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Ch. 20: Chemistry in Atmosphere
Terms in this set (144)
Composition of dry air
N2 & O2
Minor part in air, major role in global warming
Concentration of water vapor in air varies by location
Close to 0 in desert
As much as 5% in tropical rain forest
Gas that we cannot see
Steam and clouds
Condensed water vapor: tiny droplets of liquid water; Not water vapor;
6 major air pollutants
1. carbon monoxide (CO)
2. nitrogen dioxide (NO2)
3. ozone (O3)
4. sulfur dioxide (SO2)
5. particulate matter (PM: PM10 & PM2.5)
6. lead (Pb)
CO, NO2, O3, and SO2
Minuscule suspended particles
PM and Pb
dust, soot, dirt and even microscopic droplets of liquid, bacteria, or viruses
average diameter of 10 μm or less, which is on the order of 0.0004 inches
average diameter less than 2.5 μm and thus is also called fine particles
labeled EPA as air pollutants or criteria pollutants
dramatic decrease in air quality since 1980s
Environmental Protection Agency
Nicknamed as "the silent killer." It is odorless and can be detected around auto exhaust or furnace emissions in a confined space. Once in the lungs, it enters the bloodstream and disrupts the delivery of oxygen throughout the body.
Sharp odor. It can be detected around copiers, electric motors, transformers, or welding apparatus. It is toxic. Even at very low concentration it can reduce lung function in healthy people. Symptoms of ozone inhalation include chest pain, coughing, sneezing, and pulmonary congestion.
bad pollutant in troposphere, good in protecting life on Earth from harmful UV rays in the stratosphere
0-15 km; directly above the Earth's surface
15-30 km; lies above the troposphere; includes ozone layer
Region above an altitude of 50 km
Boundaries of atmosphere
Not sharp. Defined by the pattern of temperature changes. The atmosphere is a continuum with gradually changing composition, concentrations, pressure, and temperature.
Air gets thinner. 100km+ atmosphere fades into almost perfect vacuum of outer space
SO2, NO2, PM, VOC
Directly emitted into the atmosphere
Two major sources of primary pollutants
1. coal-fired electric power plants
2. motor vehicles
Major source of SO2 emissions
Major source of CO, NO2, and VOC emissions
Emissions from tailpipes of motor vehicles
Fate of SO2
Emission of SO2 from burning coal → reaction of SO2 with O2 to form sulfur trioxide, SO3 → dissolution of SO3 in water droplets in the air to produce sulfuric acid, H2SO4.
This sulfuric acid formed in the atmosphere exists in aerosol droplets, which are small enough to become trapped in the lung tissue, upon inhalation, and cause severe damage. This sulfuric acid is also one of the main contributors to acid rain.
Burning coal → SO2
SO2 + O2 → SO3
SO3 + H2O → H2SO4
Consist of particles, both liquid and solid, that remain suspended in the air rather than settling out. Smoke is a familiar aerosol.
Clean Air Act
SO2 emissions in US declining
devices installed in the exhaust systems of automobiles to reduce emissions, change CO to CO2, NO2 to N2 and O2, reducing the emissions of CO and NO2 to the air, lower the VOCs emissions by burning them with oxygen which is in exhaust stream unused in the engine cylinders
VOCs (volatile organic compounds)
Fast acceleration and high speed driving lead to incomplete combustion of gasoline (hydrocarbons), which results in the increased emissions of CO and unburned hydrocarbons
exists in many chemical forms and is very toxic, causing neurological problems, especially in children; leaded gasoline is completely banned in the US in 1997 but has yet to be banned globally
curbing the emissions of SO2 and Pb
curbing emissions of nitrogen oxides (NO, NO2, etc)
Reason for inefficient curbing of emissions of nitrogen oxides
nitrogen gas (N2) and oxygen gas (O2) are anywhere in the air. Nitrogen gas (N2) is not reactive at normal temperature. But whenever air is subjected to high temperatures (as in car engines or in coal-fired power plants) N2 and O2 combine to form NO which is very reactive, unlike N2.
Fate of NO
(1) NO + O2 → NO2 But this is not a favorable path, because it requires high concentration of NO to proceed quickly. The concentration of NO even in polluted air is not high enough to follow this path.
