Unit 1 - Chapter 4 Atmospheric Chemistry and Air Pollution
9. List four important reasons why coarse particles usually are of less danger to human health than are fine particles.
1) Since coarse particles settle out quickly, human exposure to them via inhalation is reduced. 2) When inhaled, coarse particles are efficiently filtered by the nose (including its hairs) and throat and generally do not travel as far as the lungs. In contrast, inhaled fine particles usually travel through to the lungs (which is why they are called respirable), can be adsorbed on cell surfaces there, and can consequently affect our health. 3) The ratio of surface area to mass of large particles is smaller than that of small ones, and thus, gram for gram, their ability to transport adsorbed gas molecules to any parts of the respiratory system and there to catalyze chemical and biochemical reactions is correspondingly smaller. 4) Devices such as electrostatic precipitators, spray towers, and cyclone collectors that are used to remove particulates from air are efficient only for coarse particles. Thus, although a device may remove 95% of the total particulate mass, the reduction of surface area and of respirable particles is a much lower fraction.
6. Describe the effects of acid precipitation upon (a) dissolved levels of aluminum, (b) fish populations, and (c) trees.
A) Acidified lakes characteristically have elevated concentrations of dissolved aluminum ion, Al3, and it is now known that many of the biological effects of acid rain are due to increased levels of aluminum ion dissolved in water rather than to the hydrogen ion itself. Aluminum ions are leached from rocks in contact with acidified water by reaction with the hydrogen ions; under normal, near-neutral pH conditions, the aluminum is immobilized in the rocks by their insolubility. B)Scientists believe that both the acidity itself and the high concentrations of aluminum together are responsible for the devastating decreases in fish populations that have been observed in many acidified water systems. Different types of fish and aquatic plants vary in their tolerances for aluminum and acid, so the biological composition of a lake varies as it gradually becomes increasingly acidic. Generally speaking, fish reproduction is severely diminished even at low levels of acidity that can, however, be tolerated by adult fish. Very young fish, hatched in early spring, are also subject to the shock of very acidic water that occurs when the acidic winter snow all melts in a short time and enters the water systems. C) As discussed above, acidification of the soil can leach nutrients from it and, as occurs in lakes, solubilize aluminum. This element may interfere with the uptake of nutrients by trees and other plants. Apparently, both the acidity of the rain falling on affected forests and the tropospheric ozone and other oxidants in the air to which they are exposed pose significant stresses to the trees. These two stresses alone will not kill them, but when combined with drought, temperature extremes, disease, or insect attack, the trees become much more vulnerable. Forests at high altitudes are most affected by acid precipitation, possibly because they are exposed to the base of low-level clouds, where the acidity is most concentrated. Fogs and mists are more acidic even than precipitation, since there is much less total water to dilute the acid.
1. Discuss the relationship between atmospheric particulates and haze.
Haze is produced by a high concentration of particles of diameter between 0.1 µm and 1µm. This is one of the manifestations of photochemical smog. Since most fine particles in urban air are secondary, their number can be controlled only by reducing emissions of the primary pollutant gases from which they are created.
4. Using chemical equations, explain how acid rain is neutralized by limestone that is present in soil. Describe ways in which humans have tried to neutralize acidified lakes.
If the bedrock is limestone or chalk, the acid can be efficiently neutralized ("buffered"), since these rocks are composed of calcium carbonate, CaCO3, which acts as a base and reacts with acid, producing bicarbonate ion, HCO3-, as an intermediate: CaCO3(s) + H(aq) --> Ca2+(aq) HCO3-(aq) HCO3-(aq) + H(aq) --> H2CO3(aq)--> CO2(g) + H2O(aq) The reactions here proceed almost to completion, owing to the excess of H+ that is present. Thus the rock dissolves, producing carbon dioxide and replacing the hydrogen ion by calcium ions. Neutralization by calcium carbonate and similar compounds that are commonly present as suspended particles in atmospheric dust is the mechanism by which carbonic acid in normal rainfall, and acid rain over some areas, has a greater than expected pH. In contrast, areas strongly affected by acid rain are those having granite or quartz bedrock, since the soil there has little capacity to neutralize the acid.
