Precipitation Composition in the Ohio River Valley: Spatial Variability and Temporal Trends

Abstract

Sulfate (SO 2− 4 ), nitrate (NO − 3 ) and ammonium (NH + 4 ) 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 SO 2− 4 concentrations in precipitation declined by 37-43% between 1985 and 2002, while NO − 3 concentrations decreased by 1-32%, and NH + 4 concentrations exhibited declining concentrations at some sites and increasing concentrations at others. The change in SO 2− 4 concentrations is in broad agreement with estimated reductions in sulfur dioxide emissions. Changes in NO −3 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 NO − 3 concentrations in precipitation is explicable by variations in precipitation volume. Trends in annual average log-transformed SO 2− 4 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 SO 2− 4 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 SO 2− 4 , NO − 3 and NH + 4 may have a common source which is postulated to be linked to synoptic scale variability and air mass trajectories.

Aerosol and precipitation chemistry in the southwestern United States: spatiotemporal trends and interrelationships

“…With regard to periods of data collection, it is critical to note that at least in North America, more significant reductions in SO 2 as compared to NO x over recent decades likely bias intercomparisons of ${\mathrm{NO}}_3^{-}$ : ${\mathrm{SO}}_4^{2-}$ ratios between different studies (e.g. EPA, 2003; Kvale and Pryor, 2006). Measurements in the eastern United States have pointed to reductions in precipitation sulfate unlike nitrate since the 1980s (e.g.…”

Aerosol and precipitation chemistry in the southwestern United States: spatiotemporal trends and interrelationships

“…With regard to periods of data collection, it is critical to note that at least in North America, more significant reductions in SO 2 as compared to NO x over recent decades likely bias intercomparisons of : ratios between different studies (e.g. EPA, 2003;Kvale and Pryor, 2006). Measurements in the eastern United States have pointed to reductions in precipitation sulfate unlike nitrate since the 1980s (e.g.…”