Abstract
Abstract. Online characterization of aerosol composition at the near-molecular level is key to understanding chemical reaction mechanisms, kinetics, and sources under various atmospheric conditions. The recently developed extractive electrospray ionization time-of-flight mass spectrometer (EESI-TOF) is capable of detecting a wide range of organic oxidation products in the particle phase in real time with minimal fragmentation. Quantification can sometimes be hindered by a lack of available commercial standards for aerosol constituents, however. Good correlations between the EESI-TOF and other aerosol speciation techniques have been reported, though no attempts have yet been made to parameterize the EESI-TOF response factor for different chemical species. Here, we report the first parameterization of the EESI-TOF response factor for secondary organic aerosol (SOA) at the near-molecular level based on its elemental composition. SOA was formed by ozonolysis of monoterpene or OH oxidation of aromatics inside an oxidation flow reactor (OFR) using ammonium nitrate as seed particles. A Vocus proton-transfer reaction mass spectrometer (Vocus-PTR) and a high-resolution aerosol mass spectrometer (AMS) were used to determine the gas-phase molecular composition and the particle-phase bulk chemical composition, respectively. The EESI response factors towards bulk SOA coating and the inorganic seed particle core were constrained by intercomparison with the AMS. The highest bulk EESI response factor was observed for SOA produced from 1,3,5-trimethylbenzene, followed by those produced from d-limonene and o-cresol, consistent with previous findings. The near-molecular EESI response factors were derived from intercomparisons with Vocus-PTR measurements and were found to vary from 103 to 106 ion counts s−1 ppb−1, mostly within ±1 order of magnitude of their geometric mean of 104.6 ion counts s−1 ppb−1. For aromatic SOA components, the EESI response factors correlated with molecular weight and oxygen content and inversely correlated with volatility. The near-molecular response factors mostly agreed within a factor of 20 for isomers observed across the aromatics and biogenic systems. Parameterization of the near-molecular response factors based on the measured elemental formulae could reproduce the empirically determined response factor for a single volatile organic compound (VOC) system to within a factor of 5 for the configuration of our mass spectrometers. The results demonstrate that standard-free quantification using the EESI-TOF is possible.
Highlights
Suspended particulate matter, or aerosol, is ubiquitous in the troposphere with far-reaching implications for public health, air quality, and climate (Jimenez et al, 2009; Dockery et al, 1993)
Volatile species which are not expected in the particle phase were observed, as shown in Fig. S5a– c, which may be an indication of some extent of ion fragmentation with the electrospray ionization (EESI)-TOF
While EESI-TOF particle-phase measurements suggest that C9H14Ox compounds are collectively much more abundant than either C9H12Ox or C9H16Ox compounds, Vocus-protontransfer reaction (PTR) gas-phase measurements suggest that C9H12Ox was more than 10 times more abundant than C9H14Ox, as shown in Fig. 1b and d
Summary
Aerosol, is ubiquitous in the troposphere with far-reaching implications for public health, air quality, and climate (Jimenez et al, 2009; Dockery et al, 1993). Real-time aerosol speciation is required to temporally resolve and understand aerosol dynamics To this end, an aerosol mass spectrometer (AMS) using flash vaporization and electron impact (EI) ionization serves as a reliable quantification method to determine the bulk composition of PM1 or PM2.5 (i.e. particles with an aerodynamic diameter < 1 or < 2.5 μm, respectively) over long periods of time both online (DeCarlo et al, 2006; Jimenez et al, 2009; Ng et al, 2011) and offline (Daellenbach et al, 2016). The extensive thermal and EI-induced fragmentations render the technique ill-suited to inferring the molecular identity of individual components, with very few exceptions (Alfarra et al, 2007; Budisulistiorini et al, 2013) More recent techniques such as the Filter Inlet for Gases and AEROsols (FIGAERO; Lopez-Hilfiker et al, 2014) and the Chemical Analysis of Aerosol Online (CHARON) inlet (Müller et al, 2017; Eichler et al, 2015) utilize chemical ionization mass spectrometry (CIMS) instead. The need for thermal volatilization to convert the particles to vapours before ionization may introduce artefacts from the decomposition of thermally labile compounds (Leglise et al, 2019; Stark et al, 2017; Zhao et al, 2019)
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