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 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.
Highlights
Organic aerosols (OA) account for a substantial fraction of ambient submicron aerosol mass in urban and rural/remote environments, with important impacts ranging from human health to climate forcing (IPCC, 2013; Pope and Dockery, 2006)
We evaluated the performance of both Aiken-Explicit and Aiken-Ambient methods over a large range of species, including those with higher O : C and more multifunctional moieties than originally studied by Aiken et al (2008)
The Aiken-Explicit method results reproduce actual O : C and H : C ratios for all the standard molecules with an average absolute value of the relative error of 20 and 12 %, respectively. This is consistent with the accuracies reported by Aiken et al (2008) and confirms that the Aiken-Explicit method can be used for a wide range of OA species
Summary
Organic aerosols (OA) account for a substantial fraction of ambient submicron aerosol mass in urban and rural/remote environments, with important impacts ranging from human health to climate forcing (IPCC, 2013; Pope and Dockery, 2006). Gas chromatography–mass spectrometry (GCMS) (Williams et al, 2006) and chemical ionization mass spectrometry (CIMS) with aerosol collection interface have recently been coupled to a high-resolution time-of-flight mass spectrometer to allow for determination of elemental ratios (i.e., O : C and H : C) of organic aerosols (LopezHilfiker et al, 2014; Yatavelli and Thornton, 2010; Williams et al, 2014). Each of these techniques has its own strengths and weaknesses. This manuscript evaluates the accuracy of the AMS elemental analysis approach over a wider range of OA species than has been studied before
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