(2) VOC + .OH → A
A + O2 → A'
A' + NO → A" + NO2
Through these complex paths NO converts to NO2 and this is a favorable route.
In short, if the air contains sufficient concentrations of NO, O2, VOCs, and .OH, you have the right ingredients to form NO2.
toxic and a player in the formation of ozone in the troposphere and one of the major contributors to acid rain
does not come out of automobile exhaust or of coal-burning
How is ozone produced?
It is produced from chemical reactions among two or more other pollutants, in this case, VOC and NO2. Because of these indirect sources for ozone, ozone is called a secondary pollutant.
Pathways leading to the formation of ozone
NO2 → NO + O
O + O2 → O3
crucial to the formation of ozone
Ozone formation requires..
O, which in turn is produced when sunlight splits NO2.This is the reason why ozone is linked to sunlight.
High levels of ozone
More likely to occur on long sunny summer days, especially in a congested urban area. Stagnant air can increase the buildup of air pollutants emitting from vehicles such as NO, VOCs, and NO2, all supportive to the buildup ozone. Ozone attacks rubber, thus damaging the tires of the vehicles that led to its production in the first place
Ozone when the sun sets
Once the Sun goes down, the ozone concentrations drop off sharply, because ozone is consumed in just a matter of hours. It reacts with many things, including animal and plant tissues
consists of particles called photons which propagate through the space like waves with various wavelengths. The Sun bombards earth with countless photons carrying varying amounts of energy as they have different wavelengths such as UV, Visible, Infrared, etc.
Order of wavelength of solar radiation
shorter < UV < Visible light < Infrared < longer
The shorter the wavelength of solar radiation
The greater the energy it carries
Energy of solar radiation
smaller < Infrared < Visible < UV < bigger
ultra violet radiation of sunlight having wavelength 200 nm - 400 nm; photons in the UV light are sufficiently energetic to displace electrons within neutral molecules, converting them into positively charged species. Even UV photons of shorter wavelength break bonds, causing molecules to come apart. In living things, such changes disrupt cells and create cancer
visible sunlight having wavelength 400 nm - 700 nm; photons in the visible light hit the cells of our retinas, excite electrons in the molecules in our body, which in turn trigger a series of complex chemical reactions that ultimately lead to "seeing."
infrared radiation of sunlight having wavelength 700 nm - 4000 nm; IR warms Earth, causing molecules to move, rotate, and vibrate. Photons in the IR light carry energies that are not strong enough to break bonds in chemical species.
nano meter = 10-9 m = 0.0000004 inch
a chemical species having unpaired electrons; this status of having unpaired electrons makes free radical unstable; to avoid this unstable status, free radical strives to achieve a status of having paired electrons by reacting with other chemical species; thus free radical is very reactive.
Formation of ozone in stratosphere
As oxygen gas absorbs energy from lightning, it becomes ozone: 3O2 + energy → 2O3
Role of ozone in stratosphere
Filters ultraviolet light from the Sun. So ozone protects us from damaging solar radiation.
the stratospheric region of maximum ozone concentration. Don't take the word "layer" literally as thick, fluffy blanket. The total amount of ozone in the layer is surprisingly small because at this altitude the air is very thin. Though having small concentration, however, the ozone layer still plays a critical role in keeping the UV light from reaching the earth surface.
Status quo with ozone
there is a significant depletion of ozone in the stratosphere. As a consequence, we are more exposed to dangerous UV radiation which can cause skin cancer.
Process by which ozone protects us from damaging solar radiation involves..
the interaction of matter and energy from the Sun
Solar UV radiation is greatly diminished by
passing through oxygen and particularly through ozone in the stratosphere
Types and characteristics of UV radiation
UV Radiation, Wavelength Range (nm), Energy, Damage to Us
UV-A: 320~400nm, least, least
UV-B: 280~320nm, medium, medium
UV-C: 200~280nm, most, most
Filtering of UV by oxygen
Photons in the UV wavelengths of 200 ~ 280 nm (UV-C) have the strongest energy among the UV radiations. Thus they can break the O=O bond in O2 which is stronger than the bond in O3
Eq 1: UV photons
O2 → 2O
The meaning of the photons in the UV-C light breaking the O2 bond
they are being absorbed onto O2, resulting in disappearance (filtering) of this specific UV radiation. O2 filters only the UV-C radiation.
