(5) Pollution

Major Air Pollutants (Video)

Table of Common Pollutants

Pollutants and Sources

What Are the Six Common Air Pollutants?


Pollutants can be transported in unexpected ways

The grasshopper effect

The “grasshopper effect” (also called “global distillation”) is a special case of pollutant fate and transport. The insecticide, dichlorodiphenyltrichloroethane (DDT) provides an illustration. If DDT is used in a Latin American country, it evaporates and prevailing winds blow it north. As DDT encounters cooler air, it condenses and comes to earth. On a warm day, it evaporates again. The process repeats itself, sometimes many times. Finally, in the far north, it is too cold for DDT to evaporate again, so it stays put – the Arctic is a sink for DDT and similar persistent organic pollutants (POPs).

The POPs in the Arctic accumulate in soils and water, enter the Arctic food chain, and build up in the fat of marine mammals. The Inuit, the Arctic’s indigenous people, eat the contaminated animals with the result that DDT builds up in their body fat to levels among the highest seen in the world. Worse, because DDT and other POPs concentrate in fat, high levels are found in the fatty portion of a mother’s milk. Thus, infants receive risky amounts of these pollutants as they nurse. Canada has been working to cut pollutant flow from the south and in 2008, there was good news. Canadian government studies indicate that levels of PCBs, DDT, and other POPs in the flesh of Arctic animals have either leveled off or begun to decline. This can probably be attributed to an international treaty that banned production and use of a number of POPs.

But, if the grasshopper effect totally explained DDT accumulation in the frozen North, then DDT should be evenly distributed across the Arctic. In actuality, Canadian scientists find hotspots. Sediments in certain ponds have concentrations of DDT, 60 times higher than at nearby spots serving as controls. Investigation revealed that contaminated droppings of migratory seabirds (large numbers of which nest on cliffs over the ponds) were responsible for these striking observations. How did this happen? The DDT originated in southern regions where it contaminates fish in coastal waters. Seabirds eating the fish also become contaminated.

Thereafter, the birds migrate to nesting sites over Arctic ponds where their DDT-contaminated droppings fall to ponds below. Those droppings are major sources of nutrition into Arctic pond ecosystems, promoting the growth of moss and plankton, which become contaminated with DDT. Insects eat the contaminated moss and plankton. They, in turn, are eaten by birds and other animals. Thus, these chemicals continue to spread in the food web and in humans. This is land-based bioaccumulation.

Pollutants in sediments often don’t stay buried

Sediments are composed of soil, silt, minerals, and organic materials that have been carried in rainwater runoff from surrounding land and paved surfaces into a lake, river, or other water body. By its nature, sediment is buried by additional incoming sedimentary material. Other pollutants are often buried within the sediments too – but they are not dependably buried.

▪ Bottom-feeding organisms may take in the pollutants, thus introducing them into the food web.

 ▪ Riverine and coastal area sediments are sometimes dredged. When that happens, contaminants are brought back to the surface along with the sediment.

▪Water currents, such as a strongly flowing river, also move sediments. Here, again, you see a situation where pollutants may not stay put.

Soil pollutants likewise move

Pollutants in soil also may not stay trapped. Water percolating through soil can carry pollutants down into groundwater. Rainwater can dissolve and carry off pollutants, including pesticides and fertilizers. Rainwater also erodes soil that may have pollutants absorbed within it.

The chemical fate of pollutants

Pollutants not only move. Their fate is often - as noted with acid rain precursors - to be converted into other chemicals, undergoing reactions in the atmosphere, water, and soil.

Organic pollutants

Especially in moderate and warm climes, organic pollutants can be degraded in water, soil, and the atmosphere to end products that are less risky than the parent compounds.

Microorganisms (fungi and bacteria) degrade organic wastes, including plant debris, animal remains, the organic material in trash, and also many individual organic pollutants. Microbes work in both water and soil. Microbial breakdown is a vital natural service: wastes and chemical pollutants would otherwise build up in the environment to intolerable levels.

▪ CO2 and water are the end products of microbial metabolism when oxygen is present. An organic substance degraded all the way to CO2 and water is said to be mineralized.

