Tropospheric Chemistry
• Air Pollution
Atmospheric oxidation processes and
Tropospheric Ozone
• Photochemical smog
• Sulphur chemistry and acid rain
• The greenhouse effect
Processes that affect the concentrations of
chemical species in the atmosphere
• Emissions (anthropogenic, biogenic)
• Chemistry (oxidation, photolysis)
• Transport
• Deposition (dry deposition, wet deposition)
Air Pollution
Major inorganic gaseous pollutants
•
•
•
•
Carbon monoxide (CO)
Sulfur dioxide (SO2)
Nitrogen Oxide (NO, NO2) NOx=NO+NO2
Ozone (O3)
• Primary pollutants: released directly from
sources
– Examples: CO, SO2, NOx
• Secondary pollutants: formed through
chemical reactions of the primary pollutants
and the constituents of the unpolluted
atmosphere in the air.
– Example: O3
Carbon monoxide: Health effect
CO enters the blood stream and binds
preferentially to hemoglobin, thereby replacing
oxygen.
O-O
O2
Fe
Fe
CO
C-O
Fe
320 times stronger than
hemoglobin-O2 binding
Carbon monoxide: sources and sinks
• Sources
–
–
–
–
Such as automobiles
Incomplete combustion (internal engine)
Biomass burning
Methane oxidation
Decay of plant matter
• Sink
– Reaction with OH radical
.OH + CO  CO + H.
2
H. + O2 + M  HO2. + M
– Removal by soil microorganism
Carbon monoxide: Atmospheric chemistry
CO + OH + O2  CO2 + HO2.
HO2. + NO  NO2 + OH
NO2 + hv  NO + O
O + O2 + M  O3 + M
Net: CO + 2 O2 + hv  CO2 + O3
The net reaction can be viewed as a catalytic oxidation of CO
to CO2. Net formation of O3 occurs.
Carbon monoxide: control strategies on the
automobile source
• Employ a leaner air/fuel mixture (higher
air/fuel ratio)
• Employ catalytic exhaust reactors
– Excess air is pumped into the exhaust pipe.
– Air-exhaust mixture pass through a catalytic
converter to oxidize CO to CO2.
• Addition of oxygenates to gasoline
– Examples of oxygenates: methanol, ethanol, MTBE
Sulfur dioxide: Health effect
• Produce irritation and increasing resistance in
the respiratory tract.
• Mucus secretion
• may also lead to increased mortality.
Sulfur dioxide: Sources and sinks
Sources
• Combustion of S-containing fuel in electric power
plants, vehicles.
• S (organic S + FeS2 pyrite) + O2 --> SO2
• Oxidation of H2S: 2H2S + 3 O2 --> 2 SO2 + 2 H2O
– H2S is produced as an end product of the
anaerobic decomposition of S-containing
compounds by micro organisms.
Sink
• Converted into sulphuric acid in either gas or liquid
phase
Formation of sulfuric acid and sulfate from SO2
• In gas-phase
SO2 + ·OH + M  HOSO2· + M
HOSO2· + O2  HO2· + SO3
SO3 + H2O + M  H2SO4 + M
• In aqueous phase, dissolved SO2 is oxidized to sulfate
by
– O3
(dominant pathway when pH>5)
– H2O2 (dominant pathway when pH<5)
– organic peroxides
– O2 catalyzed by iron and manganese
Nitrogen oxides: Health Effects
NO
• Cellular inflammation at very high concentrations.
• May be incorporated into hemoglobin in the blood to
interfere with the transport of oxygen around the body.
NO2
• irritate the lungs
• lower resistance to respiratory infection such as
influenza.
Nitrogen oxides: Sources and sinks
Sources
• Fuel combustion in power plants and automobiles.
