Hydrogen ion input to the hubbard brook experimental forest,New Hampshire,during the last decade

Being downwind of eastern and midwestern industrial centers, the Hubbard Brook Experimental Forest offers a prime location to monitor long-term trends in atmospheric chemistry. Continuous measurements of precipitation chemistry during the last 10 yr provide a measure of recent changes in precipitation inputs of H ion. The weighted average pH of precipitation during 1964–65 to 1973–74 was 4.14, with a minimum annual value of 4.03 in 1970–71 and a maximum annual value of 4.21 in 1973–74. The sum of all cations except H ion decreased from 51 μeq 1^?1 in 1964–65 to 23 μeq 1^?1 in 1973–74 providing a significant drop in neutralizing capacity during this period. Based upon regression analysis, the input in equivalents of H ion and nitrate increased by 1.4-fold and 2.3-fold respectively, from 1964–65 to 1973–74. Input of all other ions either decreased or showed no trend. Based upon a stoichiometric formation process in which a sea-salt, anionic component is subtracted from the total anions in precipitation, SO₄ ⁼, contribution to acidity dropped from 83% to 66%, whereas NO₃ ^? increased from 15% to 30% during 1964–65 to 1973–74. The increased annual input of H ion at Hubbard Brook during the past 10 yr is highly correlated with the increased input of nitrate in precipitation.