Environmental Chemistry
Chapter 15:
Atmospheric Chemistry
Copyright © 2011 by DBS
Contents
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Chemical and Photochemical Reactions in the Atmosphere
Free Radicals
Acid-Base Reactions in the Atmosphere
Inorganic Species in the Atmosphere
Particles in the Atmosphere
The Composition of Inorganic Particles
Carbon Oxides
Sulfur Dioxide Sources and the Sulfur Cycle
Nitrogen Oxides in the Atmosphere
Fluorine, Chlorine and Other Gaseous Compounds
Hydrogen Sulfide, Carbonyl Sulfide, and Carbon Disulfide
Organics in the Atmosphere
Organic Compounds from Natural Sources
Pollutant Hydrocarbons
Nonhydrocarbon Organic Compounds in the Atmosphere
Chemical and Photochemical Reactions
in the Atmosphere
Chemical and Photochemical Reactions
in the Atmosphere
Figure 15.1. Important Aspects of Atmospheric Chemical Processes
Chemical and Photochemical Reactions
in the Atmosphere
Important Atmospheric Chemical Species
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Inorganic oxides: CO, CO2, NO2, SO2
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Oxidants: O3, H2O2, HO• radical, HO2• radical, ROO• radicals, NO3 radical
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Reductants: CO, SO2, H2S
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Hydrocarbons: Natural CH4, pollutant alkanes, alkenes, aromatics
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Oxidized organics: Aldehydes, ketones, acids, organic nitrates
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Photochemically active species: NO2, formaldehyde
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Acids: H2SO4, H2SO3, HNO3
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Bases: NH3
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Salts: NH4HSO4
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Unstable reactive species: Electronically excited nitrogen dioxide (NO2*), HO•
Solid and liquid particles in aerosols and clouds
• Sources and sinks for gas-phase species
• Sites for surface reactions on solids
• Aqueous phase reactions in water droplets
Chemical and Photochemical Reactions
in the Atmosphere
Two Very Important Factors in Atmospheric Chemistry:
(i) Radiant solar energy
– Photons put high energy into individual molecules
(ii) Hydroxyl radical, HO•
– Most important highly reactive intermediate
Chemical and Photochemical Reactions
in the Atmosphere
Photochemical Processes
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From photons of energetic solar electromagnetic radiation, h
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Produce electronically excited species designated *
NO2 + h  NO2*
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Excited species tend to be highly reactive in the atmosphere
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Two other reactive species
– Free radicals with unpaired electrons: H3C• , HO•
– Ions such as O+ (uncommon in lower atmosphere)
Chemical and Photochemical Reactions
in the Atmosphere
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Electronically Excited Species
– Absorption of a photon, usually of ultraviolet radiation, can energize
molecules, atoms, or radicals to electronically excited states
Figure 15.2. Electronically excited states where the arrows represent
directions of electron spin
Chemical and Photochemical Reactions
in the Atmosphere
Loss of Excitation Energy from Electronically Excited Species
(i) Emission of a photon (light): NO2*  NO2 + h
Called luminescence if instantaneous, phosphorescence if slower and
Chemiluminescence when the excited species that emits a photon is formed as the
result of a chemical reaction
O3 + NO  O2 + NO2* (luminescent species)
(ii) Direct reaction of an excited species
O2* + O3  2O2 + O
(iii) Dissociation
NO2*  NO + O (very important tropospheric reaction)
O2*  O + O (important in stratosphere leading to O3)
(iv) Photoionization (formation of ions in the ionsphere)
N2*  N2+ + e-
Free Radicals
Free Radicals
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Free radicals are atoms, molecules, or molecular fragments with unpaired
electrons designated •
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Free radicals are highly chemically reactive because of the strong pairing
tendency of their unpaired electrons
– Undergo series of chain reactions generating more free radicals
– Chain termination such as H3C• + H3C•  C2H6
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Although highly chemically reactive, free radicals can be quite stable in the
upper atmosphere where there are few other species with which they can react
Free Radicals
Hydroxyl (HO•) and Hydroperoxyl (HOO•) Radicals
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HO• is the single most important intermediate species in atmospheric chemical processes
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HO• formed by photolysis of water at higher altitudes
H2O + h  HO• + H•
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As the result of ozone photolysis in lower atmosphere
O3 + h ( < 315 nm)  O* + O2
O* + H2O  2HO•
Others:
O* + CH4 → HO + CH3OH
HNO2 → HO + NO
H2O2 + h → 2HO
Free Radicals
Hydroxyl (HO•) and Hydroperoxyl (HOO•) Radicals
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HO• reacts with many electron rich (multiple bonded) species in the atmosphere
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CO
SO2
NO
HO• reacts can also react by abstraction of hydrogen, e.g.
