Elemental Ratio Measurements of Organic Compounds Using Aerosol Mass Spectrometry: Characterization, Improved Calibration, and Implications

dc.contributor.utaustinauthorRuiz, L. Hildebrandten_US
dc.creatorCanagaratna, M. R.en_US
dc.creatorJimenez, J. L.en_US
dc.creatorKroll, J. H.en_US
dc.creatorChen, Q.en_US
dc.creatorKessler, S. H.en_US
dc.creatorMassoli, P.en_US
dc.creatorRuiz, L. Hildebrandten_US
dc.creatorFortner, E.en_US
dc.creatorWilliams, L. R.en_US
dc.creatorWilson, K. R.en_US
dc.creatorSurratt, J. D.en_US
dc.creatorDonahue, N. M.en_US
dc.creatorJayne, J. T.en_US
dc.creatorWorsnop, D. R.en_US
dc.description.abstractElemental 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) over bar (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 C 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 degrees C). Thermal decomposition is observed to be efficient at vaporizer temperatures down to 200 degrees 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) over bar (C) values calculated for ambient OA by the two methods agree well, however (average relative difference of 0.06 (OS) over bar (C) units). This indicates that (OS) over bar (C) is a more robust metric of oxidation than O : C, likely since (OS) over bar (C) is not affected by hydration or dehydration, either in the atmosphere or during analysis.en_US
dc.description.departmentChemical Engineeringen_US
dc.description.sponsorshipNSF CHE-1012809, AGS-1243354en_US
dc.description.sponsorshipNASA NNX12AC03Gen_US
dc.description.sponsorshipNOAA NA13OAR4310063en_US
dc.description.sponsorshipNational Science Foundation ATM-1238109, AGS1136479en_US
dc.description.sponsorshipOffice of Basic Energy Sciences of the US Department of Energy DE-AC02-05CH11231en_US
dc.description.sponsorshipDepartment of Energy, Office of Science Early Career Research Programen_US
dc.description.sponsorshipUS Department of Energy DE-FG02-03ER83599, DE-FG02-05ER84269, DE-FG02-07ER84890en_US
dc.identifier.citationCanagaratna, M. R., J. L. Jimenez, J. H. Kroll, Q. Chen, S. H. Kessler, P. Massoli, L. Hildebrandt Ruiz et al. "Elemental ratio measurements of organic compounds using aerosol mass spectrometry: characterization, improved calibration, and implications." Atmospheric Chemistry and Physics, Vol. 15, No. 1 (2015): 253-272.en_US
dc.relation.ispartofserialAtmospheric Chemistry and Physicsen_US
dc.rightsAdministrative deposit of works to Texas ScholarWorks: This works author(s) is or was a University faculty member, student or staff member; this article is already available through open access or the publisher allows a PDF version of the article to be freely posted online. The library makes the deposit as a matter of fair use (for scholarly, educational, and research purposes), and to preserve the work and further secure public access to the works of the University.en_US
dc.subjectatmospheric aerosolsen_US
dc.subjectparticle compositionen_US
dc.subjecthydroxyl radicalsen_US
dc.subjectambient aerosolen_US
dc.subjectriver deltaen_US
dc.subjectmeteorology & atmospheric sciencesen_US
dc.titleElemental Ratio Measurements of Organic Compounds Using Aerosol Mass Spectrometry: Characterization, Improved Calibration, and Implicationsen_US

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