title: Particle-phase chemistry of secondary organic material: modeled compared to measured O:C and H:C elemental ratios provide constraints.
abstract: Chemical mechanisms for the production of secondary organic material (SOM) are developed in focused laboratory studies but widely used in the complex modeling context of the atmosphere. Given this extrapolation, a stringent testing of the mechanisms is important. In addition to particle mass yield as a typical standard for model-measurement comparison, particle composition expressed as O:C and H:C elemental ratios can serve as a higher dimensional constraint. A paradigm for doing so is developed herein for SOM production from a C(5)-C(10)-C(15) terpene sequence, namely isoprene, α-pinene, and β-caryopyhllene. The model MCM-SIMPOL is introduced based on the Master Chemical Mechanism (MCM v3.2) and a group contribution method for vapor pressures (SIMPOL). The O:C and H:C ratios of the SOM are measured using an Aerosol Mass Spectrometer (AMS). Detailed SOM-specific AMS calibrations for the organic contribution to the H(2)O(+) and CO(+) ions indicate that published O:C and H:C ratios for SOM are systematically too low. Overall, the measurement-model gap was small for particle mass yield but significant for particle-average elemental composition. The implication is that a key chemical pathway is missing from the chemical mechanism. The data can be explained by the particle-phase homolytic decomposition of organic hydroperoxides and subsequent alkyl-radical-promoted oligomerization.
authors.affiliation: School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, USA.
authors.forename: Qi
authors.initials: Q
authors.lastname: Chen
authors.forename: Yingjun
authors.initials: Y
authors.lastname: Liu
authors.forename: Neil M
authors.initials: NM
authors.lastname: Donahue
authors.forename: John E
authors.initials: JE
authors.lastname: Shilling
authors.forename: Scot T
authors.initials: ST
authors.lastname: Martin
mesh_terms: D000336:Aerosols; D013058:Mass Spectrometry; D008956:Models, Chemical; D052638:Particulate Matter / Q000737:chemistry*; D010545:Peroxides / Q000737:chemistry; D055549:Volatile Organic Compounds / Q000737:chemistry*
chemical_list: D000336:Aerosols; D052638:Particulate Matter; D010545:Peroxides; D055549:Volatile Organic Compounds