Atmospheric Environmental Concerns
problems of stratospheric ozone depletion and climate change are global
in scale, acid deposition, another air-related environmental problem,
- Acid Rain
of rain and snow depends upon the gases or other agents present in region
of the atmosphere in which the clouds are formed. When water forms clouds,
various chemicals and dust particles (both naturally-occurring and anthropogenic)
are dissolved or trapped in the droplets, and eventually deposited back
onto the ground.
snow can be acidic
due to natural causes. However, the range of acidity varies, and precipitation
may even be alkaline (or basic) in some places. "Natural" acidity
occurs because of dissolved organic oxides (like CO2) and sulfur compounds
from decaying biomass. Acidity also occurs as a result of more extreme
phenomena like volcanic eruptions, which spew large quantities of CO2,
H2S, and SO2 into the air.
gases are emitted into the air, they react with water vapor molecules
and form acid droplets. These droplets deposit as drops in what is known
as acid rain (or acid precipitation). Some dry deposition also occurs,
in which the NOx and SOx particles cling to dust and are deposited on
surfaces. These two processes are generally referred to as acid deposition.
large and routine emissions of acidic gases from human activity result
in a significant increase in acid deposition, making it a significant
regional environmental problem. Most prominent among the anthropogenic
acidic gases are oxides of sulfur and oxides of nitrogen. Both nitrogen
and sulfur have many oxides, varying in the amount of oxygen relative
to nitrogen or sulfur, and they are generally denoted by NOx and SOx.
approximately half of the compounds that add acidity to rain are anthropogenic.
One major source of these compounds is the smelting of sulfur-based ores
in metal processing. A classic case of acid deposition impacts due to
metal processing occurred in Sudbury in Ontario, Canada. There, open roasting
of nickel-copper ores released sulfur dioxide that destroyed much of the
vegetation in the area.
major source of compounds causing acid deposition is the combustion of
fossil fuels. Fossil fuels are used in the production of electricity and
in powering automobiles. All fossil fuels contain some sulfur. Coal contains
varying amounts of sulfur, depending on the region of its origin. The
sulfur in natural gas is removed during refinement.
Acid deposition has several consequences. It alters the pH in the water
cycle, thus upsetting the local ecological balance. This alteration stresses,
and can even destroy, vegetation and aquatic animals. Acid deposition
is injurious to life because of corrosive effects on the body when inhaled.
It also corrodes marble and other stones, causing "pitting"
of statues and other historical monuments. Examples of these harmful effects
seen in eastern parts of the United States include loss of some trees
and fish in Appalachian forests and streams and pitting of monuments in
the Gettysburg National Park.
of acid deposition came to be recognized in the 1970's, and since then
many laws have been passed to regulate air pollution in the U.S. and Canada.
Also, some technological measures have been taken to reduce acid precipitation,
including "clean coal" technologies, which clean coal of sulfur
before combustion, and catalytic converters in the exhaust systems of
automobiles, which transforms hydrocarbons, carbon monoxide, and nitrous
oxides into water, carbon dioxide, nitrogen, and oxygen.
- Photochemical Smog & Tropospheric Ozone
and fOG) was a phenomenon recognized in the early 1950's when thousands
of deaths and intense respiratory problems occurred in London, England;
Donora, Pennsylvania; and cities in other countries all over the industrialized
world. The city of Los Angeles and parts of Southern California have now
come to be associated with smog. There are different sources of smog.
Early incidents of smog arose primarily from the combustion of coal with
high sulfur content. The combustion resulted in the reaction of sulfur
dioxide (SO2), nitrogen oxides, and dust particles reacted
to form particles and droplets of sulfates (SO42-
) and nitrates (NO3- ). This mixture of smoke and
fog clouded the atmosphere, and irritated the mucous membranes and the
eyes of the people exposed to it.
A new version
of smog is the photochemical smog which comes from reactions between the
solar radiation (ultraviolet that gets through the stratospheric ozone
layer and visible light) and gases produced by various industrial processes.
When solar radiation acts on some of the gases emitted from some processes,
particularly exhaust from vehicles, ozone and other gases are produced
in the troposphere. This process is called a photochemical reaction because
it is a chemical reaction between light (photo) and the chemicals
in exhaust gases. Predominant among the gases undergoing photochemical
reactions are hydrocarbons (compounds of hydrogen and carbon), oxides
of nitrogen, NO (nitric oxide) and NO2 (nitrogen dioxide).
