HOM cluster decomposition in APi-TOF mass spectrometers

<p>Recent developments in mass spectrometry have brought huge advancements to the field of atmospheric science. For example, mass spectrometers are now able to detect ppq-level (10<sup>-15</sup>) concentrations of both clusters and precursor vapours in atmospheric samples (Junninen et al., 2010; Jokinen et al., 2012), as well as directly explore the chemistry of new particle formation (NPF) in the atmosphere (Kulmala et al., 2014; Bianchi et al., 2016; Ehn et al., 2014). One of the most common mass spectrometers used to measure online cluster composition and concentration in the atmosphere is the Atmospheric Pressure interface Time Of Flight Mass Spectrometer (APi-TOF MS).<br>Identification of atmospheric molecular clusters and measurement of their concentrations by APi-TOF may be affected by systematic error due to possible decomposition of clusters inside the instrument. Indeed, the detection process in the APi-TOF involves energetic interactions between the carrier gas and the clusters, possibly leading to their decomposition, and thus altering the measurement results.<br>Here we use a theoretical model to study in detail the decomposition of clusters involving so-called Highly-Oxygenated organic Molecules (HOM), which have recently been identified as a key contributor to NPF (Bianchi et al., 2019). HOM are molecules formed in the atmosphere from Volatile Organic Compounds (VOC). Some VOC with suitable functional groups can undergo an autoxidation process involving peroxy radicals, generating polyfunctional low-volatility vapors (i.e. HOM) that subsequently condense onto pre-existing particles. <br>Our study involves a specific kind of representative HOM (C<sub>10</sub>H<sub>16</sub>O<sub>8</sub>) in the APi. This elemental composition corresponds to one of the most common mass peaks observed in experiments on ozone-initiated autoxidation of α-pinene, which also fulfills the “HOM” definition of Bianchi et al. (2019). The precise molecular structure was adopted from Kurtén et al. (2016), and corresponds to the lowest-volatility structural isomer of the three C<sub>10</sub>H<sub>16</sub>O<sub>8</sub> compounds investigated in that study.<br>The main scope of this work is to determine to what extent we are able to perform measurements of atmospheric cluster concentrations using APi-TOF mass spectrometers. More specifically, we want to determine whether decomposition can possibly be responsible for the lack of observations of some HOM-containing clusters in an APi-TOF. Here, we predict both an upper bound for decomposition energy necessary for decomposition in the APi-TOF, and a lower bound for new-particle formation in the atmosphere given realistic vapor concentrations.<br>Our results show that decomposition is highly unlikely for the considered clusters, provided their bonding energy is large enough to allow formation in the atmosphere in the first place.</p>