Acid deposition, topics:
Wet, dry
Acid aerosol
pH, definition
How pH scale works: what a change in 2 pH units means in terms of acidity
Normal acidity of rainfall, why and how
Bicarbonate and carbonate ions in carbon cycle
Tall stacks, connection to long range transport
Regional aspect of acid deposition, how this make problem more difficult, technically and politically
Buffering capacity, what it is and how it is important to acid deposition
Adverse effects
Solutions to the problem
Features of the Clean Air Act acid rain control program
      Scrubber retrofit
      Emission allowances
The National Acid Precipitation Assessment Program study

Acid deposition
Acid deposition is the more inclusive term, rather than acid rain. Acid material can be deposited from the atmosphere to the surface, whether to ground or surface water. This material can be wet: acid rain, acid fog, acid mist, or acid dew. It can also be dry: aerosol, in the form of sulfates (-SO4) and nitrates (-NO3).

Wet or dry, acid material is formed from the gaseous pollutants sulfur dioxide and nitrogen dioxide. SO2 and NO2 dissolve in water to form acid rain, acid mist, etc. SO2 and NO2 react with other material in the air to form the acid aerosols, sulfate and nitrate.

pH
Acidity is a function of the concentration of hydrogen ions (H+) in a solution. The pH scale is used to express the hydrogen ion concentration because it is a more compact and convenient unit. Instead of speaking of 1 gram of hydrogen ion per 10,000,000 liters of water we can say that the pH is 7. The definition of pH is the negative logarithm of the hydrogen concentration, where the hydrogen ion concentration is grams of hydrogen per liter of water or moles of hydrogen per liter of water (same thing). In equation form:

pH = -log [H+]

Therefore 1 gram of  hydrogen ion per 10,000,000 liters of water equals 0.0000001 gram per liter or 1 x 10-7 gram per liter.
If we take the common logarithm of 1 x 10-7 (one times 10 to the minus 7) we get -7. (The log of a number is the power we need to raise 10 to in order for 10 to that power to equal the number. Well, in order to get the number 10-7 we need to raise 10 to the -7 power, or the log of 10-7 is -7.)

Back to the pH represented by 1 gram of hydrogen ion per 10,000,000 liters of water, or 1 x 10-7 gram per liter. The log of this concentration is -7, the negative of the log is -(-7) or +7. Therefore the pH is 7.

So what's the pH of a hydrogen ion concentration of 1 gram of hydrogen ion per 1,000,000 liters of water? See answer.

What then is the difference in pH between the two hydrogen ion concentrations of
1 x 10-7 gram H+ per liter and 1 x 10-6 gram H+ per liter? One is a pH of 7, the next a pH of 6, a difference of one pH unit.
By how much do the actual concentrations differ? The first is one gram per 10,000,000 liters, the second is one gram per 1,000,000 liters or
0.0000001 and
0.0000010

We see that the second (1 in 1,000,000 - pH 6) is 10 times as large as the first (1 in 10,000,000 - pH 7). So a difference of one pH unit is a difference of 10 times.

Ok, what's the pH for a hydrogen ion concentration of 1 in 100,000 or 0.0000100 grams per liter? Answer pH=5.
Notice the difference:

Therefore a difference of 2 pH units is a difference of 10 to the second power or 100 (pH 7 versus pH 5). A difference of 3 pH units is a difference of 10 to the third power (1000), etc.

Normal, unpolluted rainfall is acidic.
Acid rain is precipitation that is more acid than normal. The normal acidity of rainfall comes from carbonic acid (H2CO3). Carbonic acid forms in a water (a raindrop) when carbon dioxide gas dissolves in water. The chemical reaction is CO2 + H20 ---> H2CO3. The carbonic acid ionizes to H+ and HCO3- (bicarbonate ion) and H+ and CO3-2 (carbonate ion). The free H+ give the rain its acidity, which is roughly 5.6.

Carbon dioxide is a constituent of the atmosphere and not considered to be a pollutant (until we get to global warming, but that's another story); therefore, normal rainfall is acidic. What we call acid rain is more acidic than normal or a pH of less than about 5.6.