2. What is acid rain? What two acids predominate in it? Explain why the predominant acid in acid rain differs in eastern and western North America.
Rain with pH less than 5 is considered acid rain (also acid snow, acid fog) The main cause of acid rain are sulfur oxides and nitrogen oxides. sulfur dioxide--> sulfuric acid rain Nitric oxide--> Nitric acid rain In eastern North America, sulfuric acid greatly predominates because some electrical power is generated from power plants that use high-sulfur coal. In western North America, nitric acid attributable to vehicle emissions is predominant, since the coal mined and burned there is low in sulfur.
8. What is the difference in meaning between absorbed and adsorbed when they refer to particulates?
Substances that dissolve into the body of a particle are said to be absorbed by it; those that simply stick loosely to the surface of the particle are said to be adsorbed.
5. Explain what is meant by the expression critical load.
The regulatory scheme used in the United States of requiring reductions in sulfur dioxide emissions in certain designated geographical regions has been extended by European scientists and regulators into the concept of critical load, which recognizes that different levels of risk from acid rain are faced in different regions. Geographical areas having buffering capacity can withstand a much greater input of acid rain before damage occurs than those without the capacity. Thus, higher sulfur dioxide emissions from a particular region can be allowed if the area in which the resulting sulfuric acid is usually deposited has a high critical load. To determine the critical loads for different regions, scientists use computer models that incorporate soil chemistry, rainfall, topography, etc. The use of the concept has had great success in Sweden, for example. In using critical loads, pollution control becomes effects-based rather than source-based.
3. What is the difference between dry and wet deposition?
dry deposition, the process by which nonaqueous chemicals are deposited onto solid and liquid surfaces at ground level when air containing them passes over the surfaces.Much of the original SO2 gas is not oxidized in the air but rather is removed by dry deposition from air before reaction can occur: oxidation and conversion to sulfuric acid occurs after deposition. Wet deposition processes encompass the transfer of pollutants to the Earth's surface by rain, snow, or fog—i.e., by aqueous solutions.
7. Explain the difference between soot-and sulfur- dioxide smog and photochemical smog in terms of the chemicals involved and the health effects they cause.
soot-and sulfur- dioxide smog is chemically reducing in nature due to the presence of sulfur dioxide, photochemical smogs are oxidizing. research has shown a positive correlation between atmospheric concentrations of oxidized sulfur and ozone and hospital admissions for respiratory problems in southern Ontario. There is some evidence that the acidity of the pollution is the main active agent in causing lung dysfunction, including wheezing and bronchitis in children. Asthmatic individuals are adversely affected by acidic sulfate aerosols, even at very low concentrations. It has been speculated that pollution due to SO2 and sulfates causes a decrease in resistance to colon and breast cancer in people living in northern latitudes. The suggested mechanism of this action is a reduction in the amount of available UV-B that is necessary to form vitamin D, which is a protective agent for both types of cancers. Since sulfur dioxide absorbs UV-B and sulfate particles scatter it, significant concentrations of either substance in air will reduce the amount of UV-B reaching ground level. Photochemical smog, which arises from nitrogen oxides, is now more important than is soot-and-sulfur smog in most cities, particularly those with high population and vehicle density. it consists of gases such as ozone, and an aqueous phase containing water-soluble organic and inorganic compounds in the form of suspended particles. ozone (found in photochemical smog) produces transient irritation in the respiratory system, giving rise to coughing, nose and throat irritation, shortness of breath, and chest pains upon deep breathing. People with respiratory problems can often tell from their symptoms—such as the tightening of the chest or the beginning of a cough— when the air quality is poor. Also the subsequent rate of mortality from circulatory disease suffered by the patients increased the higher their exposure to traffic-related air pollution measured by the concentration of NO2 in the air near their homes.