Elemental ratio measurements of organic compounds using aerosol mass spectrometry: characterization, improved calibration, and implications

摘要

Elemental compositions of organic aerosol (OA) particles provide usefulconstraints on OA sources, chemical evolution, and effects. The Aerodynehigh-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) iswidely used to measure OA elemental composition. This study evaluates AMSmeasurements 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( ) for a vastly expanded laboratory data setof multifunctional oxidized OA standards. For the expanded standard data set,the method introduced by Aiken et al. (2008), which uses experimentallymeasured ion intensities at all ions to determine elemental ratios (referredto here as "Aiken-Explicit"), reproduces known O : C and H : C ratio valueswithin 20% (average absolute value of relative errors) and 12%,respectively. The more commonly used method, which uses empiricallyestimated HO and CO ion intensities to avoid gas phase airinterferences at these ions (referred to here as "Aiken-Ambient"),reproduces O : C and H : C of multifunctional oxidized species within 28 and14% of known values. The values from the latter method are systematicallybiased low, however, with larger biases observed for alcohols and simplediacids. A detailed examination of the HO, CO, andCO fragments in the high-resolution mass spectra of the standardcompounds indicates that the Aiken-Ambient method underestimates theCO and especially HO produced from many oxidizedspecies. Combined AMS–vacuum ultraviolet (VUV) ionization measurementsindicate that these ions are produced by dehydration and decarboxylation onthe AMS vaporizer (usually operated at 600 °C). Thermal decompositionis observed to be efficient at vaporizer temperatures down to 200 °C.These results are used together to develop an "Improved-Ambient" elementalanalysis method for AMS spectra measured in air. The Improved-Ambient methoduses specific ion fragments as markers to correct for molecularfunctionality-dependent systematic biases and reproduces known O : C (H : C)ratios of individual oxidized standards within 28% (13%) of the knownmolecular values. The error in Improved-Ambient O : C (H : C) values is smallerfor theoretical standard mixtures of the oxidized organic standards, whichare 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 thatare 27% (11%) larger than previously published Aiken-Ambient values; acorresponding increase of 9% is observed for OM : OC values. These resultsimply that ambient OA has a higher relative oxygen content than previouslyestimated. The values calculated forambient OA by the two methods agree well, however (average relativedifference of 0.06 units). This indicatesthat is a more robust metric of oxidationthan O : C, likely since is not affected byhydration or dehydration, either in the atmosphere or during analysis.