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

Abstract

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 oxygento-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 H 2 O + 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 H 2 O + , CO + , and CO + 2 fragments in the high-resolution mass spectra of the standard compounds indicates that the Aiken-Ambient method underestimates the CO + and especially H 2 O + 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 Published by Copernicus Publications on behalf of the European Geosciences Union. M. R. Canagaratna et al.: Elemental ratio measurements of organic compoundsOA. 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...

Role of the reaction of stabilized Criegee intermediates with peroxy radicals in particle formation and growth in air

“…Elemental analysis was performed using the ''Improved-Ambient'' method. 59 Measurements of effective density were made as a function of particle diameter using the ratio of vacuum aerodynamic diameter to SMPS mobility diameter, assuming the particles are spherical. 60 ATR-FTIR measurements.…”

Comparison of three aerosol chemical characterization techniques utilizing PTR-ToF-MS: a study on freshly formed and aged biogenic SOA

“…As stated by Canagaratna et al (2015), the overall errors observed in calculations of elemental ratios would introduce an upper uncertainty of 28 %. In contrast to AMS data, O : C ratios for the PTR-based techniques were calculated with no additional correction factors, thus explaining their values being lower than those of AMS.…”

“…Although nearly all C x H y O z ions can be identified and quantified within the AMS mass spectra, AMS O : C calculation based on Canagaratna et al (2015) has several sources of uncertainties due to correction factors applied. As stated by Canagaratna et al (2015), the overall errors observed in calculations of elemental ratios would introduce an upper uncertainty of 28 %.…”

“…2). AMS O : C values were calculated based on the method by Canagaratna et al (2015). All instruments followed similar trends.…”

“…High-resolution mass spectra were analysed using the software packages Squirrel (v1.57) and PIKA (v1.15Z). Oxygen-to-carbon ratios were calculated based on the newly developed "improved-ambient" method by Canagaratna et al (2015).…”