Filtering of UV by ozone
Radiations of UV-A and UV-B, having less energy than UV-C and yet still posing damage to us, are filtered by ozone that has less bond energy than O2.
O3 → 2O2 + O UV-A & UV-B wavelength
The meaning of photons in the UV-A & UV-B light breaking the O3 bond
they are being absorbed onto O3, resulting in disappearance (filtering) of these specific UV radiations
Rate of production and destruction in the natural cycle are the same. The overall concentration of ozone remains constant in the natural cycle.
it shows the natural steady state processes in which ozone is both formed and destroyed
Natural production of ozone in Chapman
triggered when an O2 molecule absorbs a photon of the UV light.
Natural destruction of ozone in Chapman
takes place when an oxygen atom slowly collides with an ozone molecule
Steady state distribution of stratospheric ozone by Chapman
is severely disturbed by human activities.
Average stratospheric ozone concentration
has dropped significantly in the last 20-30 years. The striking decrease in spring stratospheric ozone has been observed in Antarctica over the last 40 years.
The area having ozone less than 220 DU
Explain the depletions
The extent of these ozone depletions cannot be explained by O3 destruction in the natural cycle, because natural processes are insufficient to cause or explain the magnitude of the abnormally large decrease in ozone.
Agents mainly responsible for the stratospheric ozone depletion
CFCs (chlorofluorocarbons) which are compounds composed of the elements chlorine, fluorine, and carbon.
do not occur in nature, because there are no known natural sources of CFCs; they are 100 % man-made chemicals and are used for propellants in spray cans, cooling systems, sterilizers for surgical instruments, etc. Two of the most widely used CFCs are Freon 11 and Freon 12.
Freon 11 (CFC-11) trichlorofluoromethane (CCl3F)
Freon 12 (CFC-12) dichlorodifluoromethane (CCl2F2)
Other contributors to the destruction of ozone
the free radicals of ∙OH and ∙NO. But they are formed in the atmosphere from both natural sources and human activities.
have strong bonds, so the molecules can remain for long periods. When they eventually reach stratosphere, the high-energy photons of UV-C can break the bonds of CFCs
Free radical CL-
CFCs are known to destroy stratospheric ozone via several pathways: the free radical Cl∙ in Eq. 3 pulls away an oxygen atom away from the O3 molecule, froms chlorine monoxide, ClO∙, and leaves an O2 molecule,
2Cl∙ + 2O3 → 2ClO∙ + 2O2 (Eq. A)
Then these free radical ClO∙ join themselves to form ClOOCl,
2ClO∙ → ClOOCl (Eq. B)
Now ClOOCl decomposes in the two step sequence,
ClOOCl → ClOO∙ + ∙Cl (Eq. C)
ClOO∙ → Cl∙ + O2 (Eq. D)
Adding chemical equations (A), (B), (C), and (D) removes all species in red, leaving only the two species in the following net equation:
2O3 → 3O2 (Eq. E)
Remember free radicals are very reactive; this nature of free radicals make this sequential reactions possible
Montreal Protocol 1987
initiated regulatory measures to protect the ozone layer. Ensuing meetings in various parts of the world over the past twenty years have implemented more stringent policies.
Key initial strategy for reducing chlorine in stratosphere
stop production of CFCs
189 countries have now ratified the Montreal Protocol
They have reaffirmed that the production of CFCs and other fully halogenated CFCs is to be eliminated by 2010 by all parties, no matter the basic domestic economic needs.
(short wavelength) breaks bonds, causing molecules to come apart. This is because photons in UV light carry high energies
(long wavelength) cannot break bonds, instead it causes molecules to vibrate. This is because photons in IR light carry low energies
In the atmosphere..
molecules absorbing IR light vibrate for a while, and then reemit the light energy as heat. This is how IR light warms the atmosphere
molecules capable of absorbing and reemitting IR light, causing the atmosphere to warm up
Most important greenhouse gases
CO2 and H2O
Other greenhouse gases
CH4 (methane), N2O (nitrous oxide), O3 (ozone), CFCs, etc.
