Molecular composition and volatility of isoprene photochemical oxidation secondary organic aerosol under low- and high-NO<sub><i>x</i></sub> conditions

Abstract

Abstract. We present measurements of secondary organic aerosol (SOA) formation from isoprene photochemical oxidation in an environmental simulation chamber at a variety of oxidant conditions and using dry neutral seed particles to suppress acid-catalyzed multiphase chemistry. A high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS) utilizing iodide-adduct ionization coupled to the Filter Inlet for Gases and Aerosols (FIGAERO) allowed for simultaneous online sampling of the gas and particle composition. Under high-HO2 and low-NO conditions, highly oxygenated (O : C ≥ 1) C5 compounds were major components (∼ 50 %) of SOA. The SOA composition and effective volatility evolved both as a function of time and as a function of input NO concentrations. Organic nitrates increased in both the gas and particle phases as input NO increased, but the dominant non-nitrate particle-phase components monotonically decreased. We use comparisons of measured and predicted gas-particle partitioning of individual components to assess the validity of literature-based group-contribution methods for estimating saturation vapor concentrations. While there is evidence for equilibrium partitioning being achieved on the chamber residence timescale (5.2 h) for some individual components, significant errors in group-contribution methods are revealed. In addition, > 30 % of the SOA mass, detected as low-molecular-weight semivolatile compounds, cannot be reconciled with equilibrium partitioning. These compounds desorb from the FIGAERO at unexpectedly high temperatures given their molecular composition, which is indicative of thermal decomposition of effectively lower-volatility components such as larger molecular weight oligomers.

Flight Deployment of a High‐Resolution Time‐of‐Flight Chemical Ionization Mass Spectrometer: Observations of Reactive Halogen and Nitrogen Oxide Species

“…The University of Washington high-resolution time-of-flight chemical ionization mass spectrometer (henceforth, HRToF-CIMS) has demonstrated selectivity, sensitivity, portability, and time response capable of mixing ratio and eddy covariance flux measurements under conditions ranging from pristine to polluted atmosphere (D'Ambro et al, 2017;Lopez-Hilfiker, Mohr, et al, 2016;Mohr et al, 2017;Schobesberger et al, 2016). Here we present on the aircraft deployable version, first described in Lee et al (2014), but which has undergone substantial upgrades.…”

Estimating the saturation vapor pressures of isoprene oxidation products C<sub>5</sub>H<sub>12</sub>O<sub>6</sub> and C<sub>5</sub>H<sub>10</sub>O<sub>6</sub> using COSMO-RS

“…Estimation methods based on quantum chemistry, most notably COSMO-RS (conductor-like screening model for real solvents; Eckert and Klamt, 2002), as implemented in the COSMOtherm program (COSMOtherm, 2018), require fewer system-specific empirical parameters and could thus be expected to be more accurate for compound classes for which there are limited measurement data. COSMOtherm has recently been applied to estimate both saturation vapor pressures, Henry's law constants, and partitioning coefficients for atmospherically relevant oxidized organic molecules, including some potential autoxidation products (Wania et al, 2014;Wang et al, 2015Wang et al, , 2017.…”

Chlorine-initiated oxidation of n-alkanes under high NOx conditions: Insights into secondary organic aerosol composition and volatility using a FIGAERO-CIMS

“…It 20 was not clear if the sulfate ions measured were derived from sulfuric acid or from the decomposition of (NH4)2SO4, which could change the assumption made for aerosol acidity. A Tmax shift was also observed for isoprene SOA, where the mass and Tmax of non-nitrated OA decreased with increasing NOx concentration (D'Ambro et al, 2017). A recent investigation reported increases in Tmax for α-pinene ozonolysis SOA produced under dry conditions compared to SOA produced under humid conditions (Huang et al, 2018a).…”

“…The Tmax shift phenomena have been observed in several FIGAERO-CIMS studies (D'Ambro et al, 2017;Gaston et al, 2016;Huang et al, 2018a;Lopez-Hilfiker et al, 2015;Thompson et al, 2017). Explanations vary for the observed Tmax shifts.…”

“…Theory and operation of the CIMS are described in detail elsewhere (Aljawhary et al, 2013;Bertram et al, 2011;Lee et al, 2014;Wang and Hildebrandt Ruiz, 2017). The CIMS inlet is coupled to a filter inlet for gases and aerosols (FIGAERO), which has been 10 used in a number of studies to investigate the chemical composition and volatility of particle-phase compounds (D'Ambro et al, 2017;Gaston et al, 2016;Huang et al, 2018a;Lee et al, 2016;Lopez-Hilfiker et al, 2014Stark et al, 2017;Thompson et al, 2017). The FIGAERO system alternated between two operational modes.…”