Individual Particle TOF-SIMS Imaging Analysis of Aerosol Collected During the April 2001 Asian Dust Event.

Time of Flight Secondary Ion Mass Spectroscopy can provide information regarding the surface chemistry, including both organic and inorganic compounds, of individual atmospheric aerosol in themicrometer size range. X-ray analysis has commonly been used to analyze the composition of single particles but has several important limitations. Principally, X-ray analysis cannot be used to study organic compounds in the aerosol, it offers low sensitivity for light elements common in crustal material and it cannot distinguish isotopes. TOF-SIMS has the potential to provide superior performance in these areas. We have developed statistical image processing methods to allow extraction of individual particle mass spectra from TOF-SIMS images. In mid April 2001 a strong Asian dust event was tracked by the NASA TOMS satellite across the Pacific Ocean and into the continental United States. While Asian dust deposition is common in Hawaii, strong events characterized by significant visibility degradation have been much less frequently reported in the Rocky Mountain west. Samples were taken during and after the event at the University of Utah in Salt Lake City, Utah (SLC). Size segregated samples were collected on Al substrates using an 8 stage cascade impactor and total aerosol samples were collected with 47 mm Fluoropore filters. Surface and depth profile analysis of the particles was performed using a Phi Trift I TOF-SIMS instrument. Statistical methods, including PCA, mixture models and neural networks, were used to extract spectra of individual particles from the TOF-SIMS images and to classify particles based on their surface chemistry and depth profiles. Differences in both the chemistry and size distribution of the particles could be seen between the aerosol collected during the Asian dust event and aerosol collected post-event at the University of Utah site. Positive TOF-SIMS spectra of SLC urban aerosol were dominated by sub-micrometer organics, and negative spectra included nitrates and sulfates. The Asian Dust samples were dominated by super-micrometer mineral aerosol. Principal Components Analysis, using the positive spectra of 6386 individual particles on the Asian dust impacted sample collected April 17, 2001 and 3415 individual particles on the background sample collected June 25, 2001 clearly separated the super-micrometer mineral aerosol from the sub-micrometer organic aerosol. Principal Component 1, which was the largest difference due to the Asian dust incursion, describes Mg, Si, Ca, and Fe as anti-correlated with the other major positive ion peaks for impactor stages 4 and 5 (approximately 1-3 micrometer aerodynamic diameter). Mixture models allowed separation of the aerosol collected on all 8 stages of the impactor into 14 classes. Observation of the differences in these particle classes between the Asian dust incursion sample and background SLC aerosol indicated again that the Asian dust dominated the 1-3 micrometer aerosol. Particle classes 1 through 5 were not found in the SLC background urban samples and were dominant in the 1-3 micrometer impactor stages of the April 17, 2001 sample. We thus feel that these classes describe the Asian dust impact on the SLC aerosol. The elements making up classes 1-5 include different ratios of Ca, Na, Mg, Si, K and Fe. These elements are typical of crustal dust. The higher classes (6-14) appear to include combustion products such as the C16 and C17 amides, phthalates, and hydrocarbons typical of petroleum use. These classes were dominant in the SLC urban background aerosol.