For SO2, the Acid Rain Program places a mandatory ceiling, or cap, on emissions nationwide from electric
Percentage Changes in Total Emissions of SO2, NOX, NH3, and VOCs between 1990 and 1997 in Europe, the United States, and Asia
aData from IPCC (2000) for the years 1990 to 1997. Streets et al. (2001) report increases of 16% for SO2 and 53% for NOX for northeast Asia.
b n. a., not available.
utilities and allocates emissions to these pollution sources in the form of allowances, or a permit to emit 1 ton of SO2. Emissions from large electric power plants have been reduced through phase 1 of the Acid Rain Program. The emission trading has proven to be a cost-effective mechanism and has facilitated 100% compliance by affected sources and stimulated early emission reduction. Phase 2 was started in 2000 and will result in a reduction for a greater number of smaller plants, and emissions from larger plants will be further reduced.
Measures to limit NOx emissions in the United States have largely focused on decreasing human health effects from tropospheric ozone, of which NOx is a primary precursor. NOx abatement has also been achieved as a means to address ecosystem acidification in conjunction with much larger decreases in SO2 emissions. NOx emissions from stationary and mobile sources are limited by efforts to comply with various current and future regulations, including National Ambient Air Quality Standards, New Source Performance Standards, Title IV of the 1990 Clean Air Act Amendments, the Ozone Transport Commission NOx Budget Allowance Trading Program, state-implemented plans for NOx emissions decreases, Section 126 of the 1990 Clean Air Act Amendments, and Mobile Source Emission Limits.