A new aerosol flow reactor to study secondary organic aerosol
Authors: Pereira Kelly L. , Rovelli Grazia , Song Young C. , Mayhew Alfred W. , Reid Jonathan P. , Hamilton Jacqueline F. 
Published: August 2019

Abstract. Gas-particle equilibrium partitioning is a fundamental concept used to
describe the growth and loss of secondary organic aerosol (SOA). However,
recent literature has suggested that gas-particle partitioning may be
kinetically limited, preventing volatilization from the aerosol phase as a
result of the physical state of the aerosol (e.g. glassy, viscous). Experimental
measurements of diffusion constants within viscous aerosol are limited and
do not represent the complex chemical composition observed in SOA (i.e.
multicomponent mixtures). Motivated by the need to address fundamental
questions regarding the effect of the physical state and chemical
composition of a particle on gas-particle partitioning, we present the
design and operation of a newly built 0.3 m3 continuous-flow reactor
(CFR), which can be used as a tool to gain considerable insights into the
composition and physical state of SOA. The CFR was used to generate SOA from
the photo-oxidation of α-pinene, limonene, β-caryophyllene
and toluene under different experimental conditions (i.e. relative humidity, VOC
and VOC∕NOx ratios). Up to 102 mg of SOA mass was collected per
experiment, allowing the use of highly accurate compositional- and single-particle analysis techniques, which are not usually accessible due to the
large quantity of organic aerosol mass required for analysis. A suite of
offline analytical techniques was used to determine the chemical composition
and physical state of the generated SOA, including attenuated total
reflectance infrared spectroscopy; carbon,
hydrogen, nitrogen, and sulfur (CHNS) elemental analysis; 1H and
1H-13C nuclear magnetic resonance spectroscopy (NMR);
ultra-performance liquid chromatography ultra-high-resolution mass
spectrometry (UHRMS); high-performance liquid chromatography ion-trap mass
spectrometry (HPLC-ITMS); and an electrodynamic balance (EDB). The
oxygen-to-carbon (O∕C) and hydrogen-to-carbon (H∕C) ratios of generated SOA
samples (determined using a CHNS elemental analyser) displayed good
agreement with literature values and were consistent with the characteristic
Van Krevelen diagram trajectory, with an observed slope of −0.41. The
elemental composition of two SOA samples formed in separate replicate
experiments displayed excellent reproducibility, with the O∕C and H∕C ratios
of the SOA samples observed to be within error of the analytical
instrumentation (instrument accuracy ±0.15 % to a reference
standard). The ability to use a highly accurate CHNS elemental analyser to
determine the elemental composition of the SOA samples allowed us to
evaluate the accuracy of reported SOA elemental compositions using UHRMS (a
commonly used technique). In all of the experiments investigated, the SOA
O∕C ratios obtained for each SOA sample using UHRMS were lower than the O∕C
ratios obtained from the CHNS analyser (the more accurate and non-selective
technique). The average difference in the ΔO∕C ratios ranged from 19 %
to 45 % depending on the SOA precursor and formation conditions. α-pinene SOA standards were generated from the collected SOA mass using
semi-preparative HPLC-ITMS coupled to an automated fraction collector,
followed by 1H NMR spectroscopy. Up to 35.8±1.6 %
(propagated error of the uncertainty in the slope of the calibrations
graphs) of α-pinene SOA was quantified using this method; a
considerable improvement from most previous studies. Single aerosol droplets
were generated from the collected SOA samples and trapped within an EDB at
different temperatures and relative humidities to investigate the dynamic
changes in their physiochemical properties. The volatilization of organic
components from toluene and β-caryophyllene SOA particles at 0 %
relative humidity was found to be kinetically limited, owing to particle
viscosity. The unconventional use of a newly built CFR, combined with
comprehensive offline chemical characterization and single-particle
measurements, offers a unique approach to further our understanding of the
relationship between SOA formation conditions, chemical composition and
physiochemical properties.