Decrease in precipitation acidity resulting from decreased SO ^2-₄ concentration

The effect of decreases in SO₂ emissions on precipitation acidity has received much attention^1-10, 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 SO^2-₄ concentration in precipitation at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, USA, result from decreases in SO₂ emissions. The effect of the SO^2-₄ 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 SO^2-₄ concentration had not decreased.