ASSESSING THE EFFECT OF SO2 EMISSION CHANGES ON VISIBILITY

During the 1970s the effect of acid rain on the environment became a growing concern to scientists, public policy officials, interest groups, and the general population. The United States' Congress mandated a 10-year study, National Acid Precipitation Assessment Program (NAPAP), to examine the relationship between acid rain related emissions and environmental effects including visibility. It was found that averaging over both space and time (1 yr) that a 21% improvement in visibility could be expected from a 10-million ton reduction in SO2 emissions. The biggest improvement is expected along the Ohio River Valley. Examining the distribution of visibility improvements shows that 15 d yr-1 with lowest sulfate concentrations and least sulfate reduction there will be a just perceptible improvement, while on the other end of the spectrum 15 d yr-1 with the highest sulfate concentration and with the largest sulfate reduction there will be a 35% improvement in extinction. The paper also reviews the assumptions required to carry out the assessment and points out shortcomings in our understanding of the relationship between SO2 emissions and visibility.