Modeling kinetic partitioning of secondary organic aerosol and size distribution dynamics: representing effects of volatility, phase state, and particle-phase reaction

Abstract

Abstract. This paper describes and evaluates a new framework for modeling kinetic gas-particle partitioning of secondary organic aerosol (SOA) that takes into account diffusion and chemical reaction within the particle phase. The framework uses a combination of (a) an analytical quasisteady-state treatment for the diffusion-reaction process within the particle phase for fast-reacting organic solutes, and (b) a two-film theory approach for slow-and nonreacting solutes. The framework is amenable for use in regional and global atmospheric models, although it currently awaits specification of the various gas-and particle-phase chemistries and the related physicochemical properties that are important for SOA formation. Here, the new framework is implemented in the computationally efficient Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) to investigate the competitive growth dynamics of the Aitken and accumulation mode particles. Results show that the timescale of SOA partitioning and the associated size distribution dynamics depend on the complex interplay between organic solute volatility, particle-phase bulk diffusivity, and particlephase reactivity (as exemplified by a pseudo-first-order reaction rate constant), each of which can vary over several orders of magnitude. In general, the timescale of SOA partitioning increases with increase in volatility and decrease in bulk diffusivity and rate constant. At the same time, the shape of the aerosol size distribution displays appreciable narrowing with decrease in volatility and bulk diffusivity and increase in rate constant. A proper representation of these physicochemical processes and parameters is needed in the next generation models to reliably predict not only the total SOA mass, but also its composition-and number-diameter distributions, all of which together determine the overall optical and cloudnucleating properties.

Diffusivity measurements of volatile organics in levitated viscous aerosol particles

“…The model relies on simplifying assumptions for both (ii) and (iii): PAH evaporation from the aerosols is described by a 1/e equilibration time τ = 4.3 days, and the combined effect of PAH evaporation and oxidation in the condensed phase is described by τ = 1.4 days. These τ values, based on Zelenyuk et al (2012), are in the S. Bastelberger et al: Diffusivity measurements of volatile organics 8463 same ballpark as τ values derived from our measurements for room temperature, and the best agreement with (iii) found by Friedman et al (2014) is probably due to the strong correlation between the emission of primary organic matter and black carbon, and the emission of PAHs. However, dependences of these simple process parameterizations on particle radius r or on condensed-phase diffusivities D c (x w , T ), which is a strong function of T , are neglected.…”

“…Further, they could disable or reduce the aerosol-induced transport of certain species, which cannot be taken up by particles after becoming highly viscous, or, conversely, enable or enhance the aerosol-induced transport of species, which were locked in before becoming highly viscous. Zelenyuk et al (2012) and Shrivastava et al (2017) suggested that long-range transport of PAHs trapped in highly viscous SOA may contribute to unexpectedly high particle-bound PAH concentrations in remote regions. In addition, there have been suggestions that PAHs may be trapped irreversibly within the soot matrix (Fernández et al, 2002), but we do not further discuss this mechanism here.…”

“…Studies by Lignell et al (2014) and Hinks et al (2016) suggest that low diffusion rates can also influence photochemistry. Modeling studies demonstrate that the timescale of SOA partitioning and size distribution dynamics are, amongst other factors such as volatility, determined by bulk diffusivity (Zaveri et al, 2014). Kinetic gas-particle modeling for the photo-oxidation of dodecane under dry conditions by Shiraiwa et al (2013) revealed that experimental particle-size distributions were better represented when taking the finite condensed-phase diffusivity into account.…”

The rate of equilibration of viscous aerosol particles

“…A critical review of these models is intended to guide those with an interest in simulating particle evolution inside instruments, chamber experiments, and the ambient atmosphere. For non-equilibrium viscous particles, diffusivity (along with other properties such as volatility) determines the temporal evolution of particle composition and size-and number-distributions (Zaveri et al, 2014). These are key factors determining aerosol impact on climate and health, therefore the choice of diffusion model could have far-reaching consequences (Pöschl, 2005).…”

Constraining nucleation, condensation, and chemistry in oxidation flow reactors using size-distribution measurements and aerosol microphysical modelling

“…due to vapor-uptake limitations may be strong if coupled with particle-phase oligomerization reactions (Zaveri et al 2014). Zaveri et al (2017) found that in order to model the growth of bimodal aerosol populations formed from either isoprene or ɑ-pinene and isoprene oxidation products, the intraparticle bulk diffusivity of the accumulation mode had to be slower (an order of magnitude less) than that of the diffusivity of the Aitken mode.…”