N2 and O2
are the predominant gases in the atmosphere, they are not greenhouse gases
46 % of the incoming radiation from the Sun is absorbed by Earth's continents and oceans, warming them. Earth, in turn, radiates some of its absorbed energy back into the atmosphere in the form of IR radiation (37 %). It is this IR radiation that greenhouse gases efficiently absorb and reemit. Therefore, about 80 % [i.e., (37/46)x100] of incoming solar radiation that strikes the earth remains in the atmosphere and does not directly escape into space. IR radiation is thus trapped in the surface earth (i.e., atmosphere), warming the earth.
This trapping of IR light is known as the greenhouse effect, the process by which atmospheric gases trap and return major portion of the heat (IR radiation) radiated by the earth.
Vibrating molecules and the greenhouse gas effect
Photons in IR light do not have enough energy to break the bonds in chemical species, instead it can cause them to vibrate. Depending on the molecular structure, only certain vibrations are possible. Only when a specific IR light energy with a specific wavelength is absorbed by a molecule, a specific vibration can occur within the molecule. After the vibration the molecule reemits the absorbed energy as heat, causing the surrounding to warm up.
vibration of molecules
the vibration of bonds in molecules
Two types of molecular vibration
stretching vibration: requires more energy than bending. IR photons with shorter wavelengths cause stretching
bending vibration: IR photons with longer wavelengths cause bending
If stretching of a molecule occurs in a way that the changes in charge distributions cancel each other due to its symmetrical electron distribution, absorption of IR light into the molecule does not occur. No absorption of IR light then means no greenhouse effect. Molecules of N2 and O2 are the typical examples showing this kind of stretching. These molecules do not establish any change in charge distribution during vibration due to their symmetrical electron distributions. No absorption of IR light occurs in these molecules. Therefore, N2 and O2 are non-greenhouse gases
CO2, H2O, CH4 (methane), N2O (nitrous oxide), O3 (ozone), CFCs, etc.
are greenhouse gases because they absorb IR light and do vibrate and remit the light as heat
has been fluctuating between ice age (glacial period) and non ice age (interglacial period)
around 20,000 years ago, avg temp was about 9C below the 1950-1980 average
around 130,000 years ago, and over this period the average temperature was about 2C above the 1950-1980 average
Correlation between earth temp and CO2 concentration
when the CO2 concentration was high, the temperature was high. Interglacial atmosphere had high CO2 and glacial atmosphere had low CO2. So it is not difficult to predict that ever increasing CO2 in today's atmosphere will lead to global warming
During the first 3 billion years of the 4.5 billion year old earth history, there was no oxygen in the atmosphere, and CO2 was the major gas with its concentration perhaps 1000 times greater than now. Thus the temperature of the early earth must have been much warmer than now
Reduction of CO2 in primitive atmosphere
was also achieved by the advent of organisms capable of doing photosynthesis. Photosynthesis takes up CO2 and releases O2. Thus the high CO2 concentration in the primitive atmosphere began to decrease, and at the same time O2 began to accumulate in the atmosphere
High CO2 in primitive atmosphere
dissolved in ocean water and became incorporated in rocks such as limestone (CaCO3). And the reservoir that contains most CO2 even in today's earth system is still the deep ocean water.
Around 100 million years ago (the Jurassic period), when dinosaurs were roaming, the average temperature is estimated to have been 10 - 15 oC warmer than it is today. So, the Jurassic atmosphere must have had much higher CO2 concentration than now. Does this mean that the Jurassic earth used to have even more developed civilization than now, consuming more fossil fuels and thus releasing more CO2 than now? Obviously the answer is no, because we know there was no civilization over this period. The increased CO2 over the Jurassic was entirely due to natural processes.
Natural process during the Jurassic period
an increased upwelling of the deep ocean water to the surface, thus accelerating the release of CO2 from the ocean water to the atmosphere.
Natural processes adding CO2 to the atmosphere
decaying of organic matter (respiration); release from ocean (air-sea interaction); formation of rocks containing carbons (carbonate minerals) in the oceans.
Natural processes removing CO2 from atmosphere
photosynthesis; direct sink to ocean from atmosphere (air-sea interaction).
