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 the inclusion of cloud seeding modules. However, there is inherent uncertainty in these models. Quantifying AgI distribution in the snowpack following cloud seeding can both validate and improve model performance. The purpose of this study is to demonstrate the capacity to document the dispersion of AgI by measuring silver (Ag) enrichments in snow.

This study develops clean field and laboratory procedures to detect trace seeding signatures in alpine snowpack. Unique laboratory layout and protocols are employed to reduce contamination potential within a traditional ICP-MS laboratory setting (not housed in a Class 100 Clean Room). Using these methods, we sample a series of snow profiles within the target area of active cloud seeding in the central mountains of Idaho. The results demonstrate the ability of the new methods to reproduce distinct elevated Ag concentrations over a small scale (0.25 km ² ) and at the basin (2,400 km ² ) scale. The trace chemical analysis of snow samples from eight snow pits over an area of 0.25 km ² and six sites separated up to 65 km (basin scale) identify potential seeding signatures from two seeded storms. A localized enrichment factor is used to identify and replicate this seeding signature at all six sites within the basin. This enrichment factor can delineate a seeding signature at sites far downwind from AgI sources, where Ag concentrations are only 1-3 parts per trillion above background levels. The localized enrichment factors at all six sites contain chemical snow profiles generally corresponding to peak Ag concentrations and known cloud seeding events.