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Abstract

Certain primary biological aerosol particles (PBAPs) are known to have very high ice nucleating ability under mixed-phase cloud conditions. However, since the abundances of ice nucleation active PBAPs in the atmosphere are generally thought to be extremely small, their importance has remained uncertain. Here we present evidence for the role of PBAPs as atmospheric ice nuclei (IN) active at temperatures ranging from about -34 degrees C to -9 degrees C in a midlatitude ponderosa pine forest ecosystem in summertime. Our measurements show that the number concentrations of IN active at these temperatures were positively correlated with number concentrations of ambient fluorescent biological aerosol particles (FBAPs). Notably, the number concentrations of IN active at warmer temperatures increased quite rapidly in response to increases in the number concentrations of FBAPs. Moreover, we show that a newly-proposed parameterization related to the number concentrations of FBAPs can better reproduce the number concentrations of IN active over the entire temperature range examined, as compared with parameterizations related solely to the number concentrations of total aerosol particles with diameters larger than 0.5 mu m as proposed previously. These results suggest that certain PBAPs released from forest biota can indeed play a key role in determining atmospheric IN populations in this ecosystem, especially at warmer temperatures, potentially leading to ice initiation in nearby mixed-phase clouds.