Abstract
Abstract. The impact of aerosols on climate and air quality remains poorly understood due to multiple factors. One of the current limitations is the incomplete understanding of the contribution of oxygenated products, generated from the gas-phase oxidation of volatile organic compounds (VOCs), to aerosol formation. Indeed, atmospheric gaseous chemical processes yield thousands of (highly) oxygenated species, spanning a wide range of chemical formulas, functional groups and, consequently, volatilities. While recent mass spectrometric developments have allowed extensive on-line detection of a myriad of oxygenated organic species, playing a central role in atmospheric chemistry, the detailed quantification and characterization of this diverse group of compounds remains extremely challenging. To address this challenge, we evaluated the capability of current state-of-the-art mass spectrometers equipped with different chemical ionization sources to detect the oxidation products formed from α-Pinene ozonolysis under various conditions. Five different mass spectrometers were deployed simultaneously for a chamber study. Two chemical ionization atmospheric pressure interface time-of-flight mass spectrometers (CI-APi-TOF) with nitrate and amine reagent ion chemistries and an iodide chemical ionization time-of-flight mass spectrometer (TOF-CIMS) were used. Additionally, a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF 8000) and a new “vocus” PTR-TOF were also deployed. In the current study, we compared around 1000 different compounds between each of the five instruments, with the aim of determining which oxygenated VOCs (OVOCs) the different methods were sensitive to and identifying regions where two or more instruments were able to detect species with similar molecular formulae. We utilized a large variability in conditions (including different VOCs, ozone, NOx and OH scavenger concentrations) in our newly constructed atmospheric simulation chamber for a comprehensive correlation analysis between all instruments. This analysis, combined with estimated concentrations for identified molecules in each instrument, yielded both expected and surprising results. As anticipated based on earlier studies, the PTR instruments were the only ones able to measure the precursor VOC, the iodide TOF-CIMS efficiently detected many semi-volatile organic compounds (SVOCs) with three to five oxygen atoms, and the nitrate CI-APi-TOF was mainly sensitive to highly oxygenated organic (O > 5) molecules (HOMs). In addition, the vocus showed good agreement with the iodide TOF-CIMS for the SVOC, including a range of organonitrates. The amine CI-APi-TOF agreed well with the nitrate CI-APi-TOF for HOM dimers. However, the loadings in our experiments caused the amine reagent ion to be considerably depleted, causing nonlinear responses for monomers. This study explores and highlights both benefits and limitations of currently available chemical ionization mass spectrometry instrumentation for characterizing the wide variety of OVOCs in the atmosphere. While specifically shown for the case of α-Pinene ozonolysis, we expect our general findings to also be valid for a wide range of other VOC–oxidant systems. As discussed in this study, no single instrument configuration can be deemed better or worse than the others, as the optimal instrument for a particular study ultimately depends on the specific target of the study.
Highlights
Atmospheric aerosols, a mixture of solid and liquid particles consisting of organic and inorganic substances suspended in the air, have a significant impact on climate (Albrecht, 1989; Hallquist et al, 2009; Intergovernmental Panel on Climate Change, 2014; Twomey, 1977)
secondary organic aerosol (SOA) is predominantly formed through the gas-phase oxidation of volatile organic compounds (VOCs), producing oxygenated VOCs (OVOCs), which can subsequently condense onto preexisting aerosol particles
The COALA chamber was operated under steady-state conditions, meaning that a constant flow of reactants and oxidants were continuously added to the chamber, while chamber air was sampled by the instruments
Summary
Atmospheric aerosols, a mixture of solid and liquid particles consisting of organic and inorganic substances suspended in the air, have a significant impact on climate (Albrecht, 1989; Hallquist et al, 2009; Intergovernmental Panel on Climate Change, 2014; Twomey, 1977). They are recognized to adversely impact air quality and human health, nowadays representing the fifth-ranking human health risk factor globally (Gakidou et al, 2017). SOA is predominantly formed through the gas-phase oxidation of volatile organic compounds (VOCs), producing oxygenated VOCs (OVOCs), which can subsequently condense onto preexisting aerosol particles. Studies have provided new insights into how highly oxygenated organic molecules (HOMs) can form faster than previously expected and at high enough yields to make them a major source of condensing or even nucleating compounds (Ehn et al, 2014; Jokinen et al, 2015; Kirkby et al, 2016; Wang et al, 2018)
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