Tall stacks and long range transport
As we have pointed out, the area of concern for air quality is the first 10 feet above the ground, the human breathing zone. The air quality as it exits a smokestack aloft is not important, but instead the air concentrations at ground level are the ones that count. If pollutants are emitted from a tall smokestack then by the time they hit the ground they will be diluted more than if they had been released nearer the ground. The taller the stack, the greater the dilution.

It was common practice about 25 years ago to allow industries to build taller smokestacks as a form of pollution control. The possibility of long range transport and acid deposition was not considered, but they both began to happen. Long range transport is the movement of air pollutants perhaps several hundred miles downwind by upper level air movements. The pollutants exited the air via dry or wet acid deposition in another region. Read this article, written in 1980, to see the discussion turning to the problem of tall stacks. Notice that they got the London episodes dates wrong (London had many milder episodes but surely they meant to point to December 1952).

This regional aspect of acid deposition posed a technical and a political problem. The technical question was how to track the movement of the gases sulfur dioxide and nitrogen dioxide, their transformation to acidic species, and then their deposition hundreds of miles downwind, with the goal being to understand the link between emissions and downwind effects. This problem has not yet been solved.

The political problem was one of jurisdiction. Why should an upwind state, e.g. Ohio, take expensive steps to control its emissions when the adverse effects were not being felt in Ohio, but instead in New York? The same held true internationally. Canada accused the U.S. of dragging its feet on acid rain control because it was exporting the problem to Canada.

Buffering capacity
An area's susceptibility to acid deposition is tied to its buffering capacity, or the soil and the surface water's ability to receive an acidic input without a change in its pH. Buffering capacity can be exhausted; when it is, then additional acid input will change the pH of the soil or water and adverse effects of that change can begin. The USEPA has a primer article on acid rain damage to forests that explains buffering capacity.

The largest sources of sulfur dioxide emissions are in the Ohio River valley. Prevailing winds carry, via long range transport, the SO2 to New England and southeastern Canada. Unfortunately, this area has naturally low buffering capacity. Others areas sensitive to acidification are the mid-Appalachian Mountain region eastward to New Jersey. See a map of 1996 pH measurements across the U.S.

Adverse effects

• diminished aquatic life reproduction, best documented in Adirondack lakes (northern New York).

• metals leaching from soil and rock: aluminum toxicity. Aluminum is a common element found in soil. Under normal pH conditions aluminum will not dissolve out of soil into water but remains chemically bound in  the soil. Under acidic conditions aluminum dissolves in water and is transported via runoff into streams and lakes. High aluminum concentrations in water are toxic to wildlife.

• materials damage (covered under sulfur dioxide)

• forest dieback: red spruce damage at high elevations. Note the species- and location-specific aspect of this adverse effect. We're not saying "Acid rain hurts trees." Forest effects have been demonstrated in the U.S. on red spruce at high elevations (Appalachian Mountains). In Europe forest damage from acid deposition has been seen in parts of Poland, Czechoslovakia (the Czech Lands, Slovakia), and Germany's Black Forest. There is some evidence that this is caused by acidic precipitation and ozone acting together.

USEPA factsheet on adverse effects.

Solutions to acid deposition problem
Because sulfur dioxide is the most important gas involved in acid deposition, controls are focused on SO2 emissions reductions. Note that the first three are repeats from the listing of controls on sulfur dioxide.

• Fuel switching: low- or no-sulfur fuel (nuclear, natural gas)

• Flue gas desulfurization (scrubbers) CaCO3 + H2SO4 -> CaSO4 + H2CO3

• Fluidized bed combustion: sand, lime, ground coal. High efficiency, new technology.

• Lime treatment of lakes. This is the addition of limestone to surface water (dump it in the water from a boat). The lime dissolves, neutralizing the acidity. The approach has proved successful in the Adirondack region.

• Clean Air Act Amendments of 1990: new acid rain control program. (see below.)

Features of the new acid rain control program:

1. Retrofit older plants with scrubbers Older and dirtier power plants that emitted SO2 were forced to lower their emissions by installing scrubbers or by switching to a low-sulfur fuel.

