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Findings

Findings

Impact of Asian Dust on the Type and Amount of Precipitation

Chemical measurements of residuals in rain and snow samples from the Sugar Pine Dam site in 2009 point to a correlation of transported Asian dust and heavy snowfall.  HYSPLIT model results indicate periods of long-range transport to the Central Sierra Nevada (Sugar Pine Dam) which coincide with the heaviest snowfall periods within a large winter storm impacting the region.

Figure 1: HYSPLIT back trajectories of air masses impacting Sugar Pine reservoir site during a storm which produced the highest rates of precipitation in the form of snow, when heavy loadings of dust appeared in the precipitation residuals.  Long range transport periods (7-10) correspond to precipitation periods 4-10 shown in Fig. 2 (below).

 

Figure 2: Chemistry of residuals sampled from rainwater and melted snow collected over the course of a major storm (Periods 4-10) at Sugar Pine Dam, showing the abundance of Asian dust in precipitation during a period of heavy snow. [Asian Dust (dust), biological (Biol.), organic carbon (O.C.), and all other (other)].

Updated Research (2013)

Using data collected from aircraft, ground, and satellite platforms during the 2011 CalWater Field Season, our team was able to provide in-situ evidence for the impact of dust transported across the Pacific Ocean on the formation of ice in mixed phase cloud systems above the Sierra Nevada Mountains.  The dust act as ice crystal nuclei, and help the storm systems to precipitate, while at the same time large quantities of moisture can be delivered to the region by Atmospheric Rivers.

 

References

Ault AP, Williams CR, White AB, Neiman PJ, Creamean JM, Gaston CJ, Ralph FM, and Prather KA, "Detection of Asian Dust in California Orographic Precipitation, J. Geophys. Res. Atmos., 2011, 116, D16205.

Creamean JM and Suski KJ, Rosenfeld D, Cazorla A, DeMott PJ, Sullivan RC, White AB, Ralph FM, Minnis P, Comstock JM, Tomlinson JM, Prather KA. "Dust and Biological Aerosols from the Sahara and Asia Influence Precipitation in the Western US," Science (2013), 339, 1572-1578. doi: 10.1126/science.1227279

Understanding the Chemistry of Newly Formed Particles

Determining the major sources of particles that act as seeds for cloud drop formation, or cloud condensation nuclei (CCN), represents a critical step in the development of a more fundamental understanding of aerosol impacts on cloud formation and climate.  During our CalWater field deployments in 2009 and 2010 at Sugar Pine Dam, we were able to directly measure the CCN ability of particles formed in the atmosphere and single particle chemical composition during these events.

New particle formation (NPF) occurs spontaneously from precursor gas molecules that first form clusters, then grow rapidly into particles that are detectable by ATOFMS.  NPF events in the winter of 2009 occurred during two pristine periods following precipitation, with increases in gas-phase sulfur dioxide (SO2) and ozone (O3) concentrations each day before the events.  Amines and sulfate were detected in the particle phase throughout NPF events; the fraction of both species in the total particle population increased during each event.  We examined possible sources for the gas-phase species that played a role in the formation of new particles, including amines and O3 from the Central Valley of California and SO2 from Asia, although local sources could also contribute.

Understanding the chemical and physical properties of newly-formed particles, including their ability to act as CCN, is critical for predicting the impact of NPF on orographic cloud formation along the Sierra Nevada range.  The potential contribution of newly-formed particles in remote regions needs to be compared to that of transported urban aerosols when evaluating the impact of aerosols on clouds and climate.

Figure 1: (a) 5-minute temporally resolved SMPS size distributions with superimposed ambient relative humidity and temperature, (b) hourly precipitation and solar radiation, and (c) relative hourly SO2 and O3 mixing ratios and black carbon mass.

Figure 2: Relative fractions of ATOFMS single particle types in the 100-1000 nm size range measured during a particular period of 4 different NPF events.  Also shown is the SMPS total particle number concentrations, mean sizes, and number of ATOFMS-derived sulfate containing particles during each sub-period.  All values are averaged over the four NPF events during the period in question.  G1-G4 refer to different portions of a typical NPF event for this study.

References