New Insights into Online Molecular Characterization of Organic Aerosol in China Using Extractive Electrospray Ionization and Ultrahigh Resolution Mass Spectrometry (EESI-Orbitrap)
Qi, Lu ; Lee Chuan, Ping ; Giannoukos, Stamatios ; Wang, Dongyu ; Li, Zhiyu ; Ran, Weikang ; Ye, Penglin ; Wang, Liwei ; Wang, Meng ; Lin, Yue ; Han, Yuemei ; Wang, Qiyuan ; Baltensperger, Urs ; El-Haddad, Imad ; Cao, Junji ; Prevot, Andre; Slowik, Jay
Abstract
Despite extensive study, the sources and processes governing the formation and chemical evolution of haze pollution episodes in urban China remain unclear. Real-time molecular characterization of organic aerosol (OA) represents a promising approach to this problem. Extractive electrospray ionization (EESI) mass spectrometry allows real-time detection of individual OA components, while avoiding thermal decomposition and ionization-induced fragmentation. In previous field measurements, the EESI source was coupled with time-of-flight (TOF) mass analyzers, which are limited to a mass resolution of ~12,000 (at 200 m/z). This leads to significant uncertainty in both the identity and quantity of the measured molecules, inhibiting accurate molecular characterization, especially in polluted ambient environments. Recently, the coupling of an EESI-source with an Orbitrap mass analyzer achieves a mass resolution of ~140,000 (at 200 m/z). The EESI-Orbitrap enables unambiguous identication of the molecular ion chemical formula comprising OA and clear separation between adjacent peaks, improving both identification and quantification of ions subject to interference from neighboring molecular peaks (including background). Here we present the first EESI-Orbitrap field deployment, conducted during an intensive measurement campaign in Xian, China from March to May, 2020. Two EESI-Orbitrap systems were deployed, respectively utilizing positive and negative ion detection. The (+) EESI-Orbitrap detected oxidized organic compounds as Na+ adducts, while the (-) EESI-Orbitrap detected oxidized organic compounds, including those containing heteroatoms (e.g. organosulfates and organonitrates), as deprotonated ions. Supporting instrumentation included an EESI-LTOF, an aerosol mass spectrometer (AMS), and a Vocus proton-transfer reaction mass spectrometer (Vocus-PTR). Intercomparisons show excellent time-series agreement in both Orbitrap and TOF datasets, with the Orbitrap providing effects of transitions between day/night, haze/non-haze, and winter/summer on OA compositions. Finally, the ability of the EESI-Orbitrap to provide new insights into the dominant sources and processes governing haze formation and chemical evolution is assessed.
