Comprehensive modeling study of ozonolysis of oleic acid aerosol based on real‐time, online measurements of aerosol composition

Abstract

The chemical composition of organic aerosols profoundly influences their atmospheric properties, but a detailed understanding of heterogeneous and in‐particle reactivity is lacking. We present here a combined experimental and modeling study of the ozonolysis of oleic acid particles. An online mass spectrometry (MS) method, Extractive Electrospray Ionization (EESI), is used to follow the composition of the aerosol at a molecular level in real time; relative changes in the concentrations of both reactants and products are determined during aerosol aging. The results show evidence for multiple non‐first‐order reactions involving stabilized Criegee intermediates, including the formation of secondary ozonides and other oligomers. Offline liquid chromatography MS is used to confirm the online MS assignment of the monomeric and dimeric products. We explain the observed EESI‐MS chemical composition changes, and chemical and physical data from previous studies, using a process‐based aerosol chemistry simulation, the Pretty Good Aerosol Model (PG‐AM). In particular, we extend previous studies of reactant loss by demonstrating success in reproducing the time dependence of product formation and the evolving particle size. This advance requires a comprehensive chemical scheme coupled to the partitioning of semivolatile products; relevant reaction and evaporation parameters have been refined using our new measurements in combination with PG‐AM.

Multiphase composition changes and reactive oxygen species formation during limonene oxidation in the new Cambridge Atmospheric Simulation Chamber (CASC)

“…Gallimore and Kalberer (2013) demonstrated that the relative EESI-MS ion intensity correlated with the mass concentration of tartaric acid particles delivered into the source, suggesting that the entire particle bulk is extracted for analysis. More recently, Gallimore et al (2017) showed that the kinetics of particle-phase reactions could be monitored; loss rates derived from EESI-MS measurements compared well with other studies, and spectra were compared to Liquid Chromatography (LC) MS to confirm that the EESI-MS assignments were present in the aerosol rather than formed as artefacts in the ion source.…”

“…PG-AM is described in detail in Griffiths et al (2009) and Gallimore et al (2017). The model treats the following processes in a kinetic framework: chemical reaction in both the gas and particle phases, gas-particle exchange via uptake and evaporation, and diffusion within the particle.…”

“…EESI retains the key advantage of "soft" electrospray ionisation MS techniques, namely that quasimolecular ions are produced from aerosol analytes with minimal fragmentation. Individual molecular species can be identified and relative intensity changes monitored over time as a measure of concentration changes with particles (Gallimore et al, 2017). Gas-phase VOC components are monitored using Proton Transfer Reaction Time of Flight (PTR-ToF) MS.…”

Online molecular characterisation of organic aerosols in an atmospheric chamber using extractive electrospray ionisation mass spectrometry

“…However, as discussed above, care is required in interpreting aerosol-phase concentration changes for the many species where in-particle reaction or gas-particle partitioning may be significant. A coupled model of aerosol-and gas-phase chemistry (Gallimore et al, 2017a;Shiraiwa et al, 2010) would therefore be a desirable tool to use alongside future chamber experiments.…”

“…The detected MS peak abundance scaled in direct proportion with the aerosol mass concentration and was independent of particle diameter in the ranges studied (3-600 µg m −3 , 70-200 nm). Gallimore et al (2017a) applied the technique for the first time to characterising the kinetics of particle-phase reactions, using the ozonolysis of oleic acid particles as a model system. Changes in relative abundances of ions were used successfully as a proxy for relative concentrations and complex oligomeric species could be detected with minimal molecular fragmentation.…”