CH4 + •OH → •CH3 + H2O
NH3 + •OH → •NH2 + H2O
H2S + •OH → •SH + H2O
CH3Cl + •OH → •CH2Cl + H2O
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HO• commonly removed by reaction with CO or CH4
HO• + CO  CO2 + H•
HO• + CH4  H3C• + H2O
Free Radicals
Hydroxyl (HO•) and Hydroperoxyl (HOO•) Radicals
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Predominant fate of methyl radical is ‘add-on’ reaction with O2,
e.g. •CH3 + O2 → CH3OO•
HOO• / HO2• (hydroperoxy) and CH3OO• are called peroxy radicals
- Less reactive than other radicals - Do not readily abstract H
- Do not react with O atoms due to low conc.
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Main reactions:
HOO• + NO• → •OH + NO2•
R-OO• + NO• → RO• + NO2•
(where R = carbon chain)
Free Radicals
Oxidation of CH4
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CH4 produced in inefficient (anaerobic) burning of fuels
Predominant HC in atmosphere
No multiple bonds
Not soluble in water, does not absorb sunlight
Slow oxidation initiated by hydroxyl radical
(hydrogen abstraction reaction)
CH4 + •OH → •CH3 + H2O
•CH3 + O2 → •CH3OO•
CH3OO• + NO → CH3O• + NO2
CH3O• + O2 → H2CO + HOO•
abstraction
O2 adds forming peroxy
transfer of O
O2 abstracts H
…conversion of methane to formaldehyde
H2CO + UV-A (338 nm) → H• + HCO•
H• + O2 → HOO•
HCO• + O2 → CO +HOO•
unstable
O2 abstracts
O2 abstracts
Note: CO is a stable intermediate and can further
undergo transformations
C
O + OH• → HO-C=O
H-O-C=O + O2 → O=C=O + HOO•
….. Production of CO2 as the final product
CH4 + 5O2+ NO + 2OH• + UV-A →
CO2 + H2O + NO2 + 4HOO•
Notice the radicals consumed
and produced.
What happens to the HO2
produced?
What happens to the NO2
produced?
(see fate of free radicals)
Acid-Base Reactions in the Atmosphere
Acid-Base Reactions in the Atmosphere
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Rainwater is naturally slightly acidic
CO2(g) ⇌ CO2(aq)
CO2(aq) + H2O ⇌ H+ + HCO3-
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Stronger acids from acid gases such as SO2
SO2 + H2O ⇌ H+ + HSO3-
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Strong pollutant acids
H2SO4 HNO3 HCl
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Bases are relatively less important than acids in the atmosphere, usually from
ash and ground rock (calcium oxide, hydroxide and carbonate)
Acid-Base Reactions in the Atmosphere
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Ammonia, the most important base in the atmosphere
– NH3 from bacterial action
NO3-(aq) + 2{CH2O}(biomass) + H+ → NH3(g) + 2CO2 + H2O
– NH3 from industrial pollutant sources:
Ammonia manufacture, coke production, refrigeration systems
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Ammonia reacts with acidic gases to produce corrosive salts
NH3(aq) + HNO3(aq)  NH4NO3(aq)
Inorganic Species in the Atmosphere
Inorganic Species in the Atmosphere
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CO2 is the most abundant inorganic compound other than water in the
atmosphere
– Natural constituent
– Pollutant in the sense that excess causes excessive global
warming
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Oxides of carbon (CO), sulfur, and nitrogen are important inorganic air
pollutants
– Oxides of S and N cause pollutant acid precipitation
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Other inorganic pollutants include NH3, HCl, H2S
Particles in the Atmosphere
Particulate Matter
A complex mixture of solid particles and liquid droplets found in the air
Sizes of Common Airborne Particles
e.g NH4Cl,
SO42- / NO3- salts
Natural: forest fires,
volcanoes etc.
