Contribution of selected dicarboxylic and ω-oxocarboxylic acids in ambient aerosol to the m/z 44 signal of an aerodyne aerosol mass spectrometer

The Aerodyne aerosol mass spectrometer (AMS) employs flash vaporization (600 degrees C) followed by 70-eV electron impact ionization (EI) to detect organic and inorganic aerosols. The signal at mass-to-charge ratio (m/z) 44 (mainly CO2+) is considered the most reliable marker of oxygenated organic aerosol. This study is the first to evaluate the contribution of selected low molecular weight dicarboxylic acids (diacids) and omega-oxocarboxylic acids (omega-oxoacids) to the particle-phase m/z 44 signal of the AMS mass spectrum. Ambient measurements were conducted at a surface site in Tokyo (35 degrees 39'N, 139 degrees 40'E) during August 3-8, 2003. Diacids and omega-oxoacids were measured using a filter sampling followed by extraction, derivation, and gas chromatograph-flame ionization detector (GC-FID) analysis. The mass concentrations of diacids and (omega-oxoacids show tight correlation with the m/z 44 signal (r(2) = 0.85-0.94) during the measurement period. Laboratory experiments were also performed to determine the fragment patterns of selected diacids (C-2-C-6 diacids and phthalic acids) and omega-oxoacid (glyoxylic acid) in ambient aerosols. Here, we report for the first time that the selected organic acids could account for 14 +/- 5% of the observed m/z 44 signal on average during the measurement period. Oxalic acid (C-2) is the largest contributor, accounting for 10 +/- 4% of the observed m/z 44 signal. These results would be useful for interpreting the m/z 44 signals obtained from ambient measurements in various locations.