) → 2Fe2O3(s) + 8SO2(g)
 Also from roasting of sulfide ore during sulfuric acid (H2SO4) manufacture.
 The pollutant sources are of most concern because of their contribution to local and
regional air pollution problems.

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Oxides of sulfur
 Pollution: Pollution sources are
 The combustion of organic sulfur and iron pyrite, FeS2, in fossil fuels.
• Sulfur is present in most fuels:
• 1–2%, by weight in coal
• 2–3% in heavy fuel oils
• Lesser amount in natural gases and others.
 Pollution is the largest source of Sulfur dioxide (SO2) emission in atmosphere,
because 97% of the sulfur emitted in combustion is as SO2.
Pyrite is oxidized to produce SO2:
4FeS2(s) + 11O2(g) → 2Fe2O3(s) + 8SO2(g)
 Also from roasting of sulfide ore during sulfuric acid (H2SO4) manufacture.
 The pollutant sources are of most concern because of their contribution to local and
regional air pollution problems.
Calculating SO2 Emissions from Burning Coal:
Large quantities of coal are burned to generate electricity. A 1000-MW coal-
fired generating plant will burn 3.06 × 106 kg of coal per hour. For coal that
contains 4% sulfur by weight, calculate the mass of SO2 released (a) per hour
and (b) per year.

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 The fate of sulfur dioxide in the atmosphere is oxidation and reaction with water to
produce sulfuric acid.
 SO2 persists for an average of 10 days in the atmosphere before acid formation.

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 The fate of sulfur dioxide in the atmosphere is oxidation and reaction with water to
produce sulfuric acid.
 SO2 persists for an average of 10 days in the atmosphere before acid formation.
 The overall process is complex and not well understood:
(i) Generally SO2 is oxidized by atmospheric oxidizing agents to sulfur trioxide (SO3)
(ii) then SO3 rapidly reacts with water to produce H2SO4.
The overall reaction is thus:
2SO2 + O2 + 2H2O → 2H2SO4

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 The fate of sulfur dioxide in the atmosphere is oxidation and reaction with water to
produce sulfuric acid.
 SO2 persists for an average of 10 days in the atmosphere before acid formation.
 The overall process is complex and not well understood:
(i) Generally SO2 is oxidized by atmospheric oxidizing agents to sulfur trioxide (SO3)
(ii) then SO3 rapidly reacts with water to produce H2SO4.
The overall reaction is thus:
2SO2 + O2 + 2H2O → 2H2SO4
 A probable mechanism of SO2 → SO3 reaction involves reactions with hydroxyl radicals:
SO2 + OH· → HSO3·
HSO3· + O2 → SO3 + HO2·
SO3 + H2O → H2SO4

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 The fate of sulfur dioxide in the atmosphere is oxidation and reaction with water to
produce sulfuric acid.
 SO2 persists for an average of 10 days in the atmosphere before acid formation.
 The overall process is complex and not well understood:
(i) Generally SO2 is oxidized by atmospheric oxidizing agents to sulfur trioxide (SO3)
(ii) then SO3 rapidly reacts with water to produce H2SO4.
The overall reaction is thus:
2SO2 + O2 + 2H2O → 2H2SO4
 A probable mechanism of SO2 → SO3 reaction involves reactions with hydroxyl radicals:
SO2 + OH· → HSO3·
HSO3· + O2 → SO3 + HO2·
SO3 + H2O → H2SO4
 SO2 can be oxidized by tropospheric ozone formed due to nitrogen oxides pollution.
SO2 + O3 → SO3 + O2

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 The formation of sulfuric acid from SO2 is the main mechanism for forming acid rain,
• Acid rain is harmful to vegetation, fish, and materials (marble stones etc)
• Also SO2 forms aerosol droplets of sulfuric acid in the atmosphere (Industrial smog).
Harmful effects of Sulfur Dioxide and Sulfuric Acid

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 The formation of sulfuric acid from SO2 is the main mechanism for forming acid rain,
• Acid rain is harmful to vegetation, fish, and materials (marble stones etc)
• Also SO2 forms aerosol droplets of sulfuric acid in the atmosphere (Industrial smog).
 Sulfur dioxide has many adverse effects:
• Affects plants, causing leaf necrosis (death of leaf tissue).
• Causes chlorosis, a bleaching or yellowing of green leaves.
• Both SO2 and H2SO4 cause health hazards, for the average person SO2 at above 5 ppm
is hazardous.
• Continuous exposure can seriously affect lung function and respiratory organs.
Harmful effects of Sulfur Dioxide and Sulfuric Acid

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Straightforward means of reducing sulfur dioxide emissions  avoid having sulfur in
fuels.
 Sulfur compounds are removed from natural gas and petroleum.
 Coal often has high levels of sulfur, and so low sulfur coal in power plants is used.
 Removal of FeS2 from coal by washing.
Controlling Sulfur Dioxide pollution