N2 + O2 --> NO
2 NO + O2 --> 2 NO2
• Natural sources: electrical storms; bacterial
decomposition of nitrogen-containing organic matter
Nitrogen oxides: Atmospheric chemistry
Formation of nitric acid
Gas-phase reaction
NO2 + OH°  HNO3 daytime (dominate pathway)
Heterogeneous reaction
NO2 + O3  NO3 + O2
NO3 + NO2  N2O5
N2O5 + H2O (aq) 2 HNO3 (aq)
Minor pathway
Only operative during nighttime
Nitrogen oxides: Atmospheric chemistry
Formation of nitrate
HNO3 + NH3  NH4NO3
HNO3 + NaCl(s)  NaNO3 + HCl
Nitrogen oxides: Control strategies
1. Lower the combustion temperature of the
furnace in electric power plants
2. Install catalytic converters: catalytic
converters in automobiles can remove 76%
of NOx from tailpipes.
Three-way catalytic converter for automobile
exhaust (Remove CO, NO and HC)
HC + H2O = H2 + CO
2CO + O2 = 2CO2
2NO + 2H2 = N2 + 2 H2O
HC + 2O2 = CO2 + 2H2O
Catalyst: Rhodium
Catalyst: Platium/palladium
Heterogeneous catalyst
Prentice Hall ©2004
Automobile catalytic converter
Prentice Hall ©2004
Good Ozone and Bad Ozone
• Stratospheric ozone protect lives on Earth
from harmful effects of UV radiation.
• Tropospheric ozone:
– Causing respiratory distress and eye irritation
– Destroying plants
– Producing cracks in rubber
Ozone is a strong oxidant, reacts with molecules
containing C=C double bonds, forming epoxides.
OZONE EFFECTS ON PLANTS (III)
PIGMENTED LESIONS
BLEACHING
CHLOROSIS
IMAGES OF OZONE SYMPTOMS TO PLANTS
Agronomic crops
Vegetable crops
Woody plants
tobacco
pole bean
white pine
peanut
potato
sweet gum
white clover
water melon
althea
Formation of ozone
NO2 is capable of absorbing visible
light (<400 nm) to produce O.
NO2 + hv  NO + O
(1)
O + O2 + M  O3 + M
(2)
NO + O3  NO2 + O2
(3)
NO2 + hv  NO + O
(1)
O + O2 + M  O3 + M
(2)
HO2. + NO NO2 + OH
(4)
RO2. + NO  NO2 + RO.
(5)
No net O3
formation
O3 is
formed
Net of (1)+(2)+(4)/(5): RO2. + O2  O3 + RO.
Sources of RO2.: Oxidation of hydrocarbons
RH + OH  R. + H2O
R. + O2  RO2.
Example: Oxidation of carbon monoxide
CO + .OH + O2  CO2 + HO2.
HO2. + NO  NO2 + .OH
NO2 + hv  NO + O
O + O2 + M  O3 + M
Net: CO + 2 O2 + hv  CO2 + O3
The net reaction can be viewed as a catalytic oxidation of CO
to CO2. Net formation of O3 occurs.
Example: Oxidation of methane
CH4 + .OH + O2  CH3OO. + H2O
CH3OO. + NO  CH3O. + NO2
CH3O. + O2  HCHO + HO2.
HO2. + NO  .OH + NO2
NO2 + hn  NO + O (2x)
O + O2 + M  O3 + M (2x)
Net: CH4 + 4 O2  HCHO + H2O + 2 O3
The net reaction is that for each mole of methane oxidized,
2 moles of O3 is produced.
Necessary ingredients for ozone formation
• Sunlight
Production of O atom
• NOx (NO, NO2)
• Hydrocarbons (VOCs: volatile organic
carbon)
Production of RO2, which reacts
with NO so that O3 could
accumulate.
VOCs + NOx + hn  O3 + other pollutants
Necessary ingredients for ozone formation
VOC
CH4 + .OH + O2  CH3OO. + H2O
CH3OO. + NO  CH3O. + NO2
CH3O. + O2  HCHO + HO2.