2. Drop to 9 million tons SO2 by 2000; national emissions cap. Use SO2 emission allowances that can be traded. Of the roughly 25 millions tons of SO2 emitted annually in the U.S. in 1990, 18 million were emitted by coal-fired electricity generating plants. (The pie chart in a USEPA acid rain document shows that the utilities' share is now larger). The drop to 9 million tons is a 50% reduction. As of year 2000, the limit of coal-fired electricity generating plants' SO2 emissions is a total of 9 million tons. The USEPA is allowing the utilities to trade SO2 emission allowances. The utilities were first issued allowances that covered their current SO2 output. One allowance is the right to emit one ton of SO2 during the next year. The number of allowances in circulation will be limited to 9 million by the year 2000.

Read about the acid rain control program:

• Program overview

• Emission allowance trading

• Allowance prices Notice the bids by environmental science groups at colleges. Also note the number of allowances Enron bought!

A study that was ignored
Congress was pressured to revise the Clean Air Act in the late 1970s to regulate acid deposition directly. (Remember, sulfur dioxide and nitrogen dioxide, being criteria pollutants have been regulated strictly since the Clean Air Act of 1970.) Because the problem of acid deposition was not well understood, The National Acid Precipitation Assessment Program was established.

National Acid Precipitation Assessment Program: Authorized by Congress to study problem before amending the Clean Air Act. $535 million 1980-1990: concluded problem of moderate proportions, current controls will eventually work, recommended lime treatment. Instead, a new acid rain control program was passed as part of the Clean Air Act Amendments of 1990.
The NAPAP web pages.

Reading

• A Canadian list of  FAQs on acid rain.

• A list of acid deposition links

Answer: The pH is 6.

Introduction to ozone layer depletion and global warming

The electromagnetic spectrum, basics.
Wavelength = distance from crest to crest (or trough to trough) on an electromagnetic wave.
Frequency = the number of crests that pass by a given point in one second.
Velocity = the speed of light (300,000 kilometers per second). This is a constant (doesn't change)

Units for wavelength (length, which is not surprising; e.g., for ultraviolet, visible, and infrared, nanometers, one-billionth of a meter)
Units for frequency (the Hertz, which is per second or 1/second,)
Units for velocity (length/time, as in kilometers per second (see light speed above))

Speed of light = wavelength times frequency

Now, because the light speed is a constant, if wavelength increases then frequency must decrease because their product must remain the same. If 100 is the constant speed, then if wavelength is 20,  frequency is 5. If wavelength increases to 40 then frequency must go down to 2.5. This is called an inversely proportional relationship. Frequency and wavelength are inversely proportional.

The arrangement of the electromagnetic spectrum, by increasing wavelength, left to right:

---------------------> wavelength increasing, frequency decreasing

<--------------------  frequency increasing, wavelength decreasing

Therefore, the parts of the electromagnetic spectrum as depicted above are arranged by decreasing frequency, left to right. This is explained by wavelength and frequency being inversely proportional (as one goes up the other goes down).

Energy content of electromagnetic radiation.
The higher the frequency, the higher the energy content.

Energy = frequency times Planck's constant (doesn't change)

Units commonly used for electromagnetic energy: joule, erg, electron volts. (Joules most common); unit for Planck's constant are joule-second. So Joules = Hertz (1/sec) times Joule-second

If energy increases with frequency then the arrangement of the spectrum depicted above is by decreasing energy content, left to right.

This NASA page has a good basic discussion of the electromagnetic spectrum.

An amazing group of pages:

• Martindale's 'The Reference Desk' (the main menu page)

• Martindale's Health Science Guide

• Environment, Disaster, & Safety Courses, Tutorials, & Databases (a part of The Reference Desk)

• An electromagnetic spectrum poster and explanation.

Know:

• the relationships between wavelength, frequency, and energy content.

