Gas-to-particle partitioning of major biogenic oxidation products: a study on freshly formed and aged biogenic SOA
Authors: Gkatzelis Georgios I. , Hohaus Thorsten , Tillmann Ralf , Gensch Iulia , Müller Markus , Eichler Philipp , Xu Kang-Ming , Schlag Patrick , Schmitt Sebastian H. , Yu Zhujun , Wegener Robert , Kaminski Martin , Holzinger Rupert , Wisthaler Armin , Kiendler-Scharr Astrid 
Published: September 2018

Abstract. Secondary organic aerosols (SOAs) play a key role in climate change and air
quality. Determining the fundamental parameters that distribute organic
compounds between the phases is essential, as atmospheric lifetime and
impacts change drastically between the gas and particle phase. In this work,
gas-to-particle partitioning of major biogenic oxidation products was
investigated using three different aerosol chemical characterization
techniques. The aerosol collection module, the collection thermal desorption unit, and the
chemical analysis of aerosols online are different aerosol sampling inlets connected to a proton-transfer reaction time-of-flight
mass spectrometer (ACM-PTR-ToF-MS, TD-PTR-ToF-MS, and CHARON-PTR-ToF-MS, respectively, referred to hereafter as
ACM, TD, and CHARON). These techniques
were deployed at the atmosphere simulation chamber SAPHIR to perform
experiments on the SOA formation and aging from different monoterpenes
(β-pinene, limonene) and real plant emissions (Pinus sylvestris L.). The saturation mass
concentration C* and thus the volatility of the individual ions was
determined based on the simultaneous measurement of their signal in the gas and particle phase. A method to identify and exclude ions affected by thermal dissociation
during desorption and ionic dissociation in the ionization chamber of the
proton-transfer reaction mass spectrometer (PTR-MS) was developed and tested for each technique. Narrow volatility
distributions with organic compounds in the semi-volatile (SVOCs – semi-volatile
organic compounds) to
intermediate-volatility (IVOCs – intermediate-volatility organic compounds) regime were found for all systems studied.
Despite significant differences in the aerosol collection and desorption
methods of the proton-transfer-reaction (PTR)-based techniques, a comparison of the C* values obtained
with different techniques was found to be in good agreement (within 1 order
of magnitude) with deviations explained by the different operating
conditions of the PTR-MS. The C* of the identified organic compounds were mapped onto the
two-dimensional volatility basis set (2D-VBS), and results showed a decrease in C* with increasing oxidation state. For all experiments conducted in
this study, identified partitioning organic compounds accounted for
20–30 % of the total organic mass measured from an aerosol mass spectrometer (AMS). Further
comparison between observations and theoretical calculations was performed
for species found in our experiments that were also identified in previous
publications. Theoretical calculations based on the molecular structure of
the compounds showed, within the uncertainties ranges, good agreement with
the experimental C* for most SVOCs, while IVOCs deviated by up to a factor of
300. These latter differences are discussed in relation to two main
processes affecting these systems: (i) possible interferences by thermal and
ionic fragmentation of higher molecular-weight compounds, produced by
accretion and oligomerization reactions, that fragment in the m∕z range
detected by the PTR-MS and (ii) kinetic influences in the distribution
between the gas and particle phase with gas-phase condensation, diffusion in
the particle phase, and irreversible uptake.