▪ Some microorganisms can degrade organic substances without requiring oxygen to do so. In that case, the most common end product is methane (“swamp gas”) as seen when microbial degradation occurs in the mud of rice paddies or marshes.

▪ Some synthetic organic chemicals have structures that make it very difficult for microbes to degrade them. Included among these substances are polychlorinated chemicals such as dioxins, DDT, and PCBs, which sometimes persist in the environment for many years and, in very cold climates, indefinitely.

▪ Other factors contribute to degrading organic substances too. Atmospheric oxygen reacts with many organic substances.

▪ Heat: the higher the temperature, the more rapidly organic materials break down. In very cold conditions, the Arctic and Antarctic, organics may persist for many thousands of years, becoming deeply buried in snow and ice.

▪ Sunlight, especially the strong ultraviolet radiation of summer, contributes to the breakdown of organic pollutants.

▪ Wave motion in water assists degradation by bringing pollutants to the surface, exposing them to sunlight, heat, and oxygen.

A chemical species, the hydroxyl radical contributes to the degradation of both organic and inorganic substances.

Overwhelming the process

These processes provide natural services that are very effective in degrading organic substances. However, human activities often overwhelm natural systems. Food-processors, tanneries, and paper mills are examples of facilities that, historically, released such large quantities of pollutants and wastes into rivers and lakes that natural processes could not degrade them all. Thus water quality was severely degraded.

Inorganic pollutants

Inorganic chemicals are not mineralized to CO2 and water – they are already mineral substances. Inorganic substances do undergo chemical reactions, but are not destroyed in the same manner as organic materials. Think about a metal. Box 1.2 noted the instance of metals burned to metal oxides. Such oxidation can also occur, albeit slowly, without combustion. You may have seen a reddish bridge: the color results from the oxidation of the iron in the bridge, that is, iron reacts with atmospheric oxygen to form reddish iron oxide.

But take a sample of that iron oxide and heat it to a high enough temperature: you recover the iron while driving the oxygen back into the air It too reacts with oxygen to yield sulfur dioxide. As with iron oxide, given proper conditions, both sulfur and oxygen can be recovered.

Pollution that devastates

Sometimes a pollution event is so tragic that it changes our way of looking at the world. The deadly explosion that occurred in Bhopal India is one such event. Union Carbide, an American-owned factory in Bhopal, manufactured the insecticide carbaryl. The process used methyl isocyanate (MIC), an extremely toxic volatile liquid, which reacts violently with water. Despite this, the factory lacked stringent measures to prevent water from contacting MIC. During the night of December 2, 1984, water entered a storage tank containing 50 000 gallons (189 000 l) of MIC.

▪ The Indian government later said that improper washing of lines going into the tank caused the catastrophe. Union Carbide claimed that a disgruntled employee deliberately introduced water.

 ▪ In any case, 25-40 tons (23-36 tones) of a deadly chemical vapor settled over half this city of 800 000. About 3400 people were killed overnight, and perhaps another 15 000 died from their exposure in the following days and years. Over 40% of the women, who were pregnant at the time, had miscarriages. Tens of thousands more remained chronically ill 20-years later with respiratory infections, eye damage, neurological damage, and other ills. The catastrophe was worsened because many people lived crowded close around the factory. Moreover, poisoned residents received little medical attention at the time of the accident, at least partially because physicians didn’t know what compounds were in the toxic cloud. Thus, it was difficult to know the best mode of treatment.

 ▪ Compensation came slowly. For many years, a Bhopal court had criminal charges pending against Union Carbide’s then Chief Executive Officer, accusing him of having consciously decided to cut back on safety and alarm systems as cost-cutting measures.

▪ In 1984, Union Carbide had almost 100 000 employees, but almost went out of business and, by 1994, employed only13 000. In 2001, Dow Chemical bought the remains of Union Carbide and, not surprisingly, found that it was now held responsible for this continuing tragedy.

Tiny levels of contaminants

Bhopal represents horrendous pollution. Its opposite, levels of pollutants so low that they are barely detectable, presents a quandary – are such levels risky? Modern analytical chemistry is so sensitive that synthetic organic chemicals can be detected almost anyplace – in soil, water, air, food, animals, plants, and in our own bodies. As one scientist commented, “The analytical science has advanced just astronomically.” So how are we to think about such situations?

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