Over the past few years, a stream's pH has fallen from approximately 6.0 to approximately 4.5. The drop in pH began shortly after the construction of a coal-burning power plant several miles from the stream, and it coincided with a significant increase in sulfur dioxide levels in the local atmosphere. A spokesperson for the power plant argues that there is no evidence of a link between the power plant's activities and the stream's decreasing pH, because the power plant does not produce acids.

Is the spokesperson's argument valid? Support your answer by explaining the relationship between sulfur dioxide and acid precipitation.