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Chemistry 1002 Chapter 21Environment & Air Quality |
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Introduction
Even though it is easier for life to exist without
air than without water (many bacteria are anaerobic), humans can
survive for about three days without water, but can only survive
for about seven minutes without air (brain damage sets in after
about three minutes). For humans, then, clean air is even more
essential than clean water.
Another issue involves purification of waste gases
vs. purification of breathing air. It's far less intrusive on
quality of life to purify water supply than air supply (we don't
want to have to walk around outdoors in SCUBA gear). This relates
back to first issue. Water can be purified and distributed at
fixed locations for us to consume at our leisure. We don't have
time to travel from place to place between breaths.
In summary, it is even more imperative that we keep
the air in earth's atmosphere clean than that we keep water in
our environment clean. This chapter will focus on the chemistry
of air pollution. Supply and demand for air are issues we hopefully
will never have to face, and purification considerations will
focus strictly on purification of waste gases in order to avoid
rather than "fix" air pollution.
Air Pollution
AEROSOLS AND PARTICULATES
Very small particles of dust, salt, polymer material,
etc. suspended in air. Catalyze chemical reactions which create
polluted air by providing surface area for reactive pollution-generating
molecules to adhere to so they can meet and react with one another.
Like country clubs for air pollution molecules.
SMOG
A hybrid of the words "smoke" & "fog."
Fine liquid water droplets (mist or fog) can turn nitrogen
and sulfur oxides into acids which in turn can eventually create
acid rain.
Thermal inversion. Traps smog in cities
by not allowing cooler low-lying polluted city air to rise into
warmer overlying layer.
Photochemical smog.
Mostly produced by vehicles.
N2 + O2 Æ
2 NO
(NO stable only at hi temp)
NO + O2 Æ
NO2 (stable) + O (unstable)
O + O2 Æ
O3
(small amount only without sunlight)
NO2 + sunlight Æ
NO + O
NO now free to make more O and O can make more ozone.
A vicious cycle is set up where sunlight provides energy to make
large amts of ozone from catalytic amount of NO2.
Industrial smog.
Photochemical smog is nitrogen-based, whereas "industrial"
smog is sulfur-based.
S (in fossil fuels) + O2 Æ
SO2
2 SO2 + O2
Æ2 SO3
Slow reaction. Works better on aerosols or particulates.
ACID RAIN
Nitrogen and sulfur oxides from photochemical and industrial
smog react with water in mist or fog to produce nitric and sulfuric
acids. These acids can be carried long distances in air and return
to earth in rainfall.
2 NO2 + H2O
Æ HNO2 +
HNO3
SO3 + H2O Æ
H2SO4
STRATOSPHERIC OZONE
Although ozone is very poisonous and therefore bad
to have in troposphere (especially trapped in city air) it is
important to have ozone in stratosphere. Ozone in stratosphere
absorbs UV-B light from sun which causes skin cancer. Every 1%
reduction in stratospheric ozone increases skin cancer incidence
2% for same amount of sun exposure.
Main air pollution challenge to stratospheric ozone
seems to come from CFC's (chlorofluorocarbons)
used in modern world for refrigeration and air conditioning.
Most of world has gotten together and decided to phase out use
of these chemicals so we can avoid having to wear sunblock every
time we venture outdoors.
GREENHOUSE GASES
Greenhouse gases allow allow sunlight in visible region
of spectrum to pass through atmosphere but absorb light in infrared
(heat) region of spectrum. Visible sunlight which hits earth
is converted to heat and radiated back into space as infrared
light. Some of this heat is absorbed by greenhouse gases in stratosphere;
never makes it into space. This warms up the atmosphere.
Water vapor, carbon dioxide, and methane are major
greenhouse gases. Of these carbon dioxide seems to be most responsible
for atmosphere warming up in recent years. As we burn fossil
fuels at ever faster rate we make atmosphere warmer and change
weather patterns and water levels in oceans. Weather patterns
we are used to now will change rather significantly in years to
come (ie. flooding in midwest) until we burn up all fossil fuels.
Have to adapt to these changes.
INDUSTRIAL AIR POLLUTION
Seems to be under better control now than at any other
time in recent history except in developing countries. Civilized
world now takes environmental matters seriously and governmental
controls on industry have dramatically reduced air pollution in
recent years. Main problem now seems to come from personal automobiles.
HOME AIR POLLUTION
A variety of issues arise in this arena from time to
time.
First, as text points out, we have same polluting chemicals
in home that industry dumps into air. After all, industry is
producing these chemicals for us as consumers. "Sick building
syndrome" a recent phenomenon traced in many cases to residual
manufacturing chemicals in carpets (monomers, etc.).
A few other "home air pollution" issues which
have arisen in recent times:
Radon gas from traces of U in soil.
Airborne molds, bacteria, fungi which grow in air conditioning
filters.
Airborne asbestos from crumbling old fire-resistant
building materials.
PAH's (polynuclear aromatic hydrocarbons) and dioxins
(carcinogens) in air from fires in fireplaces, barbeque fires,
and tobacco smoke.
Buildup of toxins in air in home caused by reduced
mixing of house air with outside air (heating and cooling energy
efficiency).
Comments?
Last Revised : Wednesday, December 3, 1997
Copyright © 1997
Louisiana State University, Department of Chemistry.
All rights reserved.
http://www.chem.lsu.edu/lucid/courseinfo/chem1002/ch21.html
