Timescales of secondary organic aerosols to reach equilibrium at various temperatures and relative humidities

Abstract

Secondary organic aerosols (SOA) account for a substantial fraction of air particulate matter, and SOA formation is often modeled assuming rapid establishment of gas-particle equilibrium. Here, we estimate the characteristic timescale for SOA to achieve gas-particle equilibrium under a wide range of temperatures and relative humidities using a state-of-the-art kinetic flux model. Equilibration timescales were calculated by varying particle phase state, size, mass loadings, and volatility of organic compounds in open and closed systems. Model simulations suggest that the equilibration timescale for semi-volatile compounds is on the order of seconds or minutes for most conditions in the planetary boundary layer, but it can be longer than 1 h if particles adopt glassy or amorphous solid states with high glass transition temperatures at low relative humidity. In the free troposphere with lower temperatures, it can be longer than hours or days, even at moderate or relatively high relative humidities due to kinetic limitations of bulk diffusion in highly viscous particles. The timescale of partitioning of low-volatile compounds into highly viscous particles is shorter compared to semi-volatile compounds in the closed system, as it is largely determined by condensation sink due to very slow re-evaporation with relatively quick establishment of local equilibrium between the gas phase and the near-surface bulk. The dependence of equilibration timescales on both volatility and bulk diffusivity provides critical insights into thermodynamic or kinetic treatments of SOA partitioning for accurate predictions of gas-and particle-phase concentrations of semi-volatile compounds in regional and global chemical transport models.

Current State of Atmospheric Aerosol Thermodynamics and Mass Transfer Modeling: A Review

“…Although the Shiraiwa and Seinfeld [272] and Li and Shiraiwa [311] model results indicate equilibrium mass transfer even for semi-solid SOA in the case of SVOC, the picture is incomplete. Zhou et al [312] find that the heterogeneous reaction of benzoapyrene (BaP) with ozone in SOA coated ammonium sulfate particles is significantly affected by the viscosity of the SOA shell, which depends on RH.…”

“…Li and Shiraiwa [311] evaluate the gas-particle mass equilibrium timescales for SOA under a large range of temperature and RH conditions found in the troposphere using the KM-GAP model. They find that for semi-volatile compounds the equilibrium timescale is on the order of seconds or minutes under conditions found in the planetary boundary layer (PBL), excluding low temperature environments.…”

“…Although Shiraiwa and Seinfeld [272] did not detail the distribution of SVOC in the semi-solid condensed phase, it is possible that most of the SVOC accumulated near the surface and established a liquid phase which was subject to the Pankow [182] equilibrium mass-transfer regime. Li and Shiraiwa [311] do discuss this transient near surface bulk effect due to kinetic limitations and concentration gradients. They point to evaporation experiments [313] that indicate formation of a local thermodynamic equilibrium with the near-surface bulk that is distinct from full equilibrium with the entire bulk.…”

Mass Accommodation and Gas-Particle Partitioning in Secondary Organic Aerosols: Dependence on Diffusivity, Volatility, Particle-phase Reactions, and Penetration Depth

“…Thus, gas-phase oxidation is described kinetically, while gas-particle partitioning is often approximated by quasi-instantaneous equilibrium partitioning of the oxidation products (Pankow, 1994;Shrivastava et al, 2017a;Tsigaridis et al, 2014). The assumption of quasi-instantaneous gas-particle equilibration, however, is in question if particles are highly viscous, semi-solid or glassy -especially at low temperatures and low relative humidity (RH) (Li and Shiraiwa, 2019;Shiraiwa and Seinfeld, 2012). Experimental studies found kinetic limitations for gas uptake and particle evaporation at low RH (Liu et al, 2016;Perraud et al, 2012;Vaden et al, 2011;Yli-Juuti et al, 2017), but not for mixing in SOA at medium or high RH (Ye et al, 2016;Ye et al, 2018).…”

Non-equilibrium interplay between gas-particle partitioning and multiphase chemical reactions of semi-volatile compounds: mechanistic insights and practical implications for atmospheric modeling of PAHs

“…By solving analytical transport equations, the equilibration timescales of partitioning for volatile inorganic compounds were found to depend on the size of particles (Meng and Seinfeld, 1996). More recently, there have been numerical simulations for secondary organic aerosol (SOA) as a function of temperature and relative humidity (Shiraiwa and Seinfeld, 2012;Li and Shiraiwa, 2019). Alternatively, equilibration timescales may also be obtained experimentally.…”