HO2. + NO  .OH + NO2
NO2 + hn  NO + O (2x)
O + O2 + M  O3 + M (2x)
Net: CH4 + 4 O2  HCHO + H2O + 2 O3
Sunlight
OZONE
Photochemical smog
• Smog derives from a combination of the
words smoke and fog.
London smog and Los Angeles smog
• London smog is characterized by high SO2 and
particle concentration in the presence of fog.
– Also referred as sulfurous smog
• Los Angeles smog is characterized by high
oxidants (mainly O3). It was first recognized in
the Los Angeles area.
– The term smog is misleading in this case, as smoke
and fog are not key components.
– The appropriate term is photochemical air pollution.
Youngstown, Ohio (c. 1910)
Reading, Pennsylvania (c. 1909)
Gary, Indiana (c. 1912)
London, December 1952: 4,000 deaths
Donora, Pennsylvania, 1948: 20 deaths, 7000 respiratory illness
Jacobson et al., Atmospheric pollution
Noontime, Donora, Pennsylvania,
October 29, 1948
Copyright Photo Archive/Pittsburgh Post-Gazette, 2001. All rights reserved
Global Outlook: Photochemical Smog
in Santiago, Chile
Denver’s Brown Cloud:
Photochemical (Los Angeles) Smog
• Mixture of NOx + organic pollutants + sunlight + stagnant
meteorological conditions  ozone production
Chain initiation and propagation:
RH + OH + O2RO2 + H2O [R4]
RO2 + NO RO + NO2
[R5]
RO + O2 R’CHO + HO2
[R6]
HO2 + NO OH + NO2
[R7]
2(NO2 + hv (+O2)NO + O3)
net: RH + 4O2  R’CHO + 2O3 + H2O
Smog Bothers Pedestrians,
Los Angeles (1950s)
M. Jacobson, Air pollution
Los Angeles (July 23, 2000)
Photochemical air pollution
1
PANS and other pollutants
Volatile organic
compounds (VOCs)
Ozone (O3)
Oxygen (O2)
Nitric oxide (NO)
+
Oxygen atom (O)
Water
vapor
(H2O)
Hydrocarbons
UV radiation
Peroxyacyl
nitrates
Nitrogen dioxide (NO2)
(PANs)
Oxygen (O2)
Nitric oxide (NO)
Oxygen (O2)
Burning fossil fuels
Nitrogen (N) in fossil fuel
Sources for .OH radicals: Photolysis of O3
Photolysis of O3 forms O1D, followed by its
reaction with water.
O3 + hn  O1D + O2 l < 320 nm
O1D + H2O  2 .OH
Sources for .OH radicals: Photolysis of
H2O2
H2O2 + hn  2 .OH l  360 nm
H2O2 is formed from the reaction:
HO . + HO .  H O + O
2
2
2
2
2
Control strategies for ozone
• O3 is a secondary pollutant control of O3
requires control of its precursors.
• Control of VOCs
– In certain areas, VOCs from biological sources
could be significant.
• Control of NOx
– Difficult to control as efficient energy conversion
requires high combustion temperature.
In mid-1990s, breathing the air in
Mexico City was like smoking 2
packs of cigarettes a day.
Are we safe at home?
Indoor air pollution: Poor countries
Indoor Air Pollution and Children's Environmental Health
MOSQUITO COILS
Household use in Asia, Africa, South
America
Major Active Ingredient –pyrethrins
 0.3-0.4% of mass of coil
 “Inert” ingredients >99% of mass of coil
Long-term exposures linked to epidemiologically
 Increased asthma, persistent wheeze in children
Smoke tested contained particulates, all < 1 micron
 PAHs, carbonyls including formaldehyde
 Burning 1 coil  PM25 as if burning 75-137 cigarettes
Same amount of formaldehyde as 51
cigarettes
Indoor Air Pollution and Children's Environmental Health
Indoor Air Pollution and Children's Environmental Health
Indoor Air Pollution and Children's Environmental Health
Indoor Air Pollution and Children's Environmental Health