Chapter 14 Human Effects: Air Pollution and Heat Islands

Air pollution concerns the introduction of undesirable gases and particulates by humans. Particulates (aerosols) are solid and liquid materials in the air that are of natural or anthropogenic origin. Pollutants can be divided into two categories: Primary pollutants are emitted directly into the atmosphere. Secondary pollutants result from chemical transformations.

Amounts and sources of atmospheric pollutants.

Several different processes remove particulates from the air. Gravitational settling, the process wherein they fall from the air, effectively removes larger particulates. Scavenging is the process in which falling precipitation collides with particulates and carries them to the surface.

Carbon monoxide is a colorless, odorless gas. Exposure for three hours at 400 ppm is life-threatening, and at 1600 ppm death occurs within an hour. The most important source of CO in the U.S. is the automobile.

Threshold Levels of Carbon Monoxide

Sulfur dioxide is a primary pollutant released mainly by the burning of sulfur-containing fossil fuels, such as coal and oil. It is a colorless but highly corrosive gas that irritates human respiratory systems.

Sulfur trioxide combines with water droplets to form sulfuric acid. If this process occurs near the surface, it forms acid fog. If it occurs in clouds, subsequent precipitation of the acid compound produces acid rain.

Acid precipitation is a serious problem in the Northeast.

Nitrogen dioxide is relatively toxic, causing serious pulmonary health problems; is corrosive; and undergoes transformations that contribute to acid deposition and other secondary pollutants. It also gives polluted air a yellowish to reddish brown color, as in this photo of Hong Kong.

Volatile organic compounds (VOC) or hydrocarbons, are materials made entirely of carbon and hydrogen atoms. Industrial activities account for the greatest proportion of anthropogenic hydrocarbons in the U.S. While they pose no direct adverse health impacts, in the presence of sunlight they recombine with nitrogen oxides and oxygen to produce photochemical smog.

Photochemical smog forms when sunlight triggers reactions and transformations of gases and aerosols. Negative effects include burning eyes, sore lungs, and an unpleasant odor with poor visibility. Los Angeles-type smog (above) usually involves dry air while London-type smog combines smoke and damp air.

Ozone is the most important agent of photochemical smog, causing serious physical and environmental harm, including inflammation of air passages that can reduce lung capacity by as much as 20 percent.

Strong winds rapidly transport emissions from their source and spread them over a wide horizontal extent with the concentration of pollution inversely proportional to wind speed. High wind speeds also lower pollution concentration indirectly. Short-term variations in wind direction also affect dispersion. If wind directions are highly variable, pollutants will spread over a wider area.

In (a) the wind blows at 5 m/sec so that each puff of smoke travels 5 m before the next is released. In (b) the wind flows twice as fast as in (a), and the distance between successive puffs of smoke likewise doubles. Thus, the greater wind speed in (b) causes the same amount of pollution to be diluted within twice as large a volume of air.

Stable air resists vertical displacement and leads to higher pollutant concentrations near the ground. Unstable air enhances vertical mixing, reducing pollution concentrations near the surface. Inversions make the air extremely stable and impose the greatest restraint on vertical mixing, with the impact on pollution concentrations occurring in the morning.

The term urban heat island implies that urban areas often have higher temperatures than adjacent countrysides as a result of natural surfaces being paved and built upon, and human activities releasing heat into the local environment. The highest temperatures are normally found within the city core. Urban–rural temperature differences are greatest during the late evening and night and during the winter months.

As incoming radiation contacts a building, some is scattered in all directions and some is absorbed. The scattered radiation may in turn hit an adjacent building where further absorption can take place, lowering the urban albedo.

Increased particulates associated with urban activity can absorb and scatter incoming solar radiation and also increase the amount of absorption and reradiation of longwave energy in the atmosphere. The increase in particulates can also increase cloud cover. Precipitation can decrease downwind of urban centers as cloud water is spread over many condensation nuclei, which lessens the chance of growth to precipitation size.

The next chapter examines Earth’s climates.