Atmospheric Pollution

Carbon Monoxide

Carbon monoxide (CO) is a by-product of fuel combustion, resulting from incomplete oxidation [Page 152]of carbon in the combustion process. In North America, about 80% of CO emissions come from internal combustion engines used in transportation. Natural sources include forest fires (which may also be anthropogenically driven) and volcanoes. Indoor sources of CO include wood burning and gas stoves or furnaces.

Mexico City, which is partially surrounded by mountains, suffers from prolonged temperature inversions and severe smog episodes.

Source: Morguefile.

On a global scale, CO levels average about 100 ppb. In urban areas, the levels are elevated to a few ppm, while in traffic, values may be measured in tens of ppm. CO is a precursor to greenhouse gases such as methane and carbon dioxide.

In the human body, CO is attracted to hemoglobin in the bloodstream, replacing oxygen. Exposure to low levels may cause fatigue in healthy people and is more dangerous for persons with heart disease. High levels can be fatal and because of its colorless and odorless characteristics, it is often called a “silent killer.”

Oxides of Nitrogen

Harmful oxides of nitrogen include nitric oxide or nitrogen monoxide (NO) and nitrogen dioxide (NO2), which are collectively called NOx. High energy is required to break the strong triple bond of molecular nitrogen to generate elemental nitrogen for oxidation. Hence, the formation of nitric oxide generally results from high temperature processes such as fuel combustion in transport engines or industries, such as those involved in thermal power generation. In North America, approximately 60% of anthropogenic NOx gases result from transportation emissions and about 35% from industry, including electric power plants. Natural sources include lightening and microbial activity in soils. Nitric oxide has a lifetime of minutes, so once emitted it quickly oxidizes to NO2. NOx species play pivotal roles in the complex chemistry of the atmosphere, reacting with other gases, for example, in the creation and depletion of ground-level ozone.

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