Objective: to consider the legal definitions of air pollution.
1. air pollution is defined in many ways, but under the U.S. Clean Air Act it is:
- the presence or solid, liquids, or gases
- in the outdoor air
- in amounts injurious to humans, animals, plants, or property.
There are three aspects of this legal definition that deserve comment:
- air pollutants are not just gases;
- despite widespread concerns with indoor air, the legal defiinition of air pollution refers only to outdoors;
- and while human health effects are usually the most sensitive, they are not the only effects.
2. criteria pollutants, as defined by law, include:
SOx, NOx, CO, particulates, lead, and ozone.
These are not only the most prevalent pollutants, but thought to be the best predictors of air quality. Our discussion of criteria pollutants will add hydrocarbons because of their contribution to ozone and other effects.
3. particulates are non-gaseous pollutants suspended in air (also called solid and liquid aerosols)
This is obviously a broad category, and includes the next five definitions:
The first three are all solid materials:
4. dust = solids from various physical processes
(generally > 1 micron)
5. smoke = solids from incomplete combustion
(generally < 1 micron)
6. fumes = solids from vapor condensation
(generally < 1 micron)
The next two refer to liquids suspended in air:
7. mist = liquids from vapor condensation
(generally < 10 microns)
8. sprays = liquids from atomization of a parent liquid
(generally > 10 micron)
The reference to the size of these particulates (in microns) will be significant later on, when we discuss health effects.
9. TSP = total suspended particulates obtained from
a high-vol sampler of particulates.
TSP is actually an old fashioned, outdated term. Still, I like to introduce it anyway
because it helps clarify the history of particulate standards. Think of a high-vol sampler as a
fancy vacuum cleaner -- what makes it fancy is that we can accurately measure how much air is sucked
into the vacuum (measured in cubic meters of air per minute). We run this vacuum cleaner for 24 hours
that sucks air through a filter that collects all the liquids and solids suspended in air (i.e., the particulates).
We can then measure two things:
1. the volume of air sucked into the vacuum over 24 hours (measured in cubic meters);
2. the weight of the collected particulates (measured in grams).
We can then divide the total grams of particulates by the total cubic meters of air,
which results in grams per cubic meter. Since these numbers are usually very small, it is
often more convenient to express this as micrograms per cubic meter.
This measure is referred to as the TSP, or total suspended particulates.
10. PM-10 = Particulate Matter < 10 microns in diameter.
As I mentioned above, TSP is an outdated term. The reason for this is that we now know that
the particulates most associated with health effects are the smaller particulates. That's because they
are more efficient at penetrating deep into the lungs (but we'll discuss that in more detail later).
Since health effects are most associated with particulate matter less than ten microns in diameter,
standards have been issued for this so-called pm-10. Incidentally, you can use numbers besides 10
(e.g., pm-2.5 refers to particulate matter less than 2.5 microns in diameter).
We obviously have other criteria pollutants to discuss, but this module gets us started.
SOURCES OF PRIMARY AIR POLLUTANTS
Objective: to describe the natural as well as human sources of pollution life cycles.
1. Sulfur oxides:
Also known as SOx, its natural sources include volcanoes, oceans, and general microbial degradation. Human sources are primarily from combustion of fuels containing sulfur. We often refer to crude oils as sweet and sour (sour crude is high in sulfur, while sweet crude is low).
2. Particulates:
Also known as solid and liquid aerosols, particulates are the various non-gaseous pollutants in air. Natural sources include volcanoes, oceans (sea salt), general erosion processes (creating dust), and fires. Human sources are primarily from fuel combustion .
3. Carbon monoxide (CO):
CO is colorless, odorless, and tasteless. Due to these characteristics, CO can be undetected even at dangerous levels. Natural sources of CO are primarily from the oceans -- mostly from the partial oxidation of chlorophylls and methane. Human sources of CO are from incomplete combustion of fuels (from both motor vehicles and industrial processes). Carbon monoxide is usually, by mass, the largest of the criteria pollutants.
4. Hydrocarbons:
Hydrocarbons are in various combustible fuels. Natural sources include bacterial decomposition, and emissions from plants and animals. Human sources are primarily from the oil industry (from incomplete combustion as well as evaporation). Since it is a category (rather than a specific chemical), the source of hydrocarbons depends on the type of hydrocarbon.
5. Nitrogen Oxides:
Also known as NOx, natural emissions of NOx are mostly from microbial action. Human sources are from virtually any combustion process. The heat of combustion combines nitrogen and oxygen (both naturally present in air) by the following equation:
N2 + xO2 ---> NOx (NOx is primarily in the form of NO).
6. Lead:
Lead comes from a wide variety of natural and human sources. Since the development of unleaded gasoline, combustion emissions of lead have dropped dramatically. Nevertheless, total lead sources continue to be a major concern.
"Smog" is a term originally formed by the combination of of the words "smoke" and "fog." This can seem a little confusing, because Los Angeles, infamous for for its smog, is hardly known for foggy weather. The confusion arises because there are actually two kinds of smog: London smog (well know for its fog) and L.A. smog (also know as photochemical smog). Photochemical smog is driven by the u.v. energy from the sun, and Los Angeles is better know for its sunny weather. The differences between the two types of smog are summarized in the table below.
Name:
London smog
(New York smog, gray smog)
Photochemical smog
(L.A. smog, Denver smog, brown smog)
Weather:
cool, damp
sunny
Content:
particulates, sulfur oxides
NOx, ozone, hydrocarbons.
Sources:
coal, etc.
gasoline, combustion.
On a smoggy day, there are literally thousands of reactions that occur in the atmosphere. Fortunately, there are a few that can help us to initially understand the formation of photochemical smog. In the reactions listed below, the most important consitituents are described with a larger, bolder font.
1. NO + O2 ---> NO2 + u.v. ---> O + NO
In this first reaction, we start with Nitric Oxide (NO), which we already know is emitted from various combustion processes. It combines with oxygen in the atmosphere to form nitrogen dioxide ( NO2 ), which has a characteristic brown color that should be familiar to anyone who has lived in a smoggy region. When the u.v. rays of sunlight strike the NO2, it breaks off a single oxygen radical (O) that triggers many subsequent reactions of photochemical smog.
2. O + O2 ---> O3
In this second reaction, we see how the single oxygen radical helps form ozone ( O3 ). A variety of molecules can act as catalysts for this reaction.
3. O3 + NO ---> O2 + NO2
This third reaction is called a scavenging reaction, and it happens normally in the evening. Because it converts ozone to O2, the net result is a drop in the ozone concentration in the evenings.
4. RC + O ---> RCO + O2 ---> RCO3
The fourth reaction shifts our attention to the hydrocarbons (represented here as RC). When combined with the oxygen free radical, it forms RCO, which represents a variety of aldehydes and ketones. Some of these constiutents can combine with oxygen to form peroxide readicals ( RCO3 ).
5. O2 + RCO3 ---> O3 + RCO2
The fifth reaction demonstrates the importance of these peroxide radicals ( RCO3 ) -- it enhances the formation of ozone.
6. NO + RCO3 ---> NO2 + RCO2
The last reaction shows a more subtle role of the peroxide radicals -- by enhancing the formation of
nitrogen dioxide, we know that the nitrogen dioxide will go on to form more ozone.
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