UC San Diego

Aerosol effects on clouds contribute the largest uncertainty to predictions of anthropogenic induced climate change. Thus, chemical characterization of aerosols that nucleate cloud droplets and ice crystals is crucial to extending our knowledge of cloud formation and accurately predicting climate change. The studies presented in this dissertation utilize an aircraft aerosol time-of-flight mass spectrometer (A-ATOFMS) to study the chemical mixing state of individual aerosols, cloud droplets and ice crystal residues from two aircraft field campaigns: CalWater in the California Sierra Nevada and the Ice in Clouds Experiment-Tropical (ICE-T) in the Caribbean. Analysis of these datasets provided insights into aerosol effects on clouds and the chemical characteristics of ice nuclei (IN) as well as inlet artifacts. During CalWater two trans-Pacific transported aerosol types, dust and soot, were observed to have vastly different effects on clouds. Soot, from Asian pollution, was present in clouds with more numerous, small droplets. Clouds with small droplets in the Sierra Nevada have previously been linked to reduction of precipitation, thus increases in Asian pollution could contribute to additional precipitation reduction in the region. In contrast, dust and biological aerosols are shown to have the opposite effect and enhance precipitation in the Sierra Nevada by serving as ice nuclei in glaciated clouds that coincided with increased ice-induced precipitation. Chemically characterizing the initial ice nuclei is challenging, as numerous physical and chemical changes occur after nucleation. Using a combination of statistical analysis methods, chemical components key to ice formation were identified. Organic nitrogen and phosphate, indicators of biological species, were found to be important for ice formation in both study locations using binary logistic regression. Principal component analysis identified biological particles as key to ice formation and suggested they were of marine origin. Additionally, using in-situ cloud residual data, the ice nucleation temperature of a salt-biological particle type was identified between -10 and -13 °C. These results provide insight into the chemical composition of IN active at warmer temperatures. Lastly, a combination of in-situ and laboratory measurements identified aircraft inlet artifacts produced during sampling in mixed phase clouds and showed the artifacts were distinct from other metal- containing residues