In 1990, Congress passed an amendment to the act that included specific provisions for monitoring acid deposition in the 48 contiguous states and the District of Columbia. The acid rain amendment laid the groundwork for a cap-and-trade type of emissions trading program for sulfur dioxide, Lehmann says.
For example, fresh lemon juice, a highly acidic substance, has a pH of 2. Rainwater that is considered pure is closer to 5.6. Precipitation with a pH lower than 5 is called acid rain, Lehmann says.
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