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Effect of Vaporizer Temperature on Ambient High-Resolution Time-of-Flight Aerosol Mass Spectrometer Organic Mass Spectra

Kenneth S. Docherty (1,2), John A. Huffman (2), and Jose L. Jimenez (1,2)

(1) CIRES, (2) Department of Chemistry and Biochemistry, University of Colorado-Boulder

Thermal vaporization is commonly used in many real-time aerosol chemical speciation instruments to desorb components of the aerosol prior to ionization and mass selection. Due to the wide variety of chemical species present in ambient aerosols combined with their wide range of volatilities, elevated vaporization temperatures are routinely used to ensure complete desorption of low volatility species such as ammonium sulfate. However, these high temperatures can cause thermal fragmentation of organic species, particularly for labile compounds, thereby lessening the quality of data with respect to identification of complex organic species. The high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) normally vaporizes species at 600oC during ambient sampling, which can cause degradation of organic mass spectra as a result of thermal fragmentation. An HR-ToF-AMS was modified to allow automated vaporizer temperature stepping from 600oC to a minimum of 200oC with 4 steps over a period of ~25 minutes. This instrument was deployed during the summer 2005 Study of Organic Aerosols in Riverside (SOAR-1) and also during the 2007 Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX) to characterize the impact of vaporizer temperature on HR-ToF-AMS organic aerosol mass spectra. Here we present the results of positive matrix factorization (PMF) of organic aerosol mass spectra collected during periods of vaporizer temperature cycling. PMF results are presented both from the analysis of individual vaporizer temperatures (i.e., for each cycling temperature) and from the analysis of all cycling temperatures combined. The potential for gaining information regarding the volatility of individual organic aerosol components using PMF analysis of variable vaporizer temperature data will also be presented.