Man-made: fossil-fuel
combustion, industry
Sources
Mineral dust from
weathering of rocks
and soils
Chemical composition
can be used to ID
source and fate
Particles in the Atmosphere
Table 15.1 Terms Used to Describe Atmospheric Particles
Term
Condensation aerosol
Meaning
Formed by condensation of vapors or reactions of gases
Aerosol
Colloidal-sized atmospheric particles
Dispersion aerosol
Formed by grinding of solids, atomization of liquids, or
dispersion of dusts
Fog
Denotes high level of water droplets
Haze
Decreased visibility due to particles
Mists
Liquid particles
Smoke
Particles from incomplete fuel combustion
Particles in the Atmosphere
Chemical Processes for Inorganic Particle Formation
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Solid oxides from inorganic solids in fuels, e.g. coal
3FeS2 + 8O2  Fe3O4(s) + 6SO2(g)
CaCO3 + heat  CaO(s) + CO2(g)
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Gases reacting to produce liquid-forming compounds
2SO2(g) + O2 + 2H2O  2H2SO4(aq)
(hydroscopic, forms aerosol droplets)
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Salt formation
H2SO4(droplet) + 2NH3(g)  (NH4)2SO4(droplet)
Particles in the Atmosphere
Reactions Involving Particles
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Provide active surfaces upon which heterogeneous reactions can occur
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Nucleation bodies for the condensation of water vapor
Particles in the Atmosphere
Reactions Involving Particles (Figure 15.3)
The Composition of Inorganic Particles
The Composition of Inorganic Particles
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Example: Formation of particulate bound
atmospheric Na2SO4:
2SO2(g) + O2 + 2H2O  2H2SO4(aq)
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Followed by reaction with particulate NaCl
from ocean spray containing sea salt
H2SO4 + 2NaCl(particulate) 
Na2SO4(particulate) + 2HCl
Particulate high in SO4- but low in Cl- would
indicate ocean spray origin, plus man-made
S content
Figure 15.4. Inorganic Materials and their Origins in Particles
Chemical composition can be
used to ID source and fate
The Composition of Inorganic Particles
Origins of Some Atmospheric Particle Constituents
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Al, Fe, Ca, Si: Soil erosion, rock dust, coal combustion
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C: Incomplete combustion of carbonaceous fuels
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Na, Cl: Marine aerosols, chloride from combustion of organohalide polymers
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Sb, Se: Volatile elements from combustion of oil, coal, refuse
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V combustion of some kinds residual petroleum
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Zn: Combustion sources
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Pb: Combustion of lead-contaminated materials
The Composition of Inorganic Particles
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Fly ash
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Much of the mineral particulate matter in polluted atmosphere is in the form of oxides (aluminum,
calcium, iron and silicon) and other compounds (soot and carbon black) from combustion of fossil
fuels
Toxic metals
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Lead formerly in leaded fuels
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Cadmium and mercury in batteries that are burned
Radioactive particles
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Largely polonium from decay of naturally occurring radon gas
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Man-made from combustion of fossil fuels (fly ash)
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Above ground nuclear testing
Carbon Oxides
Carbon Oxides
Carbon monoxide, CO
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From partial combustion of carbon-containing fuels
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Toxic by reaction with blood hemoglobin
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Localized pollutant, such as in high-traffic areas
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Destroyed by reaction with hydroxyl radical: CO + HO•  CO2 + H•
Carbon dioxide, CO2
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Natural atmospheric constituent, from aerobic respiration, volcanoes, rocks
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IR absorption responsible for atmospheric greenhouse effect
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Excessive amounts of fossil fuel burning will probably cause global warming
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Now at about 380 ppm volume
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Increasing more than 1 ppm/year
Global Warming and the Greenhouse Effect
1958: Keeling began measuring CO2
at Mauna Loa, HI
Carbon Oxides
What’s Up With the Weather (2000)
Carbon Oxides
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What is the significance of the Keeling curve?
What could be
responsible for this
seasonal up-down
fluctuation?
