Use a simple box model of the United States planetary boundary layer (roughly the lowest 2 km of the atmosphere) to estimate an emission factor of sulfur dioxide from coal combustion (kg SO2 emitted / ton coal burned) in the year 2000. Assume that air resides in the US boundary layer for 3 days on average. Sulfur dioxide reacts to form sulfate with a chemical lifetime of 4.5 days and has a depositional lifetime of about 6 days. The US burned 109 tons of coal in 2000. Mixing ratios of SO2 vary substantially from place to place within the US but a national "average" value would be approximately 500 pptv.