UC San Diego

Atmospheric aerosols have significant implications for human health and climate. For instance, aerosols impact climate directly by scattering and absorbing solar and terrestrial radiation and indirectly by acting as cloud condensation nuclei (CCN) and ice nuclei (IN), which facilitate cloud droplet and ice crystal formation, respectively. Changes in chemistry, size, and number concentrations between different locations and over time alter how aerosols impact air quality and cloud formation, and can have broader implications on precipitation efficiency and phase. Further, aerosol composition largely depends on meteorology, which influences sources and chemical transformation in the atmosphere. Aerosol-cloud- precipitation interactions represent one of the largest sources of uncertainty in climate science; therefore, a better understanding of the aerosols that contribute to these effects is needed. To address this source of uncertainty, the chemical composition of individual ambient aerosols and aerosols as insoluble residues in precipitation samples was determined using aerosol time-of -flight mass spectrometry (ATOFMS) and provided insight into their potential to serve as cloud seeds at three different locations over time. A three-year summer study (2005-2007) in Riverside, CA afforded information on the inter-annual variability of the urban aerosol due to changes in aerosol transport and meteorological conditions. In the summer of 2008 in Atlanta, GA, tropical cyclones shifted the representative aged urban aerosol to a less-aged, less-CCN active aerosol population, having implications on regional cloud formation after extreme weather events. At a remote site in the Sierra Nevada Mountains in the winter of 2009, observations of newly- formed aerosols presented a new source of CCN. Inter- annual trends in precipitation at the same remote site showed how IN transported from the Sahara and Asia potentially influenced precipitation processes during three winter seasons (2009-2011). Investigating changes in cloud seeds represents a longer-term goal to reduce uncertainties associated with modeling aerosol-cloud- precipitation interactions. Larger spatial and temporal coverage is needed to better understand trends in cloud formation and precipitation and to provide more detail for regional and global model parameterization. The results presented herein represent a noteworthy advancement towards understanding variation in composition and sources of cloud seeds in different regions and in most cases long time periods