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Acid Rain in New England

Acid Rain Monitoring

Kendall Street Station, Cambridge, MA - Click for larger image

Continuous emissions monitors (CEMs) tell us how much of a pollutant a power plant (or other affected facility) emits. Under the Acid Rain Program, each affected unit must monitor emissions of sulfur dioxide and nitrogen oxides. Most also measure carbon dioxide. CEMs are the most common way to monitor emissions, but in some cases, Utilities may use an approved alternate method. For example, if a unit burns oil and they know the sulfur content of the oil and the amount they used, they can compute the amount of sulfur dioxide they will emit.

Each plant in the acid rain program must submit a monitoring plan to EPA and revise it as their monitoring system changes. This document tells which monitors will be used, where they will be located, and how the data will be gathered and sent to EPA. If a plant’s monitoring system changes, they must revise their monitoring plan.

There are several ways to monitor emissions from a plant. We measure nitrogen oxides by taking a sample of the gas emitted thought the plant’s smokestack and analyzing it. In some cases sulfur dioxide is measured this way, but we can also compute sulfur emissions from the fuel’s sulfur content, as noted above. When plants use natural gas, which contains only traces of sulfur, we may estimate the plant’s emissions by assuming the gas contains a certain low amount of sulfur. Sulfur emissions from such plants are so low that an actual measurement of sulfur dioxide is impractical.

Power plants must submit data whenever they are operating. If their monitor is not operating, they must report a "default" value, which is generally the maximum amount of the pollutant they can emit. EPA gathers the data, and tabulates the emissions form each plant. This information is in the annual acid rain progress report, which can be found on the Clean Air Markets Division webpage.

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