Particle Size Distribution Dynamics Can Help Constrain the Phase State of Secondary Organic Aerosol

Abstract

Particle phase state is a property of atmospheric aerosols that has important implications for the formation, evolution, and gas/particle partitioning of secondary organic aerosol (SOA). In this work, we use a size-resolved chemistry and microphysics model (Statistical Oxidation Model coupled to the TwO Moment Aerosol Sectional (SOM-TOMAS)), updated to include an explicit treatment of particle phase state, to constrain the bulk diffusion coefficient (D b) of SOA produced from α-pinene ozonolysis. By leveraging data from laboratory experiments performed in the absence of a seed and under dry conditions, we find that the D b for SOA can be constrained ((1–7) × 10–15 cm2 s–1 in these experiments) by simultaneously reproducing the time-varying SOA mass concentrations and the evolution of the particle size distribution. Another version of our model that used the predicted SOA composition to calculate the glass-transition temperature, viscosity, and, ultimately, D b (∼10–15 cm2 s–1) of the SOA was able to reproduce the mass and size distribution measurements when we included oligomer formation (oligomers accounted for about a fifth of the SOA mass). Our work highlights the potential of a size-resolved SOA model to constrain the particle phase state of SOA using historical measurements of the evolution of the particle size distribution.

A computationally efficient model to represent the chemistry, thermodynamics, and microphysics of secondary organic aerosols (simpleSOM): model development and application to α-pinene SOA

“…By leveraging a more complete set of gas and aerosol measurements including those gathered through novel experimentation, we argue that these measurements -old and new -when integrated with the simpleSOM model have the potential to better constrain process-specific SOA parameters. For instance, several studies have been able to leverage the evolution of the aerosol size distribution with kinetic SOA models to constrain the oxidation chemistry, 110 volatility, 33,111 and phase state 67,100 of SOA. Analogous to the use of the aerosol size distribution, the detailed speciation of gas-and particle-phase organic compounds (e.g., D'Ambro et al, 28 Isaacman-VanWertz et al 12 ) made possible by advanced mass spectrometry techniques could be put to similar use.…”

“…When particle-phase reactions are considered, the equations take on a slightly modified form depending on the effective reaction rate and these equations have been covered in our previous work. 67…”

“…This would appear to be the case for -pinene SOA, which has recently been shown to exhibit a D b in the range of 10 -15 cm 2 s -1 . 67,100,103 Second, the atmospheric SOA mass yields are sensitive to the D b value of the absorbing OA, but this sensitivity is limited to the first few hours of photochemical aging if this D b value is higher than 10 -15 cm 2 s -1 , extending to a few days for D b values lower than 10 -15 cm 2 s -1 . The phase state of atmospheric OA is largely uncertain since there are only a handful of observations 38,104,105 and modeling studies 37,106 that have attempted to do so.…”

“…Symbols in panels (b) through (d) show the measured, end-ofexperiment SOA mass yield and the estimated VBS SOA mass yield at an OA mass concentration of 10 µg m -3 .4.4 Particle Phase StateThe phase state of SOA might be important to account for when modeling laboratory-based SOA experiments since these experiments are typically performed under low relative humidities (<20%) that may encourage the production of semisolid/viscous aerosol. Building on our recent work,67 we performed a case study to understand the impact of assuming the phase state in chamber experiments on SOA mass yields in the real atmosphere. Results from this case study are shown in Figure4for the same low NO X experiment shown in earlier figures.…”

“…The largest D b,chamber value (10 -6 cm 2 s -1 ) was chosen to represent a liquid aerosol that reflects the current approach to modeling gas/particle partitioning of SOA with no limitations to mass transfer. The intermediate D b,chamber value (3×10 -15 cm 2 s -1 ) was chosen based on our recent work where we showed that observations of SOA mass concentration and the aerosol size distribution in an -pinene chamber experiment could only be reproduced for a narrow range of D b values around the chosen value 67. And finally, the smallest D b,chamber value (10 -17 cm 2 s -1 ) was chosen to represent an even more viscous SOA that is consistent with direct measurements of viscosity for -pinene SOA 38,39.…”