AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Effects of Thermal Decomposition and Ion Fragmentation on Elemental Ratios and Chemical Compositions Measured with High Resolution Aerosol Mass Spectrometry
MANJULA CANAGARATNA, Paola Massoli, Leah Williams, Sean Kessler, Edward Fortner, John Jayne, Kevin Wilson, Jesse Kroll, Douglas Worsnop, Aerodyne Research, Inc.
Abstract Number: 642 Working Group: Aerosol Chemistry
Abstract Aerosol mass spectrometry has been widely used to characterize aerosol chemical composition in both laboratory and ambient environments. In thermal vaporization based aerosol mass spectrometers particulate material is first heated and the resulting gas is ionized and detected. Thus, the mass spectra obtained from these instruments reflect both thermal decomposition and ion fragmentation. Oxidized organic aerosol (OOA) species, which are known to account for a significant fraction of the organic aerosol mass measured in most ambient environments, are expected to undergo significant thermal decomposition and ion fragmentation.
In this study we use several commercially available oxidized organic molecules to evaluate the Aerodyne aerosol mass spectrometer's (AMS's) response to different types of OOA. A distinguishing feature of ambient OOA mass spectra measured with the AMS is a peak at m/z 44 (CO$_(2)$^(+)), which is broadly known to result from thermal decomposition and/or ion fragmentation of carboxylic acid containing moieties. Thus high resolution AMS spectra of several types of organic acid species (polyacids, keto and hydroxy acids and aromatic acids) as well as other mixed oxygen-containing functionalities are measured as part of this work. Thermal decomposition effects in AMS spectra were examined with oven temperatures ranging from 200 C to typical operating temperatures of 600 C. Ion fragmentation effects were further examined by comparing AMS spectra from conventional electron impact ionization (70 eV) with AMS spectra obtained using softer photoionization from vacuum ultra-violet light (8.5 eV-12.5 eV). Taken together, these measurements provide insight into how thermal decomposition and ion fragmentation processes affect the elemental ratios and chemical functionality information that is extracted from high resolution AMS spectra of OOA. The temperature dependent decomposition behavior of oxidized organic species is also of broader interest to other aerosol measurement techniques that utilize thermal methods to volatilize particulate material.