Evaluation of Glaciogenic Cloud Seeding using Trace Chemistry

Glaciogenic cloud seeding with silver iodide (AgI) has been used to enhance precipitation for over 60 years. Assessments of AgI impact and dispersion are often quantified using atmospheric processes models with impact assessed by comparing models with and without inclusion of cloud seeding modules.  However, there is inherent uncertainty in these aerosol models and physical validation of AgI distribution is of value to both validate and improve model performance. The purpose of this study is to demonstrate the capacity to physically validate the dispersion of AgI by measuring silver enrichments in snow. Field and laboratory methods were developed to detect trace seeding signatures in snowpack. Unique laboratory layout and protocols were developed to reduce contamination potential within a traditional ICP-MS laboratory setting (not housed in a Class 100 Clean Room). Using these methods, we sampled a series of snow profiles within the target area of active cloud seeding in the central mountains of Idaho. Our results demonstrate the ability the ability to reproduce distinct evidence of elevated Ag at concentrations at a hillslope (0.25 km 2 ) and at the basin (2,400 km 2 ) scale. The construction of 8 snow pits at one site (hillslope scale) and 6 sites along a 65 km transect (basin scale) reliably identified both of the seeded storm layers sampled. The location of the peaks in Ag concentration within the snow profiles generally corresponds in timing to known cloud seeding events. Distinct seeded storm layers were reliably identified seeding signatures more than 60 km from the AgI sources, where silver concentrations were only enhanced 1-3 parts per trillion. Ag enriched snow in these chemical profiles generally correspond to downwind target locations and AgI seeding times.