Measuring tiny amounts of atmospheric pollutants

Several gases pollute the lower atmosphere as a result of things people do. Ozone, carbon monoxide, nitrogen oxides, and hydrocarbons are all emitted from automobiles and trucks. Sulfur oxides, carbon monoxide, and finely divided particulates are emitted from coal-burning power plants. Home heating and industrial sources add other gases and finely divided solids. Nature contributes its own atmospheric pollutants when lightening starts a forest fire, or when a volcano erupts, or when the sun bakes organic compounds out of the leaves of trees. Carbon dioxide, a gas produced when any carbon-containing material burns, was not considered a pollutant until recent years. Now that it is implicated in global warming, steps are being taken to reduce CO₂ emissions wherever possible.

The dangers of polluting our atmosphere were not fully realized until scientists figured out how to measure small amounts of diverse substances dispersed in very large volumes of atmospheric gases. Remember that sometimes pollutants are present in concentrations low enough to be measured in parts-per-million and even parts-per-billion. The time-honored technique of dealing with small amounts of material to be measured is to find ways of concentrating it. Solids, for example, could be trapped by pumping air through a filter. Reactive gases could be trapped by pumping air through solutions that react with specific gases. The material on the filter paper would then be analyzed, or the amount of chemical change in the solution would be determined.

Modern electronics, innovative separation techniques, and sophisticated detectors make simpler, faster measurements possible. One often used experimental method is gas chromatography, diagrammed in the following figure. In a gas chromatographic experiment, the sample to be analyzed is mixed with an inert carrier gas, and then blown through a long column. This "column" is actually a coiled tube, shown in dark green in the picture below. It is packed with a material to interact with (but not react with) the components of the gas being analyzed. Each solute gas moves through the column at a different speed. So the gases separate. Gases that move faster move through the column exit the column first, and those that move more slowly emerge later. Since the time to pass through the column is different for each gas, gases can be identified by how long their trip through the column takes. A detector at the end of the column measures how much of each gas is present as it emerges from the column. It records that information as a peak on a plot. The more of a gas is present, the higher its peak will be. Therefore, by measuring the time a gas takes to pass through the column we know what kind of gas it is, and by the size of the peak it produces on the plot, we know how much is present.

Schematic of a gas chromatograph. After Harris, D. C. Quantitative Chemical Analysis, 3rd ed. New York: Freeman, 1991.

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Schematic of a gas chromatograph: Courtesy Centre for Atmospheric Science, University of Cambridge.