Gas uptake and chemical aging of semisolid organic aerosol particles

Abstract

Organic substances can adopt an amorphous solid or semisolid state, influencing the rate of heterogeneous reactions and multiphase processes in atmospheric aerosols. Here we demonstrate how molecular diffusion in the condensed phase affects the gas uptake and chemical transformation of semisolid organic particles. Flow tube experiments show that the ozone uptake and oxidative aging of amorphous protein is kinetically limited by bulk diffusion. The reactive gas uptake exhibits a pronounced increase with relative humidity, which can be explained by a decrease of viscosity and increase of diffusivity due to hygroscopic water uptake transforming the amorphous organic matrix from a glassy to a semisolid state (moisture-induced phase transition). The reaction rate depends on the condensed phase diffusion coefficients of both the oxidant and the organic reactant molecules, which can be described by a kinetic multilayer flux model but not by the traditional resistor model approach of multiphase chemistry. The chemical lifetime of reactive compounds in atmospheric particles can increase from seconds to days as the rate of diffusion in semisolid phases can decrease by multiple orders of magnitude in response to low temperature or low relative humidity. The findings demonstrate that the occurrence and properties of amorphous semisolid phases challenge traditional views and require advanced formalisms for the description of organic particle formation and transformation in atmospheric models of aerosol effects on air quality, public health, and climate.

Fig. 1.

Characteristic time of bulk diffusion (τ_cd) in liquid, semisolid, and solid particles as a function of diffusion coefficient and particle diameter. In the size range of the atmospheric aerosol accumulation mode (d_p ≈ 10² nm), τ_cd in semisolid particles varies from seconds to years (light green arrow).

Fig. 2.

(A) Viscosity (ν) and self-diffusion coefficient (D_org) of the protein BSA estimated as a function of relative humidity (RH) using the Stokes-Einstein relation with published viscosity data and hygroscopic growth factor data for BSA. As RH increases, the protein phase changes from solid over semisolid to viscous liquid, while D_org increases from ∼10^-21 cm² s^-1 to ∼10^-10 cm² s^-1. (B) Diffusion coefficient of ozone (D_ox) in BSA estimated as a function of RH by theoretical calculations using hygroscopic growth factor data. D_ox is ∼5 × 10^-10 cm² s^-1 in solid BSA at RH < 20%, but it increases up to ∼10^-6 cm² s^-1 in a viscous liquid aqueous phase at RH > 95%. The shaded areas represent uncertainties of estimation. The red stars show the D_org and D_ox values derived from the ozone uptake experiments, and the error bars indicate corresponding uncertainties.

Fig. 3.

Ozone uptake coefficients (γ_O3) on protein films (BSA, 246 nm) observed (A) at 50% RH and different gas-phase O₃ concentrations (volume mixing ratios) and (B) at ∼140 ppb O₃ and different relative humidities. The decrease of γ_O3 over time exhibits a double-logarithmic slope close to -0.5, which is characteristic for bulk diffusion-limited gas uptake. The time, RH, and O₃ concentration dependence of γ_O3 can be reproduced by the kinetic multilayer model KM-SUB (dotted lines).

Fig. 4.

Kinetic model results for the ozone uptake by semisolid protein (BSA) at 42 ppb O₃ and 50% RH. (A) Ozone uptake coefficients (γ_O3) as observed (data points) and simulated with a multilayer model (KM-SUB, solid line) and with a resistor-based double-layer model (K2-SUB, dashed line). (B) Radial profile of ozone bulk concentration ([Ox]) calculated with KM-SUB; r/r_p is the distance from the particle center normalized by the particle radius (r/r_p = 1 at the surface).

Fig. 5.

Chemical half-life (t_1/2) of reactive amino acids in a protein particle of 200 nm diameter calculated as a function of gas-phase ozone concentration (volume mixing ratio, 1 atm). (A) t_1/2 at 25 °C for different relative humidities between 20% and 90%. (B) t_1/2 at 50% RH for different temperatures between 25°C and -60 °C. The shaded area shows the range of t_1/2 at -60 °C estimated for supercooled water vs. ice.

Fig. 6.

Atmospheric processing of amorphous organic aerosol particles from primary emissions or secondary formation in the atmosphere. The phase state can fluctuate between glassy solid, semisolid and liquid depending on ambient relative humidity and temperature (physical transformation). Depending on phase state and diffusivity, the chemical transformation and aging upon interaction with atmospheric oxidants and other trace gases (small red circles) proceeds differently: relatively slow via surface adsorption and reaction on solid particles or relatively fast via bulk absorption and reaction in liquid particles. Upon cloud formation, solid aerosol particles are more likely to be activated as ice nuclei (IN) forming ice crystals, whereas liquid aerosol particles are more likely to be activated as cloud condensation nuclei (CCN) forming water droplets.