Abstract. Heterogeneous ice nucleation is a crucial process for forming ice-containing clouds and subsequent iceinduced precipitation. The importance for ice nucleation by airborne desert soil dusts composed predominantly of minerals is widely acknowledged. However, the potential influence of agricultural soil dusts on ice nucleation has been poorly recognized, despite recent estimates that they may account for up to 20-25 % of the global atmospheric dust load. We have conducted freezing experiments with various dusts, including agricultural soil dusts derived from the largest dustsource region in North America. Here we show evidence for the significant role of soil organic matter (SOM) in particles acting as ice nuclei (IN) under mixed-phase cloud conditions. We find that the ice-nucleating ability of the agricultural soil dusts is similar to that of desert soil dusts, but is clearly reduced after either H 2 O 2 digestion or dry heating to 300 • C. In addition, based on chemical composition analysis, we demonstrate that organic-rich particles are more important than mineral particles for the ice-nucleating ability of the agricultural soil dusts at temperatures warmer than about −36 • C. Finally, we suggest that such organic-rich particles of agricultural origin (namely, SOM particles) may contribute significantly to the ubiquity of organic-rich IN in the global atmosphere.

In this study, we determined the conditional stability constant (log K′) of copper for the gills of rainbow trout (Oncorhynchus mykiss; RBT) and brook trout (Salvelinus fontinalis; BT). Using toxicity‐based complexation bioassays, which measure the effect of competing organic ligands on copper toxicity, the RBT gill copper log K′ range was 6.4 to 7.2. Using a Scatchard analysis of gill Cu accumulation, the RBT log K′ was 7.50 and the BT log K′ was 7.25. The close agreement in RBT log K′ values between these two methods suggests that measurement of gill copper accumulation is an acceptable alternative for determining a toxicity‐based gill copper binding affinity. Our results also suggest that there is either a single gill copper binding component or, more realistically, multiple components with similar binding properties that function collectively to define a single toxicologically relevant copper conditional stability constant. These results suggest analytical approaches to measuring bioavailable metal concentrations, such as geochemical modeling where biological ligands are included in speciation calculations, may adequately simulate complex biological ligands. A method to convert gill copper accumulation to a bioavailable water criterion is also discussed.

No abstract

No abstract

No abstract