Trace Metal Concentrations in Fog and Aerosol Samples Collected from Davis, Fresno and Bakersfield, CA: 2010–2013

Aerosol particles can enter the atmosphere from natural sources (mineral dust, sea spray,…) as well as from anthropogenic sources (combustion sources,…).  Once in the atmosphere, aerosols can undergo short and long range transport, where they can be processed by clouds, photochemical reactions, and interactions with anthropogenic gases. It is important to understand these atmospheric processes to assess the fate of the metals in the atmosphere and potential impacts on human health, climate, and the biosphere.  In this study, the processing of metals and trace elements in aerosols was investigated in field studies of radiation fogs in the Central Valley of California (Fresno, Davis, and Bakersfield).  Trace element concentrations were monitored in the atmospheric aqueous phase of the fogs as well as in pre, post and interstitial aerosol samples. Fog water collected in the urban areas of Fresno and Davis had similar concentrations for many elements like Mn and Ni (2-20 µg L^-1 and 0.5-10 µg L^-1, respectively).  Fog water collected in Fresno, however, had much higher concentrations of Fe and Pb (4-159 and 0.3-5 µg L^-1, respectively) as compared to Davis (Fe = 0.2-51 µg L^-1 and Pb <0.003-0.6 µg L^-1).  A major focus of this work was on fog processing of iron, specifically its solubilization during fog and high humidity events. The soluble iron fraction was highest in PM_2.5 samples collected from Fresno, which had an average of 5.2 ng m^-3 soluble Fe, while Bakersfield and Davis had averages of 0.4 and 0.5 ng m^-3 soluble Fe, respectively.  The observations will be discussed relative to the likely source materials and ageing processes.