Characterization of Individually Identified Ice Nucleating Particles within an Ambient Particle Population: Implications for Ice Nucleation Description
Tuesday, 16 December 2014: 4:00 PM
One of the greatest challenges in atmospheric sciences is predicting heterogeneous ice nucleation, where one out of one million ambient aerosol particles induces ice crystal formation resulting in mixed-phase and cirrus clouds. A novel multi-modal methodology is presented that allows optical and micro-spectroscopic imaging of individual identified field-collected ice nuclei (IN) active in immersion freezing and deposition ice nucleation. The identified IN from an ambient aerosol population sampled in central California dominated by urban and marine aerosols belong to the most common particle types: organic coated sea salt, and Na-rich, secondary, and refractory carbonaceous particles. Based on these observations, the IN represent not particles with unique chemical composition and exceptional ice nucleation propensity; rather, they are common particles in the ambient particle population. The results suggest that particle-type abundance and surface area, together with particle-type ice nucleation efficiency, is crucial for determining ice formation within the particle population. Complementary to our findings, we introduce a new stochastic and time-dependent immersion freezing simulation able to reproduce a wide range of laboratory data in which all particles in an aerosol population are treated explicitly as IN accounting for variable surface area. Of particular interest is the impact of the uncertainty of particle surface area on interpretation of laboratory and field derived ice nucleation data. The model simulation and experimental results corroborate classical nucleation theory with important consequences for cloud modeling studies and field measurement strategies.