Human perturbation adding CO2 to atmosphere
burning fossil fuels (coal, petroleum, natural gas); deforestation
Human perturbation removing CO2 from atmosphere
Accumulation in atmosphere of CO2 caused by
Natural removal processes do not respond quickly enough to the increased amount of CO2
H2O, CH4 (methane), N2O (nitrous oxide), O3 (ozone), CFCs, and others are greenhouse gases because
they absorb IR light and vibrate and remit the light as heat.
Greenhouse gases with global warming potential
CO2, CH4, and N2O
Ability to trap IR light
Methane (CH4) is at least 20 times more effective than CO2 in its ability to trap IR light
CO2 more important role in global warming than CH4 (methane) because
(1) CO2 concentration (385 ppm) is much higher than CH4 (1.75 ppm), and (2) CO2 has much longer life time (50 ~ 200 years) in atmosphere than CH4 (12 years)
methane's effect on temperature will be less pronounced than CO2's effect, adding only perhaps a few tenths of a degree to the average temperature of earth in the next 100 years. This is in sharp contrast to the major effect predicted for CO2, a temperature rise of at least 1.0 ~ 3.5 oC by the end of this century
Methane in the atmosphere comes from a variety of sources
Methane leaking from the deposits of natural gas during exploration and from the refining of petroleum is considered to be a major source. Another major source of methane is agriculture such as rice paddies, cattle and sheep. Methane is formed and released through bacterial actions on rice roots and through belching and flatulence of the animals that chew their cud.
Nitrous oxide (N2O, laughing gas)
296 time more effective than CO2 in its ability to trap IR energy, and accordingly affecting global warming.
CO2 more important role in global warming than N2O because
(1) CO2 concentration (385 ppm) is much higher than N2O (0.31 ppm)
Majority of N2O molecules in atmosphere
come from the bacterial removal of nitrate ion (NO3-) from soils. Major human-caused sources are automobile catalytic converters, ammonia fertilizers burning of biomass, etc
Ozone can act like a greenhouse gas
Since depletion of ozone may have a slight cooling effect, however, ozone is clearly not a principal cause of climate change.
CFCs, HCFC, and halons also absorb IR light and reemit it as heat
these are greenhouse gases, although their concentrations are still very low
a number that represents the relative contribution of a molecule of the atmospheric gas to global warming. This number takes into account both atmospheric lifetime of greenhouse gases and their effectiveness in absorbing IR light.
As the reference value, CO2 has GWP value of 1, and all other greenhouse gases are indexed with respect to it. No GWP values are assigned to water vapor, ozone, aerosols, and other ambient air pollutants, because they have short lifetimes and are distributed unevenly around the world. Among the three greenhouse gases that have global warming potential, CO2, CH4 and N2O, nitrous oxide has the greatest GWP value. However, an absolute effect of nitrous oxide on global warming should be the least, because it concentration is the lowest among the three gases.
Sulfur hexafluoride (SF6)
has the greatest GWP value, 22,000 times more potent as a greenhouse gas than CO2, but its atmospheric concentration is extremely low. SF6 is used for electrical insulation in transformers and a cover gas for smelting operations.
Any effort to remove CO2 is called by the term sequestration of CO2 which literally means keeping something apart.
Examples of sequestration
Planting trees would an example of CO2 sequestration as they remove CO2 through photosynthesis. But some researchers concluded that this is not very efficient at sequestering carbon. An idea of capturing CO2 from a power plant and of subsequent liquefying and pumping it deep into the ocean was proposed. This type of sequestration was implemented off the coast of Norway
to substantially reduce the amount of greenhouse gases released into the atmosphere. The Kyoto Protocol has set binding emission targets for the industrialized countries to reduce emissions of greenhouse gases from 1990 levels. As of 2007, the U.S. has continued to opt out of the Kyoto Protocol because of two reasons: 1) the reductions required by the protocol would cause serious harm to the U.S. economy, and 2) there is the lack of restrictions for developing nations. With the long delay in ratification and implementation of the Kyoto Protocol, the targets for 2012 most likely cannot be met without further restrictions.
Global warming and ozone depletion
both involve the atmosphere. However, effects of ozone on global warming, whether they are positive or negative, are small compared with the observed effects of the major greenhouse gases on global warming
Region of atmosphere: mostly troposphere
Major Substances: H2O, CO2, CH4, N2O
Interaction with light: Gases vibrate and return heat to earth when they absorb IR radiation
Nature of problem: Increase in greenhouse gases increases avg. global temp.
