Reductions in haze across the United States since the early 1990s

摘要

The widespread decrease in the haziest conditions and the dramatic improvement in scenic clarity, especially in the eastern United States, points to the success of the combined regulatory activity that has resulted in reduced SO_2 and NO_x emissions and therefore secondary aerosol concentrations. However, because conditions in the West were initially cleaner and thus closer to natural background levels (～20 Mm~(-1) in the West and ～30 Mm~(-1) in the East for the haziest conditions), improvements in the haziest bext have not been as significant; in fact, at some sites the haziest conditions have actually increased. As emissions from regulated sources continue to decline, the relative contributions to visibility degradation from other sources such as wildfire emissions, dust, unregulated sources, or international contributions are likely to increase. The role of these sources requires more research to fully understand and predict in the context of regional haze goals.