• the parts of the electromagnetic spectrum, in (wavelength or frequency) order

• units for the electromagnetic spectrum

Ozone layer depletion
The ozone layer is found in the stratosphere, the segment of the atmosphere just above the troposphere. In the troposphere, which extends from the ground to about seven miles high, air temperature declines with increasing altitude. At the boundary between the troposphere and the stratosphere, the temperature of the (thin) air quits declining and begins to increase with altitude. The stratosphere extends to about 35 miles high. The area called the ozone layer is a portion of the stratosphere, from about 12 to16 miles high. This chart shows the layers of the atmosphere. Question: From the chart, what is the altitude for the tropopause (beginning/bottom of the stratosphere)? What is the altitude for the stratopause (end/top of the stratosphere)?

Ozone is formed in the ozone layer by oxygen molecules absorbing ultraviolet radiation. The O2 molecules split into oxygen atoms (O); then the atoms combine with molecules to form ozone: O + O2 --> O3; O3 molecules themselves absorb ultraviolet solar energy, causing them to split into O + O2. So the ozone layer contains a mixture of O, O2, and O3, combining and splitting. The result is the absorption of ultraviolet radiation and its conversion into infrared (heat) energy, which dissipates in the upper atmosphere. Without this ultraviolet absorption in the stratosphere the ultraviolet would penetrate to ground level. Life at ground level relies on this ultraviolet shield for survival. If there were no shield at all, little life could exist. If the shield performs less efficiently (as a consequence of ozone layer depletion) then adverse effects would begin. By the way the absorption of ultraviolet energy and its conversion to heat in the ozone layer is the reason why air temperatures in the stratosphere rise with altitude, as shown in the chart you just looked at.

Ultraviolet radiation
The ultraviolet part of the electromagnetic spectrum is divided into three segments.

Know: The names and properties of the three parts of the ultraviolet part of the electromagnetic spectrum.

It's UVB that is strongly absorbed in the ozone layer and it is UVB that would increase if the ozone layer contained a lower concentration of ozone molecules (ozone layer depletion).

Human threats to the ozone layer

The chlorofluorocarbons, a family of synthetic organic chemicals, were first identified as a threat to the ozone layer in a 1974 report in Nature, by Sherwood Rowland and Mario Molina (who, with Paul Crutzen, recently were given the Nobel Prize for their work). The CFCs contain carbon, fluorine, and chlorine atoms. They are (were, they are being phased out) used as refrigerants (in air conditioners, freezers, refrigerators), solvents (cleaners), foaming agents (to make pillows, automobile seats), and as aerosol propellants (the gas that carries the aerosol out of a spray can).  This and subsequent research proposed that the CFCs harmed stratospheric ozone by the following mechanism:

• This article also shows the mechanism of ozone depletion.

• A recent article by Sherwood Roland on ozone depletion.

Good properties of CFCs: They are nonflammable, nontoxic, noncorrosive, nonreactive. Note, for example, that CFC refrigerants (brand name Freon) replaced ammonia and sulfur dioxide, which were used as refrigerants until the 1940s. They replaced highly toxic and irritating gases.

One of these good properties is a problem for CFCs as ozone layer depleters. Being nonreactive synthetic molecules, the CFCs have an life in the atmosphere of between 60 and 80 years. This means that they have plenty of time to mix upward in the troposphere and even find their way into the lower stratosphere to destroy ozone.

Adverse effects of significant ozone layer depletion

Note: None of these effects has been shown to be happening as a result of ozone depletion; instead, this is a list of known adverse effects of excessive exposure to ultraviolet radiation in general.

• Increased incidence of skin cancer in susceptible groups (light-skinned)

• Possible increased risk of cataracts

• Decreased crop yields

• Adverse effects on certain fish larvae, phytoplankton

• Weakening of plastic products

• More on adverse health effects of UVB.

• More on adverse environmental effects of UVB.

Solutions to the ozone layer depletion problem

1. International agreements to phase out ozone-depleting chemicals. A series of agreements began toward banning CFCs with the Vienna Convention for the Protection of the Ozone Layer in 1985. The first phaseout agreement was drafted in 1987, called the Montreal Protocol on Substances that Deplete the Ozone Layer. This was an extension of   the Vienna Convention for the Protection of the Ozone Layer. Subsequent changes to the Montreal Protocol  in 1990, 1992, 1995, and 1997 have accelerated the phaseout of ozone-depleted chemicals. The text of the treaty, as amended. Although the Vienna Convention is the original document and the Montreal Protocol has been changed many times, the shorthand term for the agreement to phase out CFCs and other ozone-depleting chemicals is the Montreal Protocol.