Photochemical smog is an increasing problem in cities with high traffic.
It is often associated with Los Angeles and Mexico City because of the
large amounts of traffic and bright sunlight there.
reactions produce a variety of gases, many of which are harmful to health.
They often cause simple irritation of the mucous membranes and eyes because
of their acidic nature, and sometimes result in more serious respiratory
problems because they overpower the immune and respiratory systems.
gases produced in the photochemical smog are ozone and peroxyacetyl nitrate
(often referred to as PAN). The following reactions produce ground-level
+ uv NO + O
O + O2 (+ catalyst) O3
other reactions also occur, producing a variety of highly reactive compounds,
and recycling nitrogen dioxide to produce more of the reactions! A catalyst
is a compound that helps speed a reaction while maintaining its own amount
and composition being the same before and after the reaction. Carbon monoxide,
which is present in plenty in vehicle exhaust, is a good catalyst for
the above reaction.
that sunlight is a requisite for this reaction. Figure 10 shows the time
course of the tropospheric ozone formation in a typical high traffic,
sunny city. It shows the ozone buildup, after the exhaust gases and sunlight
have had time to "cook" the reactive mixture that makes up the
13: Rise of ozone smog toward mid day.
whose presence in the stratosphere has a protective effect on us, becomes
a health problem when it is present in our layer of the atmosphere and
we breathe it in. Because of this, the tropospheric ozone is often referred
to as "bad ozone" and the stratospheric ozone as "good
- Urban Heat Islands
heat islands" are a sort of localized enhanced greenhouse phenomenon.
They are simply built-up areas of city that are significantly warmer than
the surrounding area of countryside. The difference in temperature comes
from the fact that buildings, paved surfaces, and other man-made structures
absorb higher amounts of sunlight than most natural objects. This energy
is re-radiated at longer wavelengths during the night, and atmospheric
pollution in the form of heat-absorbing gases form a "local"
atmosphere much like the glass of a greenhouse, trapping in the heat.
have noticed that metropolitan areas are creating their own weather patterns
at night due to the collision of cool air from the surrounding area with
the warmer city air. It is important to note that urban heat islands are
a localized effect, whereas the general atmospheric greenhouse effect
is global in extent.
of Air Pollution
pollution has numerous impacts on ecosystems and human health. At an extreme
is the devastation of areas like Sudbury, Canada, from acid rain and large
areas of the Black Forest regions in East Germany from decades of unchecked
industrial pollution. Human health effects include respiratory problems
as well as effects on the eyes and skin. Different effects are associated
with different concentrations of the pollutant. Although people react
much more sharply to odors in the air and early air pollution standards
were set by aesthetic conditions, there are odorless but dangerous pollutants
such as CO. Maximum allowed (or permissible) concentrations (MAC or MPC)
are usually set on a citywide, regional, or statewide basis to control
air pollution. Emergency measures, such as closing industrial plants,
limiting auto use, and advising children or people with respiratory problems
to remain indoors, are sometimes taken when there are dangerous pollutant
oxides such as Sox and NOx cause corrosions of many materials
such as metals and limestone and can cause damage to structures. Typical
MAC of SO2 is about 0.3 ppm. NO2 changes vegetation. Both of
these can contribute to the development of respiratory disease.
monoxide is formed by rapid burning of carbon in an environment with insufficient
oxygen. CO in concentrations of 2000 ppm causes death by interfering with
the distribution of oxygen in the body. Hemoglobin is the molecule in
the blood that carries O2 to all parts of the body. The CO molecule has
the same overall shape as the O2 and fits into the part of the hemoglobin
that normally carries O2, thus making the space unavailable for O2. The
compound carboxyhemoglobin can affect the ability to track and see clearly
if breathed at 30 ppm for 8 hours. At that rate of inhalation, 10% of
the hemoglobin can become carboxyhemoglobin.
has a strong odor even at 0.02 ppm, and can cause damage to biological
tissues and to some materials like rubber. Ozone irritates the eyes and
upper respiratory tract at concentrations of 0.1 ppm.
dust, soot, and other materials ejected in various processes can become
airborne. Particulates in the air can also cause health problems. Visibility
is reduced when particulate concentration are high. Asthma, an in creasing
problem in children and in populations and other respiratory problems
are aggravated by inhaled particulates.