A model framework to retrieve thermodynamic and kinetic properties of organic aerosol from composition-resolved thermal desorption measurements
<p>Chemical ionization mass spectrometer (CIMS) techniques have been developed that allow for quantitative and composition-resolved measurements of organic compounds as they desorb from secondary organic aerosol (SOA) particles, in particular during their heat-induced evaporation. One such...
| Main Authors: | , , , , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-10-01
|
| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/18/14757/2018/acp-18-14757-2018.pdf |
| Summary: | <p>Chemical ionization mass spectrometer (CIMS) techniques have been developed
that allow for quantitative and composition-resolved measurements of organic
compounds as they desorb from secondary organic aerosol (SOA) particles, in
particular during their heat-induced evaporation. One such technique employs
the Filter Inlet for Gases and AEROsol (FIGAERO). Here, we present a newly
developed model framework with the main aim of reproducing FIGAERO-CIMS
thermograms: signal vs. ramped desorption temperature. The model simulates
the desorption of organic compounds during controlled heating of
filter-sampled SOA particles, plus the subsequent transport of these
compounds through the FIGAERO manifold into an iodide-CIMS. Desorption is
described by a modified Hertz–Knudsen equation and controlled chiefly by the
temperature-dependent saturation concentration <i>C</i>*, mass accommodation
(evaporation) coefficient, and particle surface area. Subsequent transport is
governed by interactions with filter and manifold surfaces. Reversible
accretion reactions (oligomer formation and decomposition) and thermal
decomposition are formally described following the Arrhenius relation. We use
calibration experiments to tune instrument-specific parameters and then apply
the model to a test case: measurements of SOA generated from dark ozonolysis
of <i>α</i>-pinene. We then discuss the ability of the model to describe
thermograms from simple calibration experiments and from complex SOA, and the
associated implications for the chemical and physical properties of the SOA.
For major individual compositions observed in our SOA test case (<i>#</i><i>C</i> = 8
to 10), the thermogram peaks can typically be described by assigning
<i>C</i>*<sub>25 °<i>C</i></sub> values in the range 0.05 to
5 µg m<sup>−3</sup>, leaving the larger, high-temperature fractions
(<i>></i>50 %) of the thermograms to be described by thermal
decomposition, with dissociation rates on the order of ∼ 1 h<sup>−1</sup> at
25 °C. We conclude with specific experimental designs to better
constrain instrumental model parameters and to aid in resolving remaining
ambiguities in the interpretation of more complex SOA thermogram behaviors.
The model allows retrieval of quantitative volatility and mass transport
information from FIGAERO thermograms, and for examining the effects of
various environmental or chemical conditions on such properties.</p> |
|---|---|
| ISSN: | 1680-7316 1680-7324 |