Long- and short-term changes in sulfate deposition: Effects of the 1990 Clean Air Act Amendments

Annual, volume-weighted concentrations of SO42- in bulk precipitation have declined steadily (-0.44 mu mol/liter-yr) since 1965 at the Hubbard Brook E xperimental Forest (HBEF), NH in response to decreases in regional SO2 emis sions (r(2) = 0.74). Similar declines in concentrations have occurred in we t-only precipitation at HBEF and at nearby sites since 1978. However, decre ases in SO42- concentrations following passage of the U.S. Clean Air Act Am endments in 1990, were not unusual from the perspective of long-term data f rom the HBEF. Statistically significant declines (-5.6 mol/ha-yr) in bulk d eposition of SO42- also have occurred since 1965 in relation to decreases i n SO2 emissions (r(2) = 0.58), but annual variations in deposition also are strongly related to amount of precipitation and other factors.