Increasing trends in wintertime particulate sulfate and nitrate ion concentrations in the Great Plains of the United States (2000–2010)

Trends in wintertime particulate sulfate ion and nitrate ion concentrations were computed for 2000–2010 using data from remote and rural sites across the United States from the Interagency Monitoring of Protected Visual Environments (IMPROVE) program. Several sites in the northern and central Great Plains had increasing sulfate and nitrate ion concentrations in December at the rate of over 5% yr−1. The positive trends are in contrast to the decreasing national annual trends in nitrogen oxides (NOx) and sulfur dioxide (SO2) emissions as reported by the Environmental Protection Agency. Increasing trends in particulate nitrate and sulfate ion concentrations have important implications for air pollution mitigation strategies, considering concentrations at these sites counter the reductions in emissions from controlled sources across the United States.