Combining Mass Spectrometry and ATR-FTIR Spectroscopy to Study Phase, Diffusion and Composition of Secondary Organic Aerosol from the Ozonolysis of α-pinene

Abstract:

Secondary organic aerosol (SOA) is ubiquitous in the atmosphere and composes a large fraction of the total aerosol budget. Recent reports from field measurements and laboratory studies show that some SOA particles are better represented by a semi-solid low viscosity tar-like material rather than a ideal liquid often assumed in regional and global models. Characterizing the phase of SOA is crucial to understanding how particles interact with trace gases and how it ultimately impacts their growth and evolution in the atmosphere. We report here laboratory studies carried out in the unique UCI large-volume, slow-flow, aerosol flow reactor. Particles from the ozonolysis of a-pinene were formed at various relative humidities (RH from < 3% to 90%) and collected onto a custom ATR-FTIR impactor. The observed impaction pattern provided insights into changes in phase/viscosity of the SOA as a function of relative humidity. In addition, attenuated total reflectance FTIR and mass spectrometry measurements provided information on simultaneous changes on composition. Application of ATR-FTIR combined with PTR-MS provided additional data on the volatility of the SOA at room temperature and diffusion coefficients of two key components pinonaldehyde and acetic acid present in the SOA. Implication for modeling the growth and ultimately the lifetime of SOA in the atmosphere will be discussed.