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| Titel |
Elemental ratio measurements of organic compounds using aerosol mass spectrometry: characterization, improved calibration, and implications |
| VerfasserIn |
M. R. Canagaratna, J. L. Jimenez, J. H. Kroll, Q. Chen, S. H. Kessler, P. Massoli, L. Hildebrandt Ruiz, E. Fortner, L. R. Williams, K. R. Wilson, J. D. Surratt, N. M. Donahue, J. T. Jayne, D. R. Worsnop |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 15, no. 1 ; Nr. 15, no. 1 (2015-01-12), S.253-272 |
| Datensatznummer |
250119296
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| Publikation (Nr.) |
 copernicus.org/acp-15-253-2015.pdf |
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| Zusammenfassung |
| 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. |
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