α-Pinene secondary organic aerosol at low temperature: chemical composition and implications for particle viscosity
Authors: Huang Wei , Saathoff Harald , Pajunoja Aki , Shen Xiaoli , Naumann Karl-Heinz , Wagner Robert , Virtanen Annele , Leisner Thomas , Mohr Claudia 
Published: February 2018

Abstract. Chemical composition, size distributions, and degree of
oligomerization of secondary organic aerosol (SOA) from α-pinene
(C10H16) ozonolysis were investigated for low-temperature
conditions (223 K). Two types of experiments were performed using two
simulation chambers at the Karlsruhe Institute of Technology: the Aerosol
Preparation and Characterization (APC) chamber, and the Aerosol Interaction
and Dynamics in the Atmosphere (AIDA) chamber. Experiment type 1 simulated
SOA formation at upper tropospheric conditions: SOA was generated in the AIDA
chamber directly at 223 K at 61 % relative humidity (RH; experiment
termed “cold humid”, CH) and for comparison at 6 % RH (experiment
termed “cold dry”, CD) conditions. Experiment type 2 simulated SOA
uplifting: SOA was formed in the APC chamber at room temperature (296 K) and
< 1 % RH (experiment termed “warm dry”, WD) or 21 % RH
(experiment termed “warm humid”, WH) conditions, and then partially
transferred to the AIDA chamber kept at 223 K, and 61 % RH (WDtoCH) or
30 % RH (WHtoCH), respectively. Precursor concentrations varied between
0.7 and 2.2 ppm α-pinene, and between 2.3 and 1.8 ppm ozone for type 1 and
type 2 experiments, respectively. Among other instrumentation, a chemical
ionization mass spectrometer (CIMS) coupled to a filter inlet for gases and
aerosols (FIGAERO), deploying I− as reagent ion, was used for SOA
chemical composition analysis. For type 1 experiments with lower α-pinene concentrations and cold SOA
formation temperature (223 K), smaller particles of 100–300 nm vacuum
aerodynamic diameter (dva) and higher mass fractions
(> 40 %) of adducts (molecules with more than 10 carbon
atoms) of α-pinene oxidation products were observed. For type 2
experiments with higher α-pinene concentrations and warm SOA formation
temperature (296 K), larger particles (∼ 500 nm dva) with
smaller mass fractions of adducts (< 35 %) were produced. We also observed differences (up to 20 ∘C) in maximum desorption
temperature (Tmax) of individual compounds desorbing from the
particles deposited on the FIGAERO Teflon filter for different experiments,
indicating that Tmax is not purely a function of a compound's
vapor pressure or volatility, but is also influenced by diffusion limitations
within the particles (particle viscosity), interactions between particles
deposited on the filter (particle matrix), and/or particle mass on the
filter. Highest Tmax were observed for SOA under dry conditions
and with higher adduct mass fraction; lowest Tmax were observed for SOA under
humid conditions and with lower adduct mass fraction. The observations
indicate that particle viscosity may be influenced by intra- and
inter-molecular hydrogen bonding between oligomers, and particle water
uptake, even under such low-temperature conditions. Our results suggest that particle physicochemical properties such as
viscosity and oligomer content mutually influence each other, and that
variation in Tmax of particle desorptions may have
implications for particle viscosity and particle matrix effects. The
differences in particle physicochemical properties observed between our
different experiments demonstrate the importance of taking experimental
conditions into consideration when interpreting data from laboratory studies
or using them as input in climate models.