Articles | Volume 17, issue 1
Atmos. Chem. Phys., 17, 159–174, 2017
Atmos. Chem. Phys., 17, 159–174, 2017

Research article 04 Jan 2017

Research article | 04 Jan 2017

Molecular composition and volatility of isoprene photochemical oxidation secondary organic aerosol under low- and high-NOx conditions

Emma L. D'Ambro1,2, Ben H. Lee1, Jiumeng Liu3, John E. Shilling3,4, Cassandra J. Gaston1,a, Felipe D. Lopez-Hilfiker1,b, Siegfried Schobesberger1, Rahul A. Zaveri3, Claudia Mohr1,c, Anna Lutz5, Zhenfa Zhang6, Avram Gold6, Jason D. Surratt6, Jean C. Rivera-Rios7, Frank N. Keutsch7, and Joel A. Thornton1,2 Emma L. D'Ambro et al.
  • 1Department of Atmospheric Sciences, University of Washington, Seattle, WA 98195, USA
  • 2Department of Chemistry, University of Washington, Seattle, WA 98195, USA
  • 3Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, WA 99352, USA
  • 4Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352, USA
  • 5Department of Chemistry, Atmospheric Science, University of Gothenburg, Gothenburg, Sweden
  • 6Department of Environmental Sciences and Engineering, Gillings School of Global and Public Health, University of North Carolina, Chapel Hill, NC 27599, USA
  • 7John A. Paulson School of Engineering and Applied Sciences and Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA, USA
  • anow at: Rosenstiel School of Marine & Atmospheric Science, University of Miami, FL 33149, USA
  • bnow at: Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, Zurich, Switzerland
  • cnow at: Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, Germany

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.

Short summary
We studied the formation and properties of secondary organic aerosol produced from isoprene. We find that a significant fraction (~50 %) of the mass is composed of low-volatility, highly oxidized compounds such as C5H12O6. A significant fraction of the remainder appears to be in the form of oligomeric material. Adding NOx maintained or decreased SOA yields while increasing the fraction of low-volatility material, possibly due to oligomers.
Final-revised paper