ALBERT

Description: Sulfate (SO4 2−), nitrate (NO3 −) and ammonium (NH4 +) concentrations in precipitation as measured at NADP sites within the Ohio River Valley of the Midwestern USA between 1985 and 2002 are quantified and temporal trends attributed to changes/ variations in (i) the precipitation regime, (ii) emission patterns and (iii) air mass trajectories. The results indicate that mean SO4 2− concentrations in precipitation declined by 37–43% between 1985 and 2002, while NO3 − concentrations decreased by 1–32%, and NH4 + concentrations exhibited declining concentrations at some sites and increasing concentrations at others. The change in SO4 2− concentrations is in broad agreement with estimated reductions in sulfur dioxide emissions. Changes in NO3 − concentrations appear to be less closely related to variations in emissions of oxides of nitrogen and exhibit a stronger dependence on weekly precipitation volume. Up to one quarter of the variability in log-transformed weekly NO3 − concentrations in precipitation is explicable by variations in precipitation volume. Trends in annual average log-transformed SO4 2− concentrations exhibit only a relatively small influence of variability in weekly precipitation amount but at each of the sites considered the variance explanation of annual average log-transformed SO4 2− by sampling year was increased by removing the influence of precipitation volume. Annual mean log-transformed ion concentrations detrended for precipitation volume (by week) and emission changes (by year) exhibit positive correlations at all sites, indicating that the residual variability of SO4 2−, NO3 − and NH4 + may have a common source which is postulated to be linked to synoptic scale variability and air mass trajectories.