The San Joaquin Valley is a large agricultural and industrial valley located in central California.  Conventional aerosol measurements in the region have found that the ambient particulate budget is strongly influenced by the geography and meteorology of the region.  Winter stagnation periods lead to elevated particulate mass concentrations (PM) and the formation of secondary aerosol.  This study is part of a multi-year campaign to better characterize the sources and processes leading to these elevated PM levels in central California.
 

    Aerosol analysis is performed using aerosol time-of-flight mass spectrometry (ATOFMS), which simultaneously measures particle size and composition on a individual particle basis.  The field transportable ATOFMS (affectionately known as Jake) collects both positive and negative ion mass spectra from single particles to allow for the analysis of both cationic and anionic species, respectively.  Measurements of PM2.5 and particle light scattering coefficient were also taken using a tapered-element oscillating microbalance (TEOM) and a Radiance Research nephelometer.  Aerosol sampling occurred over a three-week period in January 1999 in Bakersfield, CA. (see map below)
 

SAN JOAQUIN VALLEY
 
 

    Particle mass concentrations show higher levels during the stagnation episode in which a heavy and extensive fog is present.  ATOFMS particle counts compare to the mass concentrations due to instrumental biasing towards larger particles.  This provides a semi-quantitative description of ambient aerosol mass with high temporal resolution.  There is a large drop in PM2.5 and particle counts at the transition from the stagnation to the clearing episode and a dramatic reduction in particle mass concentrations during a rain period in which the particles are precipitation scavenged.
 
 
 
 
 
 

    Particle composition was assigned by the presence of peaks in the single particle mass spectra corresponding to sources and reaction processes.  The major types present in Bakersfield were organic carbon (a), marine (b), dust (c), and elemental carbon (d) particles.  Variations in single particle composition during differing meteorological conditions are shown to the right.  Organic particles dominate the ambient aerosol composition during the stagnation period.  A dramatic shift in aerosol composition occurs during the clear period when the organics decrease and the majority of particles become marine.  The haze period midway through the sudy has similar mass concentrations to the initial stagnation period but the composition of these particles is dramatically diferent.  Marine particles contribute to the elevated mass concentrations and decrease in visibility for this time period.  This observation can only be made by aquiring real-time data.

    Overall, the results from this study provide new insight into aerosol chemistry in an environment with unique meteorological conditions and source contributions.