The Need to Understand Environmental Problems

 So, it is clear that the human population is dramatically increasing and is expected to continue to increase. More human beings means an increased demand for energy, water, food, and other resources needed to sustain our species, and this leads to increasing levels of pollution in the air, fresh and salt waters, soils, subsurface, and biosphere. Past environmental problems, when the world populations were smaller, occurred primarily in urban centers. London’s use of coal in the early seventeenth and eighteenth centuries allowed it to grow, but also produced significant levels of air pollution, which was dominated by soot and sulfur dioxide emissions resulting in reduced visibility. The term “smog” was first used in London because these air pollution events looked like heavy smoky fogs (smoke + fog = smog). The high-density population and traffic in Los Angeles, CA in the 1950s led to the formation of “photochemical smog,” which was determined to be due to the emissions of nitrogen oxides and unburned volatile organic hydrocarbons from the internal combustion engine reacting to form ozone. Details on these air pollution issues will be discussed further in Chapter 5. These observations led to research which developed a fundamental understanding of the chemistry involved in the processes. This understanding has been used to develop control strategies to reduce the levels of these pollutants in many urban areas. While these problems are still affecting all major cities, we have begun to reduce the levels of key air pollutants, such as ozone and carbon monoxide, in many of the megacity environments.

The need to fully understand the chemistry of any material added to our environment is emphasized by cases where a chemical species that was developed to solve a specific problem was found later to lead to more severe environmental and health impacts and to the eventual regulation or banning of the materials. One example of this is the use of chlorofluorocarbons (CFCs), whose chemistry will be described in detail in Chapter 4. One of the first commercial CFCs was dichlorodifluoromethane (CCl2F2), with the trademark name Freon. It was developed by Thomas Midgely, Jr. and Albert Henne, who worked for General Motors Research in the late 1920s. The compound was specifically developed as an operating fluid to replace the toxic and flammable gases of ammonia, sulfur dioxide, propane, and chloroform (chloromethane) that were being used in refrigeration at that time. Midgely, shown in Figure 1.8, received awards for the development of Freon, as it was thought to be environmentally benign due to its low reactivity.

At the time, we did not understand the chemistry of the stratosphere, nor did we under-stand atmospheric chemical transport. A better understanding of these atmospheric processes would soon lead to the discovery that Freon and other CFCs would build up in the lower atmosphere, be transported to the upper atmosphere, and lead to the potential catalytic destruction of the stratospheric ozone that was shielding all life from harmful ultraviolet (UV) radiation. Later work also discovered that these same CFCs were very strong infrared-radiation absorbers, allowing them to act as potent greenhouse gases. These problems made the CFCs environmentally unsound for long-term usage, and led to their worldwide ban by the Montreal Protocol. Thus, a better understanding of the chemistry and physics of the atmosphere led to the determination that these apparently safe compounds had other significant impacts on the global systems that were unforeseen at the time.

Another example is the use of alkyl lead products, tetramethyllead ((CH3)4Pb) and tetraethyl-lead ((CH3CH2)4Pb), chemical additives that were added to gasoline to improve motor vehicle operation. These fuel additives were developed to reduce knocking in motor vehicles by enhancing and stabilizing the fuel energy content. At the time, little attention was paid to the impacts of the alkyl lead combustion products. We now know that combustion of these additives leads to the formation of lead-containing aerosol particles emitted to the atmosphere in the vehicle’s exhaust. The emission of these lead aerosols results in elevated lead levels in the nearby environment approaching harmful levels, especially for young children. This led to their eventual elimination and ban as a fuel additive, followed by the development of unleaded gasolines now used primarily throughout the United States and the world. These new unleaded gasolines are more refined with a higher octane content, which solves the engine-knocking problem while avoiding the toxic lead combustion products. Interestingly, both the alkyl lead gasoline additives and the CFCs were developed by Thomas Midgely, Jr.

The increasing need for food production led to the development and widespread use of agricultural pesticides to prevent crop losses. One of the most well known of these pesticides is 1,1′-(2,2,2-trichloroethane-1,1-diyl)bis(4-chlorobenzene) or dichlorodiphenyltrichloroethane (DDT). DDT was first synthesized in the late nineteenth century, but its toxicity to insects and, in particular, disease-carrying mosquitos was first discovered by chemist Paul Hermann Müller in 1939, who won the Nobel Prize in Medicine for his work. Later, it was discovered that DDT was bioaccumulated in the food chain and caused serious problems with bird reproduction. This problem was publicized by marine biologist Rachel Carson in her book Silent Spring (Carson, 1962), which led to an environmental movement and the eventual ban of the use of DDT except in the most serious situations. Since that time, chemists involved in the development of pesticides are required to investigate their toxicity and chemical reactivity in a broad range of environmental systems to insure that the compounds do not lead to similar problems.

Once it was recognized that the emission of chemicals into our air, water, and soils could lead to significant short- and long-term impacts that were unintentional, regulations and con-trol strategies were initiated to attempt to limit the impacts. However, in many cases control strategies were implemented before a thorough understanding of the chemical and physical impacts of the chemicals were well understood with regard to their transport, transformation, and removal processes on the various scales involved in the environmental systems. An example of a pollution control strategy that solved a local problem but led to a regional one was the use of tall stacks for the release of combustion gases from power plants and industrial sites. These tall stacks were designed to release pollutant gases at a higher altitude in the atmosphere. The gases then entered the atmosphere’s mixing layer, resulting in the dilution of the gases by mixing them together with uncontaminated air. This also kept them above ground level, where they would not be a direct risk to local populations. Although the release of sulfur dioxide from coal-fired power plants at mixing layer height reduced the immediate damage to the local area, this high-altitude release allowed the sulfur dioxide and associated air pollutants to be transported long distances from the source. During the transport time, the sulfur dioxide was converted to sulfuric acid, which was eventually deposited on the ground in rain, leading to “acid rain” on a regional scale. We now recognize that the solution to pollution is not dilution and, in most cases, we work to either trap pollutants before release into the atmosphere or to convert any toxic or environmentally damaging pollution to as inert a form as possible before release in order to minimize the environmental impacts.

These examples and others, which we will discuss in the following chapters, clearly indicate why a thorough understanding of environmental problems and how their chemistry is linked to all the environmental systems is essential to preventing additional, otherwise unforeseen impacts. Our past problems have given us the opportunity to learn from our mistakes and we are now at a point in time when the chemistry involved in these environmental processes is fairly well understood, allowing for the prediction of possible negative feedbacks of new technologies, materials, and chemicals before they happen in order to propose more effective mitigation or remediation strategies. The past limitations of our understanding of the environmental chemistry of the various environmental systems led to an inability to consider the “cradle to grave” consequences of the use of chemicals and their release to the environment. However, our cur-rent understanding of the processes has progressed far enough to allow us to predict impacts and work toward using environmental chemistry to limit these impacts as much as possible as we examine the overall lifecycles of chemicals in the environment. The fundamental principles of environmental chemistry require that we examine all of the potential systems where the chemicals can have impacts, so that we do not repeat our mistakes from the past and do not create additional unforeseen problems. Thus, we can now work toward a proactive approach to environmental chemistry that considers all of the Earth’s environmental systems.