Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
Atmos. Chem. Phys., 15, 253-272, 2015
https://doi.org/10.5194/acp-15-253-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
12 Jan 2015
Elemental ratio measurements of organic compounds using aerosol mass spectrometry: characterization, improved calibration, and implications
M. R. Canagaratna1, J. L. Jimenez2, J. H. Kroll4,3, Q. Chen3, S. H. Kessler4, P. Massoli1, L. Hildebrandt Ruiz5, E. Fortner1, L. R. Williams1, K. R. Wilson6, J. D. Surratt7, N. M. Donahue8, J. T. Jayne1, and D. R. Worsnop1 1Aerodyne Research, Inc., Billerica, MA, USA
2Department of Chemistry and Biochemistry, and Cooperative Institute for Research in the Environmental Sciences (CIRES), University of Colorado, Boulder, CO, USA
3Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA
4Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA
5McKetta Department of Chemical Engineering, and Center for Energy and Environmental Resources, The University of Texas at Austin, Austin, TX, USA
6Lawrence Berkeley National Lab, Berkeley, CA, USA
7Department of Environmental Science and Engineering, University of North Carolina, Chapel Hill, NC, USA
8Center for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, PA, USA
Abstract. Elemental compositions of organic aerosol (OA) particles provide useful constraints on OA sources, chemical evolution, and effects. The Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) is widely used to measure OA elemental composition. This study evaluates AMS measurements of atomic oxygen-to-carbon (O : C), hydrogen-to-carbon (H : C), and organic mass-to-organic carbon (OM : OC) ratios, and of carbon oxidation state (OS C) for a vastly expanded laboratory data set of multifunctional oxidized OA standards. For the expanded standard data set, the method introduced by Aiken et al. (2008), which uses experimentally measured ion intensities at all ions to determine elemental ratios (referred to here as "Aiken-Explicit"), reproduces known O : C and H : C ratio values within 20% (average absolute value of relative errors) and 12%, respectively. The more commonly used method, which uses empirically estimated H2O+ and CO+ ion intensities to avoid gas phase air interferences at these ions (referred to here as "Aiken-Ambient"), reproduces O : C and H : C of multifunctional oxidized species within 28 and 14% of known values. The values from the latter method are systematically biased low, however, with larger biases observed for alcohols and simple diacids. A detailed examination of the H2O+, CO+, and CO2+ fragments in the high-resolution mass spectra of the standard compounds indicates that the Aiken-Ambient method underestimates the CO+ and especially H2O+ produced from many oxidized species. Combined AMS–vacuum ultraviolet (VUV) ionization measurements indicate that these ions are produced by dehydration and decarboxylation on the AMS vaporizer (usually operated at 600 °C). Thermal decomposition is observed to be efficient at vaporizer temperatures down to 200 °C. These results are used together to develop an "Improved-Ambient" elemental analysis method for AMS spectra measured in air. The Improved-Ambient method uses specific ion fragments as markers to correct for molecular functionality-dependent systematic biases and reproduces known O : C (H : C) ratios of individual oxidized standards within 28% (13%) of the known molecular values. The error in Improved-Ambient O : C (H : C) values is smaller for theoretical standard mixtures of the oxidized organic standards, which are more representative of the complex mix of species present in ambient OA. For ambient OA, the Improved-Ambient method produces O : C (H : C) values that are 27% (11%) larger than previously published Aiken-Ambient values; a corresponding increase of 9% is observed for OM : OC values. These results imply that ambient OA has a higher relative oxygen content than previously estimated. The OS C values calculated for ambient OA by the two methods agree well, however (average relative difference of 0.06 OS C units). This indicates that OS C is a more robust metric of oxidation than O : C, likely since OS C is not affected by hydration or dehydration, either in the atmosphere or during analysis.

Citation: Canagaratna, M. R., Jimenez, J. L., Kroll, J. H., Chen, Q., Kessler, S. H., Massoli, P., Hildebrandt Ruiz, L., Fortner, E., Williams, L. R., Wilson, K. R., Surratt, J. D., Donahue, N. M., Jayne, J. T., and Worsnop, D. R.: Elemental ratio measurements of organic compounds using aerosol mass spectrometry: characterization, improved calibration, and implications, Atmos. Chem. Phys., 15, 253-272, https://doi.org/10.5194/acp-15-253-2015, 2015.
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Short summary
Atomic oxygen-to-carbon (O:C), hydrogen-to-carbon (H:C), and organic mass-to-organic carbon (OM:OC) ratios of ambient organic aerosol (OA) species provide key constraints for understanding their sources and impacts. Here an improved method for obtaining accurate O:C, H:C, and OM:OC with a widely used aerosol mass spectrometer is developed. These results imply that OA is more oxidized than previously estimated and indicate the need for new chemical mechanisms that simulate ambient oxidation.
Atomic oxygen-to-carbon (O:C), hydrogen-to-carbon (H:C), and organic mass-to-organic carbon...
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