Since 1958 atmospheric
carbon dioxide has risen
by more than 15%
http://www.cmdl.noaa.gov/ccgg/index.html
Carbon Oxides
Fate of Atmospheric Carbon Dioxide (sinks)
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Incorporated into biomass by photosynthesis
CO2 + H2O + sunlight energy  {CH2O} + O2
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Dissolving and reacting in water
CO2 + H2O  H+ + HCO3-
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Reaction with carbonate minerals
CO2 + H2O + CaCO3  Ca2+ + 2HCO3-
Sulfur Dioxide Sources
and the Sulfur Cycle
Sulfur Dioxide Sources
and the Sulfur Cycle
Sulfur Dioxide Sources
and the Sulfur Cycle
Sulfur Dioxide Reactions in the Atmosphere
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Influenced by many factors
– Temperature, humidity, light intensity, atmospheric transport
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Results in formation particulate matter
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Ultimately oxidized to sulfuric acid and sulfates
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Effects of Atmospheric Sulfur Dioxide
– Affects respiration
– Phytotoxic
– Acidifies precipitation
– Particulate sulfuric acid and sulfate salts
Nitrogen Oxides in the Atmosphere
Nitrogen Oxides in the Atmosphere
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Three oxides of N:
– Nitrous oxide N2O
– Nitrogen monoxide NO
– Nitrogen dioxide NO2
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Microbially generated N2O is relatively unreactive in the atmosphere
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Photolyzed in the stratosphere:
N2O + hv → N2 + O
Also:
N2O + O* → N2 + O2
N2O + O* → NO + NO
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NO participates in stratospheric ozone cycles (see later)
NOx Cycle (simplified)
NOx Cycle
Removes O3
SINK
SOURCE
HNO3 (inert)
N2O + O → 2NO
i.e. inactive until
transported
RAIN OUT
RESERVOIR
Removal negligable (few
reactions except O)
N2O, N2
Agriculture NO3-, NO2-
Mankind can alter stratospheric O3 without leaving the ground
Nitrogen Oxides in the Atmosphere
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NO and NO2 = NOx
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Natural sources – lightning, biological
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Man-made – burning fossil fuels, automobiles
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Most NO2 is derived from NO generated at high temperatures from internal
combustion engines
N2 + O2 → 2NO
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Atmospheric conversion from NO to NO2 is rapid
Nitrogen Oxides in the Atmosphere
Atmospheric Reactions of NOX (Figure 15.9)
Fluorine, Chlorine, and Other Gaseous
Compounds
Fluorine, Chlorine, and Other Gaseous
Compounds
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HF and F2 are extremely toxic
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Gaseous fluorides are highly phytotoxic
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Silicon tetrafluoride, SiF4, generated by reactions used in metal smelting
operations
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Sulfur hexafluoride, SF6, is a powerful greenhouse gas used as an atmospheric
tracer
– Global warming potential per molecule around 24,000 x CO2
– Extremely stable, lifetime around 3000 years
– Probably destroyed in the ionsphere by reaction with free e– Now at levels of around 0.3 parts per trillion
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Elemental chlorine, Cl2, is very toxic
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Hydrogen chloride, HCl, contributes to acid rain
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Produced in combustion of organochlorine polymers
Hydrogen Sulfide, Carbonyl Sulfide, and
Carbon Disulfide
Hydrogen Sulfide, Carbonyl Sulfide, and
Carbon Disulfide
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Hydrogen sulfide, H2S, from microbial decay of organosulfur compounds, microbial
reduction of sulfate, natural gas contaminant
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H2S is very toxic
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22 people killed in Poza Rica, Mexico, 1950
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242 fatalities in China in 2003
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Very phytotoxic to some plants
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H2S damages some materials such as exposed copper metal
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Eventually oxidized to sulfate in the atmosphere
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Carbonyl sulfide, COS, emitted by marine phytoplankton
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• Greatest source of atmospheric sulfur
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• Reacts only slowly, eventually producing sulfate
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Carbon disulfide, CS2, produced by some marine phytoplankton
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• Does not last long due to reaction with HO•
Organics in the Atmosphere
Organics in the Atmosphere
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Two important aspects
– Reactions resulting from photon absorption, h
– Importance of reactions with hydroxyl radical, HO•
Organic Compounds from Natural
Sources
Organic Compounds from Natural
Sources
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Natural sources contribute 7/8 ths of total organics to atmosphere
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Huge amounts of methane produced by anaerobic bacteria
2{CH2O} (bacterial action) →CO2(g) + CH4(g)
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Also methane from domesticated animals
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Methane from rice fields
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HC’s produced by living sources are called biogenic hydrocarbons
– Vegetation, microorganisms, forest fires, animal wastes, volcanoes
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Vegetation most important source of non-methane hydrocarbons (NMHCs)
Organic Compounds from Natural
Sources
