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Measurements of ice nucleating particles in the atmosphere : method development and results from field campaigns in North America and Europe Mason, Ryan H.

Abstract

Ice nucleating particles (INPs), which are a small fraction of the total aerosol population, are capable of catalyzing ice formation under atmospheric conditions. INPs may therefore influence the development, albedo, and lifetime of mixed-phase and ice clouds, and ultimately indirectly effect climate. As this aerosol indirect effect represents one of the largest sources of uncertainty in our understating of climate processes, measurements that quantify and characterize the atmospheric INP population are needed. The micro-orifice uniform deposit impactor-droplet freezing technique (MOUDI-DFT) was developed to measure INP concentrations in the atmosphere as a function of size and temperature in the immersion mode. The first campaign using the MOUDI-DFT was conducted in a Colorado forest. The concentrations of INPs and bioparticles were increased and correlated during and following rainfall events, and their size distributions were similar. This indicates that rainfall-associated mechanisms of bioparticle release may influence the abundance and efficiency of INPs in this region. The MOUDI-DFT was next used at a coastal site in Western Canada. INP concentrations were strongly correlated with those of fluorescent bioparticles and the size distributions of these particles were similar, suggesting bioparticles were an important source of INPs during this study. Despite the predominance of marine air masses, no evidence of a marine INP source was found. Six parameterizations of ice nucleation were tested and found to be poor predictors of the measured INP concentrations, identifying a need to develop INP parameterizations appropriate for coastal environments. Finally, size-resolved INP measurements from six ground-level sites in North America and one in Europe were presented, covering Arctic, alpine, coastal, marine, agricultural, and suburban environments. On average, 78 % of INPs were supermicron in size and 53 % were in the coarse mode (> 2.5 micrometers). Large particles were therefore a significant component of the ground-level INP in these diverse locations. The results presented in this dissertation increase our understanding of atmospheric INP concentrations, composition, and size. This information can be used to constrain INP sources, improve modeling of their long-distance transport and related indirect climate effects, and determine the ability of existing instrumentation to capture the full INP population.

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Attribution-NonCommercial-NoDerivs 2.5 Canada

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