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Straightforward means of reducing sulfur dioxide emissions  avoid having sulfur in
fuels.
 Sulfur compounds are removed from natural gas and petroleum.
 Coal often has high levels of sulfur, and so low sulfur coal in power plants is used.
 Removal of FeS2 from coal by washing.
 Fluidized bed combustion: Pulverized coal is blasted into a hot bed of calcium oxide, where
the coal is burned, and sulfur dioxide is trapped by the following reaction:
CaO + SO2 → CaSO3
Controlling Sulfur Dioxide pollution

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Straightforward means of reducing sulfur dioxide emissions  avoid having sulfur in
fuels.
 Sulfur compounds are removed from natural gas and petroleum.
 Coal often has high levels of sulfur, and so low sulfur coal in power plants is used.
 Removal of FeS2 from coal by washing.
 Fluidized bed combustion: Pulverized coal is blasted into a hot bed of calcium oxide, where
the coal is burned, and sulfur dioxide is trapped by the following reaction:
CaO + SO2 → CaSO3
 Use of lime (Ca(OH)2) scrubbing to remove sulfur dioxide from stack gas:
Ca(OH)2 + SO2 → CaSO3 + H2O
The calcium sulfite product of this process is oxidized,
CaSO3 + 1/2O2 + 2H2O → CaSO4,2H2O
to generate gypsum, CaSO4,2H2O, which serves as a raw material for building construction.
Controlling Sulfur Dioxide pollution

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Oxides of Nitrogen
 Nitrous oxide (N2O): colorless, slightly sweet odor
 nitric oxide (NO): colorless, odorless
 nitrogen dioxide (NO2): pungent-smelling, red-brown
 NO and NO2 are together called NOx – major source of air pollution
 N2O is released from fertilizer-treated soil: non-toxic and rather harmless.

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Oxides of Nitrogen
Pollution sources of NOx gases:
 A major pollutant source of these gases is the internal combustion engine, e.g.,
automobile engine.
 Road vehicle fuels contain negligible nitrogen but
 NOx pollution arises from high temperature combustion of molecular nitrogen and oxygen
producing NO.
N2 + O2 → 2NO

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Oxides of Nitrogen
Pollution sources of NOx gases:
 A major pollutant source of these gases is the internal combustion engine, e.g.,
automobile engine.
 Road vehicle fuels contain negligible nitrogen but
 NOx pollution arises from high temperature combustion of molecular nitrogen and oxygen
producing NO.
N2 + O2 → 2NO
 Combustion of fuels that contain organically bound nitrogen, such as coal, also produces
NO.
 Some of the nitric oxide is converted to NO2 in the atmosphere.
NO + O2 → 2NO2
 Road transport contributes approximately 50% and power stations 25% of the NOx
emissions.

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An extremely important aspect of nitrogen dioxide in the troposphere is that
 When exposed to solar radiation of wavelength of 398 nm (visible light) it photodissociates:
NO2 + hν → NO + O

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An extremely important aspect of nitrogen dioxide in the troposphere is that
 When exposed to solar radiation of wavelength of 398 nm (visible light) it photodissociates:
NO2 + hν → NO + O
 NO2 is one of a very few atmospheric molecules that absorb & photolyze in the visible range.
Photolysis of NO2 generates ozone in the troposphere:
NO2 + hv → NO + O(triplet) (1)
O(triplet) + O2 + M → O3 + M (third body to remove energy) (2)

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An extremely important aspect of nitrogen dioxide in the troposphere is that
 When exposed to solar radiation of wavelength of 398 nm (visible light) it photodissociates:
NO2 + hν → NO + O
 NO2 is one of a very few atmospheric molecules that absorb & photolyze in the visible range.
Photolysis of NO2 generates ozone in the troposphere:
NO2 + hv → NO + O(triplet) (1)
O(triplet) + O2 + M → O3 + M (third body to remove energy) (2)
 The ozone can be consumed by reacting with NO to form the original reactant—nitrogen
dioxide, which can undergo photodissociation again to start the cycle over.
O3 + NO → NO2 + O2 (3)
 The net effect of these three reactions (1-3) is absorption of energy without any net chemical
change.
 But, both O3 molecules and O atoms are very reactive – react with other pollutants 
photochemical smog formation.

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Photochemistry of NO2
Absorption spectrum of NO2
 At λ > 398 nm NO2 still partially photodissociates (in 398 – 415 nm range) but efficiency
declines rapidly with wavelength.

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Photochemistry of NO2
Absorption spectrum of NO2
 At λ > 410 nm, photoexcitation of NO2 is the major process with following consequences:
NO2* + hv → NO2* Absorption
NO2* → NO2 + hv Fluorescence
NO2* + O2 → NO2 + O2* Electronic energy transfer

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Ozone as Secondary Pollutant in troposphere
 Tropospheric ozone is formed by the combination of molecular oxygen and atomic
oxygen, the latter is obtained from the photodissociation of NO2.
NO2 + hv → NO + O(triplet)
O(triplet) + O2 + M → O3 + M (third body to remove energy)
O3 + NO → NO2 + O2 (slow and inefficient in case of large NO2 cocn)
 Creation and destruction of O3 are thus in equilibrium.
 However, when NO2 pollution is high due to vehicle emission, O3 destruction reduces.
 Ozone pollution depends on a number of factors, primarily automobile emission of
NO2 and intense sunlight.