Major sources: Release of CO2 from burning fossil fuels, deforestation; CH4 from agriculture
Consequences: Climate change and effects on habitats and sea level rise
Remedies: Decrease use of fossil fuels, slow deforestation
International response: Kyoto Protocol
U.S. response: Not ratified; but regulatory actions by several states
Region of atmosphere: Stratosphere
Major Substances: O3, O2, CFCs
Interaction with light: gases break apart into smaller molecules or atoms when they absorb UV radiation
Nature of problem: Decrease in O3 increases exposure to UV radiation
Major sources: Release of CFCs from cooling systems. CFCs release ∙Cl that destroys O3
Consequences: Increase in skin cancer and damage to phytoplankton
Remedies: Eliminate use of CFCs, find suitable replacements
International response: Montreal Protocol
U.S. response: Signed Montreal Protocol and fully participate
pH of natural rain water (uncontaminated)
Natural water such as rain and river/lake water has pH slightly less than 7, i.e., it is not neutral. This is because natural water contains dissolved CO2 which actually causes the water to be acidic by the following reaction:
CO2(g) + H2O(l) → H+(aq) + HCO3-(aq)
pH of 5.3 caused by
Carbon dioxide in the atmosphere dissolves to a slight extent in water and reacts with it to produce a slightly acidic solution of carbonic acid:
CO2(g) + H2O(l) H2CO3 (aq) carbonic acid
H2CO3 (aq) H+(aq) + HCO3-(aq)
The carbonic acid dissociates slightly leading to rain with a pH around 5.3
Acid rain can have pH levels lower than 4.3
Acid rain comes from
The extra acidity must be originating somewhere in this heavily industrialized part of the country. The most acidic rain falls in the eastern third of the United States, with the region of lowest pH being roughly the states along the Ohio River valley.
Contributors to acid rain
The main contributors to acid rain are sulfur dioxide (SO2), sulfur trioxide (SO3), nitrogen monoxide (NO), and nitrogen dioxide (NO2). These compounds are collectively designated SOx and NOx, better known as "sox and nox."
How do the oxides of sulfur and nitrogen, sox and nox, behave as acids?
They are acid anhydrides, meaning "acids without water." When an anhydrid is added to water, an acid is generated:
SO2(g) + H2O(l) → H2SO3(aq)
SO3(g) + H2O(l) → H2SO4(aq)
H2SO3(aq) and H2SO4(aq) are acids because they can now release H+.
4NO2(g) + 2H2O(l) + O2(g) → 4HNO3(aq)
HNO3 is an acid because it can now release H+.
How does sulfur get into the atmosphere?
It is mainly from the burning of coal. Coal contains 1-3% sulfur and coal burning power plants usually burn about 1 million metric tons of coal a year!
Burning of sulfur with oxygen produces sulfur dioxide gas, which is poisonous.
S(s) + O2(g) → SO2(g)
Once in the air, the SO2 can react with oxygen molecules to form sulfur trioxide, which acts in the formation of aerosols.
2SO2(g) + O2(g) → 2 SO3(g)
How does NOx get into atmosphere?
It is mainly from automobiles. Although gasoline does not contain nitrogen, the air which is 70% N2 is inevitably involved in the gasoline burning inside the car engines.
Because of the high temperature condition at the car engines, the N2 which is usually unreactive at room temperature becomes reactive with O2. This produces NO2.
This is not completely filtered by the automobile tailpipes and eventually reach out to the atmosphere.
Once in the atmosphere NO2 continues to react to form nitric acid:
4 NO2(g) + 2 H2O(l) + O2(g) → 4 HNO3(aq)
Impacts of acid rain
May be a bigger threat to human welfare than carbon emission causing global warming
•acidification of fresh water that causes significant reduction in fish population
•death of trees leading to deforestation
•deteriorating human health conditions
•damage of infra-structures such as dissolution of sculptures, and corrosion of bridge structures
chiefly of environmental pollution and pollutants originating in human activity
each of two or more different physical forms in which an element can exist. Graphite, charcoal, and diamond are all allotropes of carbon.
What atmospheric component is responsible for the natural acidity of rain?