AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
Abstract View
Quantifying Oligomerization in Organic Aerosol through Desorption Thermogram Modeling
SIEGFRIED SCHOBESBERGER, Felipe Lopez-Hilfiker, Emma L. D'Ambro, Olli-Pekka Tikkanen, Joel A. Thornton, University of Eastern Finland
Abstract Number: 260 Working Group: Aerosol Chemistry
Abstract Recent studies have indicated a major role of oligomer formation and decomposition in controlling the volatility of secondary organic aerosol (SOA). State-of-the-art mass spectrometric techniques now allow us to measure aerosol molecular composition at a sufficiently high time resolution to access such molecular-scale mechanisms. Using the Filter Inlet for Gas and AEROsols (FIGAERO), coupled to a time-of-flight chemical ionization mass spectrometer, composition-resolved thermograms can be measured for a major fraction of an SOA sample through temperature-controlled evaporation.
In our study, we developed a model to simulate these thermograms and applied it to chamber experiments of SOA formation from monoterpenes. The model describes desorption of aerosol constituents using a modified Hertz-Knudsen equation. Additional terms, with rates modeled after Arrhenius’ equation, describe the formation and dissociation of oligomers. Interactions of desorbed molecules with inner instrument surfaces are accounted for as well, and a series of special experiments provided additional constraints.
Our model produces thermogram peaks similar to those observed in FIGAERO experiments. Disregarding oligomerization at first, peak position is sensitive to particle-phase diffusivity and the compound’s vapor pressure, whereas the enthalpy of vaporization also affects peak width. When sampling SOA however, a substantial fraction of the signal for most compounds desorbed at higher temperatures than their elemental composition suggests, supporting the hypothesis of the pivotal role of oligomer formation and decomposition in determining SOA properties. The corresponding thermograms usually exhibited an initial peak, followed by a plateau or second peak towards higher temperatures. We are able to simulate these peak shapes, when we include the oligomer formation and dissociation terms, and use the controlling rate constants and activation energies as free parameters. By using a state-of-the-art optimization algorithm, we fit all free parameters to the observed thermograms, thus gaining quantitative insights into the dynamics of the reversible oligomerization reactions occurring in SOA.