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Regional & Local Atmospheric Environmental Concerns

While the problems of stratospheric ozone depletion and climate change are global in scale, acid deposition, another air-related environmental problem, is regional.

Regional - Acid Rain

The composition 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.

Rain or 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.

When acidic 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.

Increasingly 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.

Currently, 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.

The other 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.

The problem 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.


Local - Photochemical Smog & Tropospheric Ozone

Smog (SMoke 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.

Photochemical 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.

Among the gases produced in the photochemical smog are ozone and peroxyacetyl nitrate (often referred to as PAN). The following reactions produce ground-level ozone:

NO2 + uv NO + O
O + O2 (+ catalyst) O3 (+ catalyst)

Numerous 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.

Also note 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 photochemical smog.

Figure 13: Rise of ozone smog toward mid day.

So, ozone 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 ozone".


Local - Urban Heat Islands

"Urban 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.

Figure 14: Graph of temperatures showing urban heat island effect.
(from Lawrence Berkeley National Laboratory)

Meteorologists 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.

Effects of Air Pollution

Air 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 levels.

Acidic 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.

Carbon 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.

Ozone 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.

Lead, 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.




  ©Copyright 2003 Carnegie Mellon University
This material is based upon work supported by the National Science Foundation under Grant Number 9653194. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.