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 Thus O3 and NO2 concentrations vary with volume of automobile emission and intensity of
sunlight.
 Therefore O3 and NO2 concentrations vary with the time of day as well as with the time of
year.
 The morning rush of traffic produces an NO and hydrocarbon peak.
 Oxidation of the NO to NO2 then leads to a rise in O3 during the middle of the day,
 O3 generation decreases as the sun goes down and further NO from the evening rush
contributes to its removal (O3 + NO → NO2 + O2).

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 Ozone photodissociates at any altitude to give an O atom and an oxygen molecule
whenever it is struck by photons in the near-ultraviolet range (240 to 330 nm).
O3 + hv → O2 + O(singlet)
 In the troposphere, O atoms react with water to produce hydroxyl radicals (OH·).
O(singlet) + H2O → 2OH·

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 Ozone photodissociates at any altitude to give an O atom and an oxygen molecule
whenever it is struck by photons in the near-ultraviolet range (240 to 330 nm).
O3 + hv → O2 + O(singlet)
 In the troposphere, O atoms react with water to produce hydroxyl radicals (OH·).
O(singlet) + H2O → 2OH·
 In the daytime, hydroxyl radicals can react with nitrogen dioxide to produce nitric acid.
2OH· + NO2 → HNO3
 This three-step process is the primary means of removing NO2 from the atmosphere.
Net reaction:
O3 + 2NO2 + H2O → 2HNO3 + O2

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 Ozone photodissociates at any altitude to give an O atom and an oxygen molecule
whenever it is struck by photons in the near-ultraviolet range (240 to 330 nm).
O3 + hv → O2 + O(singlet)
 In the troposphere, O atoms react with water to produce hydroxyl radicals (OH·).
O(singlet) + H2O → 2OH·
 In the daytime, hydroxyl radicals can react with nitrogen dioxide to produce nitric acid.
2OH· + NO2 → HNO3
 This three-step process is the primary means of removing NO2 from the atmosphere.
Net reaction:
O3 + 2NO2 + H2O → 2HNO3 + O2
Ozone is strongly oxidizing and so
• Ozone is highly toxic causing adverse health effects in humans even below 1 ppm.
• High ozone concentration lead to crop degradation and incur huge losses in agricultural sector.

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Carbon Monoxide (CO)
 Carbon monoxide, CO, is an air pollutant with direct toxicity to humans.
 Carbon monoxide binds to blood hemoglobin and prevents the hemoglobin from transporting
oxygen
 Hemoglobin’s affinity for CO is 200–300 times greater than its affinity for oxygen.
 CO is removed from the atmosphere only slowly by reaction with hydroxyl radicals
OH· + CO → CO2 + H·
 The hydrogen radical then may react with O2 and NO to regenerate the hydroxyl radical:
H· + O2 + M → HOO· + M (third body to remove energy)
HOO· + NO → NO2 + OH·
 Highly congested urbanized areas where the air changes are slow concentrations of CO may
reach 100 ppm, which then causes health hazard.

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Acid Rain
 Hydrogen chloride, HCl, emitted by the combustion of chlorine-containing organic
compounds,
 H2SO4 from sulfur dioxide,
 HNO3 produced from nitrogen oxides,
Contributes to acid rain.
2SO2 + O2 + 2H2O 2H2SO4
SO2 + O3 → SO3 + O2
SO3 + H2O → H2SO4
N2 + O2 → 2NO; 2NO + O2 → 2NO2
2NO2 + H2O → HNO2 + HNO3
 Incorporated into rainwater, these acids fall to the ground as acid rain.

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Acid deposition: the effects of
 atmospheric strong acids,
 acidic gases (SO2),
 acidic salts (NH4NO3 and NH4HSO4), etc.
 Acid deposition is a major air pollution problem.
 There have been some incidents where localized release of acid, (as sulfur dioxide from
metal ore smelting operations) devastating vegetation within several kilometers of the
source.

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Harmful Effects of Acidic Precipitation or Acid Rain
a) Direct effects: reduced and distorted visibility due to the presence of sulfuric acid droplets
and solutions/solid particles of acidic salts, e.g. NH4HSO4.
b) Phytotoxicity (toxicity to plants) and destruction of forests.
• Direct: resulting from exposure of plant leaves and roots to acidic gases, SO2 and NO2.
• Indirect: liberation of phytotoxic Al3+ ion in soil by the action of acid rain.
c) Direct effects on humans and other animals – respiratory problems.
d) Effects upon plants and fish in acidified lake water.
e) Damage to materials. Stone (especially acid-soluble limestone and marble) and metal used
in building can be corroded and etched by acidic precipitation.
• Taj Mahal, a principally marble structure, is at constant threat from acid rain due to
industrial activity in nearby areas.
f) Electrical equipment, particularly relay contacts and springs can be corroded by acidic
precipitation.