Trends in atmospheric ammonium concentrations in relation to atmospheric sulfate and local agriculture

Ammonium (NH4+) concentrations in air and precipitation at the Institute of Ecosystem Studies (IES) in southeastern New York, USA declined over an 11-year period from 1988 to 1999, but increased from 1999 to 2001. These trends in particulate NH4+ correlated well with trends in particulate SO42− over the 1988–2001 period. The NH4+ trends were not as well correlated with local cattle and milk production, which declined continuously throughout the period. This suggests that regional transport of SO42− may have a greater impact on concentrations of NH4+ and subsequent deposition than local agricultural emissions of NH3. Ammonium concentrations in precipitation correlated significantly with precipitation SO42− concentrations for the 1984–2001 period although NH4+ in precipitation increased after 1999 and SO42− in precipitation continued to decline after 1999. The correlation between NH4+ and SO42− was stronger for particulates than for precipitation. Particulate NH4+ concentrations were also correlated with particulate SO42− concentrations at 31 of 35 eastern U.S. CASTNet sites that had at least 10 years of data. Air concentrations of NH4+ and SO42− were more strongly correlated at the sites that were located within an agricultural landscape than in forested sites. At most of the sites there was either no trend or a decrease in NH4+ dry deposition during the 1988–2001 period. The sites that showed an increasing trend in NH4+ dry deposition were generally located in the southeastern U.S. The results of this study suggest that, in the northeastern U.S., air concentrations of NH4+ and subsequent deposition may be more closely linked to SO42− and thus SO2 emissions than with NH3 emissions. These results also suggest that reductions in S emissions have reduced NH4+ transport to and NH4+-N deposition in the Northeast.