Answer:
Acidity and alkalinity are measured using a pH scale, for which 7.0 is neutral. The lower a substance's pH (less than 7), the more acidic it is; the higher a substance's pH (greater than 7), the more alkaline it is. Average rain has a pH of about 5.6; it is slightly acidic because carbon dioxide (CO2) dissolves into it, forming weak carbonic acid. Acid rain usually has a pH between 4.2 and 4.4. The most common nitrogen-related compounds emitted into the air by human activities are nitrogen oxides. Ammonia is another nitrogen compound emitted into the atmosphere, primarily from agricultural activities and fossil fuels. Most of the nitrogen oxides released in the U.S. due to human activity are from the burning of fossil fuels associated with transportation and industry.
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