Cloud chemistry in the eastern United States, as sampled from three high-elevation sites along the Appalachian Mountains

Atmospheric deposition of acidic cloud water is thought to be one of the causes for the recent forest decline in industrialized areas of the world. The present paper presents results from the Mountain Acid Deposition Program (MADPro), a part of EPA's Clean Air Status and Trends Network, (CASTnet). We used automated cloud water collectors at three selected mountain sites (Whiteface Mt., NY; Whitetop Mt., VA; and Clingman's Dome, TN) to take hourly samples from non-precipitating clouds during temperate (non- freezing) seasons of each year from 1994 to 1997. Samples were promptly analyzed for pH, conductivity, and concentration of dissolved ions. Cloud liquid water content (LWC) and meteorological parameters were measured at each site. Mean cloud frequencies and LWC of clouds were higher at Whiteface Mt., NY, than in the Southern Appalachians. The four most prevalent ions found in cloud water samples were usually, in order of decreasing concentration: sulfate (SO²⁻₄) hydrogen (H⁺), ammonium (NH₄⁺), and nitrate (NO₃⁻). Within cloud events the concentration of these major ions tended to co-vary. Typically there was an inverse relationship between LWC of the cloud and ionic concentration of the cloud water. During the sampling season, the highest ionic concentrations were seen during mid-summer. Ionic concentrations of samples from the southern sites were significantly higher than samples from Whiteface Mt., but further analysis indicates that this is at least partially due to the north–south difference in the LWC of clouds. MADPro results are shown to be comparable with previous studies of cloud chemistry in North America.     

Temporal evolution of redox processes and free Cd dynamics in a metal-contaminated soil after rewetting

Soil testing procedures to address metals bioavailability currently use air-dried soil rewetted almost until saturation. Such practices may influence the redox state of soil and the related dynamics of metals. To assess this potential impact, a metal-contaminated soil was air-dried and rewetted to 90% water holding capacity. We monitored over a 21-day incubation period the temporal changes of soil redox potential and solution Cd concentration (either total or free). Other physico-chemical parameters were followed notably pH, ionic strength (I) and the concentrations of NO(3)(-), Mn, Fe and SO(4)(2-) in solution. Soil redox potential showed the progressive establishment of strong reducing conditions in soil, in agreement with the temporal changes of NO(3)(-), Mn, Fe and SO(4)(2-) concentrations. It decreased by 13 pe units over the culture period leading to sulphate-reducing conditions (pe<-3) within only 21days. Solution Cd concentration increased transitorily over the first 100-150h of incubation (2-fold increase) in relation with the parallel increase in the concentration of competing cations for adsorption (Ca, Mg). It steeply decreased over the last 300h of incubation (30-fold decrease) as a result of Cd precipitation as Cd sulphides. This biphasic evolution of Cd dynamics was related to the temporal changes of Cd resupply from the solid phase. Using the technique of DGT we described the kinetics of Cd resupply over time and needed to invoke the existence of two pools of Cd.