Contribution of Selected Dicarboxylic and ω-Oxocarboxylic Acids in Ambient Aerosol to the m/z 44 Signal of an Aerodyne Aerosol Mass Spectrometer

Abstract

The Aerodyne aerosol mass spectrometer (AMS) employs flash vaporization (600°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 CO ₂ ⁺ ) 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 ω-oxocarboxylic acids (ω-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°39 ′ N, 139°40 ′ E) during August 3–8, 2003. Diacids and ω-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 ω-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 (C₂–C₆ diacids and phthalic acids) and ω-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 (C₂) 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.

Acknowledgments

The authors thank M. Koike and K. Asano for their support in the filter sampling. They also thank J. D. Allan for providing the AMS analysis software and M. R. Alfarra, J. L. Jimenez, and D. R. Worsnop for useful information on the fragment patterns of some organic compounds. This study was funded by the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) and the Japanese Science and Technology Agency (JST).

Notes

^a The parameters were taken from the International Chemical Safety Cards (ICSC) of the International Labour Organization (ILO) (http://www.ilo.org/).

^b The parameters were taken from Chemical Dictionary (1994).

^a Total organics and M ₄₄ were measured by AMS. Dicarboxylic acids (diacids) and ω-oxocarboxylic acids (ω-oxoacids) were measured using a filter sampling followed by GC-FID analysis. The AMS data were averaged over the integration time of the filter sampling.

^b Selected diacids represent the sum of the selected diacids listed in this table.

^c Total diacids and ω-oxoacids represent the sum of all diacids (C₂–C₁₂) and ω-oxoacids (C₂–C₉), respectively, identified by the GC analysis. These are listed by Kawamura and Yasui (2005).

* Base peak.

^a Assuming that one of the carboxyl groups is detected as the m/z 44 signal.

^b Values in parenthesis are the relative uncertainties (i.e., ratio of the standard deviation to the average). N/A means that the m/z 39 signal is not assigned as a fragment of the compound.