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Most HCs emitted by plants are terpenes
Fig. 15.8: Some common terpenes emmitted to the atmosphere by vegetation
Pollutant Hydrocarbons
Pollutant Hydrocarbons
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Hydrocarbons are abundant air pollutants because of their widespread use
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• Fuels
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Alkanes, such as 2,2,3-trimethylbutane
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Alkenes such as ethylene and propylene
• Polymer manufacture
• Industrial chemicals
• Solvents
Pollutant Hydrocarbons
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Alkynes such as acetylene
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Aromatics such as napthalene
Hydrocarbons emitted as engine exhaust byproducts tend to be relatively more reactive because
of the unsaturated compounds in them
Pollutant Hydrocarbons
Figure 15.12. Aromatic hydrocarbons found in the atmosphere; the first six compounds are among the
top 50 chemicals manufactured and the last two are multicyclic aromatics (PAHs)
Nonhydrocarbon Organic Compounds in
the Atmosphere
Nonhydrocarbon Organic Compounds in
the Atmosphere
Aldehydes and Ketones
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Formaldehyde (right) is the simplest aldehyde, a photochemically reactive
species that may be emitted directly or formed by secondary atmospheric
reactions
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Unlike most air pollutants, some aldehydes may undergo direct photochemical
reactions by absorbing h
Formaldehyde
Nonhydrocarbon Organic Compounds in
the Atmosphere
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Aldehydes and Ketones
Figure 15.13. Some Aldehydes and Ketones that May be Found in the Atmosphere
Nonhydrocarbon Organic Compounds in
the Atmosphere
Miscellaneous Oxygen-Containing Compounds
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• Aliphatic alcohols (R-OH) • Phenols (Ar-OH) • Ethers (R-O-R’)
• Carboxylic acids (R-COOH)
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Among alcohols, methanol, ethanol, isopropanol and ethylene glycol rank
among top 50 chemicals
– Methanol and ethanol in atmosphere because of volatility, but are removed
due to high water solubility
– Increased levels of ethanol because of use in gasoline
Nonhydrocarbon Organic Compounds in
the Atmosphere
Miscellaneous Oxygen-Containing Compounds
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Phenol is among the top 50 chemicals produced
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Two examples of common ethers that may be found in the atmosphere (right);
MTBE is being phased out of use in gasoline due to water solubility
Nonhydrocarbon Organic Compounds in
the Atmosphere
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Organo-oxygen Compounds (Cont.)
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Ethylene oxide and propylene oxide are among the top 50 produced chemicals; ethylene
oxide is relatively toxic
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Carboxylic acids (R-COOH)
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Final oxidation products of hydrocarbons in the atmosphere
– Relatively non-volatile, so they tend to occur in particles
– Lower acids (formic and acetic acids) are water-soluble and removed with rainfall
Nonhydrocarbon Organic Compounds in
the Atmosphere
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Organohalides (Figure 15.14)
• Dichloromethane is a very volatile solvent
– Vinyl chloride used to make polyvinylchloride plastic, carcinogen
– Trichloroethylene widely used as a dry cleaning solvent
• PCBs are notorious water pollutants
– Once widely used in electrical applications, as hydraulic fluids, other
– Highly persistent and bioaccumulative, manufacture now banned
– Hudson River sediments badly contaminated due to dumping from industrial
equipment manufacture
Nonhydrocarbon Organic Compounds in
the Atmosphere
Chlorofluorocarbons (Freons)
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Compounds such as dichlorodifluoromethane in which all Hs have been replaced by Cl and F
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Extreme stability
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Very low toxicity
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Now constituents of the global atmosphere
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Banned because of destruction of stratospheric ozone
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Replaced by compounds with at least one H-C bond
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H-C bond attacked by tropospheric HO•
Halons, such as Halon-1211, CBrClF2
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Fire extinguishers on aircraft
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Damage stratospheric ozone
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Much lower production levels than past chlorofluorocarbons
Nonhydrocarbon Organic Compounds in
the Atmosphere
Organosulfur Compounds
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Some, such as thiols, notable for bad odors
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Dimethylsulfide released in large quantities by marine phytoplankton
Figure 15.13. Common Organosulfur Compounds Associated with Air Pollution
Nonhydrocarbon Organic Compounds in
the Atmosphere
Organonitrogen Compounds
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• Amines
• Amides
• Nitriles
• Nitro compounds
• Heterocyclics
• Some amines, such as the methylamines, are widely used, toxic, noxious substances
– Some aromatic amines are known human carcinogens
– Some oxygenated -NO2 compounds, such as peroxyacetyl nitrate (PAN, Chapter 16), are
strong oxidants that are harmful air pollutants
Figure 15.16. Potential Air Pollutant Organonitrogen Compounds
Nonhydrocarbon Organic Compounds in
the Atmosphere
Organonitrogen Compounds
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Some oxygenated -NO2 compounds, such as peroxyacetyl nitrate (PAN, Chapter 16), are
strong oxidants that are harmful air pollutants
(acetyl)
(peroxyacetylnitrate)
Peroxyacetylnitrate is eye irritant and toxic to plants