Ice crystals in clouds frequently form upon a subset of aerosol particles called ice nuclei (IN). IN influence cloud ice crystal concentrations, consequently affecting cloud lifetime and reflectivity. The present understanding of these effects on climate is hindered by limited data on the global distribution of IN. This thesis presents measurements of deposition-mode IN concentrations under conditions relevant to mid-level clouds, 238 K and 138% RHi. at two Canadian sites: Toronto, a major city, and Whistler, a pristine coniferous rainforest. In Toronto, chemically-resolved surface areas were estimated by single-particle mass spectrometry and regressed against IN concentrations to identify a significant relationship between IN concentrations and both carbonaceous aerosols (EC and/or OC) and dust. In Whistler, IN concentrations during a biogenic secondary organic aerosol (SOA) event did not increase from background levels (0.1 /L), suggesting that biogenic SOA particles do not nucleate ice under these conditions.

Ice crystals in clouds frequently form upon a subset of aerosol particles called ice nuclei (IN). IN influence cloud ice crystal concentrations, consequently affecting cloud lifetime and reflectivity. The present understanding of these effects on climate is hindered by limited data on the global distribution of IN. This thesis presents measurements of deposition-mode IN concentrations under conditions relevant to mid-level clouds, 238 K and 138% RHi. at two Canadian sites: Toronto, a major city, and Whistler, a pristine coniferous rainforest. In Toronto, chemically-resolved surface areas were estimated by single-particle mass spectrometry and regressed against IN concentrations to identify a significant relationship between IN concentrations and both carbonaceous aerosols (EC and/or OC) and dust. In Whistler, IN concentrations during a biogenic secondary organic aerosol (SOA) event did not increase from background levels (0.1 /L), suggesting that biogenic SOA particles do not nucleate ice under these conditions.