Alteration of Heterogeneous Ice Nucleation Properties Induced by Particle Aging
Abstract:Aerosol particles that can serve as ice nuclei frequently experience rapid and extensive chemical aging during atmospheric transport. This is known to significantly alter some ice nucleation modes of the few types of ice nucleation particle systems where aging effects have been simulated, such as for mineral dust. Yet much of our understanding of atmospheric particle freezing properties is derived from measurements of fresh or unaged particles. We know almost nothing regarding how atmospheric aging might alter the freezing properties of biomass burning aerosol or biological particle nucleants.
We have investigated the effects of simulated aging using a chamber reactor on the heterogeneous ice nucleation properties of biomass burning aerosol (BBA) and ice-active bacteria particles. Some types of aging were found to enhance the freezing ability of BBA, exhibited as a shift in a portion of the droplet freezing curve to warmer temperatures by a few °C. Ice-active bacteria were found to consistently loose their most ice-active nucleants after repeated aging cycles. The bacterial systems always retained significantly efficient ice active sites that still allowed them to induce freezing at mild/warm temperatures, despite this decrease in freezing ability.
A comprehensive series of online single-particle mass spectrometry and offline spectromicroscopic analysis of individual particles was used to determine how the aging altered the aerosol’s composition, and gain mechanistic insights into how this in turn altered the freezing properties. Our new ice nucleation framework that uses a continuous distribution of ice active site ability (contact angle) was used to interpret the droplet freezing spectra and understand how aging alters the internal and external variability, and rigidity, of the ice active sites.