2. National bans on CFCs.
The United States banned the use of chlorofluorocarbons as a propellant in aerosol sprays in 1978. There are two important facts here: one the CFC is the propellant; it's not the aerosol. Also, no CFCs have been used as propellant in the US for 20 years. I still hear people say they won't use aerosol sprays because they want to protect the environment! By the way, the substitute for the propellant in aerosol sprays is quite commonly the flammable gas propane (read the label), so the trade did not eliminate risk but just changed its character from one of chronic worldwide increased ultraviolet exposure to acute personal fire hazard.

Following international and national bans and phaseouts of CFC production, smuggling of CFCs has become a major problem.

Additional smuggling links:

• Environmental Investigation Agency

• U.S. enforcement of CFC ban, example

3. Substitutes for the CFCs

a.  Add a destabilizing hydrogen atom to the chlorofluorocarbon molecule to create a hydrochlorofluorocarbon (HCFC). For example:

CF2Cl2 --> CF2ClH (one of the two chlorine atoms is replaced by a hydrogen atom).

What this does is greatly shorten the atmospheric residence time of the molecule (it falls apart in the lower atmosphere before it has a chance to mix upward to the ozone layer).

So to replace CFC-12 (Freon 12), HCFC-22 is being used; one of the chlorine atoms on Freon 12 is replaced by a hydrogen atom.

b. Produce synthetic refrigerants that have no chlorine at all. These chemicals are call hydroflurocarbons (HFCs).

For example, HFC-134a is being used in new automobile air conditioners.

See the USEPA pages on substitutes.

Uncertainties

1. Ozone layer concentrations fluctuate naturally with the sunspot cycle and perhaps for other reasons. Our data set is fairly short and limited; therefore we do not know for certain that the ozone layer is being depleted. There is also the question of depletion by latitude and by time of year. (Recent depletion measurements using satellites have begun to show a more discernible downward trend.)

2. Ground level measurements of ultraviolet radiation have not shown a significant increase. If the ozone layer were getting thinner, then more ultraviolet should be getting through. One explanation is that ground level ozone (smog) is absorbing the ultraviolet before the UVB monitors can read it. See United Nations FAQs on ozone on this uncertainty which agrees that the data aren't available. The USEPA says that ultraviolet radiation is increasing at ground level, but only shows that ultraviolet is stronger at the South Pole during the so-called ozone hole. The Toronto Study, which is the source cited by the USEPA to show increased ultraviolet levels at ground level, is inconclusive (see the Science article in the library; I also have a copy.)

3.  Fred Singer takes a contrarian view of ozone layer depletion and global warming. Question: What is Singer's answer to this question in the article, "Is There an Increasing Trend in Ultraviolet Radiation at the Earth’s Surface?" What did the reanalysis of the Kerr/McElroy data show?

Suggested uncertainties that probably aren't

1. Volcanoes put more chlorine in the atmosphere than CFCs.
A USEPA article explains how this may not be true.

2. CFCs are heavier than air; they can't get to the stratosphere.
The atmosphere is turbulent and will mix gases such as the CFCs readily.

See these ozone depletion documents, written by Robert Parson of the University of Colorado, which is in question and answer format, for an excellent overview.
Ozone Depletion FAQ Part I: Introduction to the Ozone Layer
Ozone Depletion FAQ Part II: Stratospheric Chlorine and Bromine
Ozone Depletion FAQ Part III: The Antarctic Ozone Hole
Ozone Depletion FAQ Part IV: UV Radiation and its Effects

Other Ozone layer depletion links
 Studies, outline, more links
Our Ozone Shield, from the National Oceanic and Atmospheric Administration 

This is then end of the material for examination 4.

Copyright © 1998-2002 Bruce Wyman, Ph.D.
Last modification:6/30/2004
http://www.faculty.mcneese.edu/wyman/102w

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