The Contribution of Trans-Pacific Submicron Aerosols and Local Particle Nucleation Bursts to California's Air Quality as Seen from the Pacific Coast Mountain Range

Abstract:

The long-range transport of dust and anthropogenic aerosols to the Western US has received considerable attention due to the growing disparity between North American and Asian air quality. Using MODIS and space-borne LIDAR measurements some have argued that the transcontinental transport of dust from Asia, Africa, and Europe outweighs that of locally produced combustion aerosols (Yu et al. 2012). This study seeks to compare the aerosol composition, number, and size distribution of locally derived submicron aerosols (including particle nucleation events) vs. long-range transported aerosols observed at a remote mountain site near the Pacific Coast. Toward this aim, rotating drum impactor (RDI) and scanning mobility particle size (SMPS) measurements of size-segregated elemental compositions and size spectra were collected from February to November of 2012 at Chews Ridge (elevation 1450 m) in Monterey County, California. This mountaintop site experiences two main wind modes. The main mode is ohshore-directed winds from the southwest, which are most likely to bring trans-Pacific aerosols to the site; and offshore-directed, northeasterly winds that bring continental aerosols to the site from the interior of California. Elemental ratios (normalized to Al), matrix factorization, and a k-cluster analysis of these data suggest distinct crustal, combustion, and marine sources with considerable seasonal as well as short-term variability. HYSPLIT model back trajectories support the hypothesized sources of these submicron aerosols. Locally, SMPS data reveal consistent nucleation bursts and subsequent growth in the 20-60 nm range during the afternoons. A distinct but weaker diel cycle was observed in the 70 – 100 nm range, corresponding to the smallest RDI impactor stage. Finally, the Pb isotopic composition (206Pb/207Pb and 208Pb/207Pb) of aerosol samples from selected dates will be measured by MC-ICPMS to further identify aerosol origins (e.g. Ewing et al. 2010).