The effect of decreases in SO2 emissions on precipitation acidity has received much attention1-10, but there has been no direct quantification of how recent decreases in SO2 emissions in the northeastern and midwestern United States have affected precipitation acidity at a local site. Yet such quantification is an Simportant step in assessing the effectiveness of control measures for SO2 emissions. It is thought that recent decreases in SO2- 4 concentration in precipitation at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, USA, result from decreases in SO2 emissions. The effect of the SO2- 4 decrease on precipitation acidity is obscured by long-term trends in other ions, which also influence acidity. Here we show that, given the observed trends in concentration of other ions, the H+ concentration in 1983 would have been nearly two-thirds higher than the measured values if the SO2- 4 concentration had not decreased.
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