Organic Aerosol Analysis with a High Resolution Time-of-Flight Aerosol Mass Spectrometer in Riverside, CA (SOAR 2005)

During the summer months ambient aerosols reach peak concentrations in the L.A. Basin, with the highest downwind of downtown L.A. and the Chino agricultural area. For this reason, a newly developed version of the Aerodyne Aerosol Mass Spectrometer, the High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS), was deployed during July and August in Riverside, CA for the Study of Organic Aerosols in Riverside (SOAR 2005). The main advantage of the HR-ToF-AMS is its ability to resolve the elemental composition of most mass fragments, especially for the low m/z where the majority of the signal occurs in the AMS. This allows the direct separation of organic mass fragments from the mass spectra to be separated into an increased numzber of organic categories (CxHy+, CxHyOz+, CxHyNz+, CwHxNyOz+) while still retaining quantitative mass concentrations. Combining elemental composition with the time series analysis, it is shown that most of the organic aerosol in Riverside was highly oxygenated in nature (OOA), likely mostly secondary organic aerosol (SOA). Time series of mass concentrations are shown from Riverside, demonstrating a diurnal difference between morning and evening organic particulate compositions. Further analysis of the organic spectra includes deconvolution of the mass spectra by category. SOA formed from chamber reactions was also analyzed with the HR-ToF-AMS, and the spectra are compared with that of the ambient OOA. Spectra from lab analysis of organic standards are used to help the interpretation of the ambient data. The HR-ToF-AMS should lead to more accurate source apportionment and increased characterization of ambient organic aerosols.