Measurements and Parameterization of Ice Nuclei Concentrations at a Forested Site in Colorado: Results from the BEACHON-RoMBAS Campaign

Atmospheric ice nuclei (IN) and cloud condensation nuclei (CCN) of biological origin (e.g., bacteria, fungal spores, pollen) emitted from forested sites may play a key role in linking biogeochemical and water cycles in the semi-arid Western U.S. In this study, we examine the concentrations and compositions of IN measured during the BEACHON-RoMBAS (Bio-hydro-atmosphere interactions of Energy, Aerosols, Carbon, H₂O, Organics, and Nitrogen – Rocky Mountain Biogenic Aerosol Study) campaign conducted at the Manitou Forest Observatory in July-August 2011. IN number concentrations active under mixed-phase cloud conditions were obtained as a function of temperature using a continuous flow diffusion ice chamber (CFDC). We also demonstrate the first successful use of an aerosol concentrator to observe very low IN number concentrations at temperatures warmer than -20°C. Associated measurements of aerosol number-size distributions and fluorescent biological particle concentrations are used to demonstrate the sometimes strong role of biological IN emissions in such forested environments. We also use these data to develop a new parameterization to quantify IN number concentrations, in dependence on aerosol number concentrations and temperatures, at the semi-arid Western U.S. forested location. Finally, we investigate meteorological factors affecting the variation of the concentrations and compositions of IN.