Kinetic limitations in gas-particle reactions arising from slow diffusion in secondary organic aerosol

Abstract

The potential for aerosol physical properties, such as phase, morphology and viscosity/ diffusivity, to affect particle reactivity remains highly uncertain. We report here a study of the effect of bulk diffusivity of polycyclic aromatic hydrocarbons (PAHs) in secondary organic aerosol (SOA) on the kinetics of the heterogeneous reaction of particle-borne benzo[a]pyrene (BaP) with ozone. The experiments were performed by coating BaP-ammonium sulfate particles with multilayers of SOA formed from ozonolysis of alpha-pinene, and by subsequently investigating the kinetics of BaP loss via reaction with excess ozone using an aerosol flow tube coupled to an Aerodyne Aerosol Mass Spectrometer (AMS). All reactions exhibit pseudo-first order kinetics and are empirically well described by a Langmuir-Hinshelwood (L-H) mechanism. The results show that under dry conditions (RH < 5%) diffusion through the SOA coating can lead to significant mass transfer constraints on the kinetics, with behavior between that previously observed by our group for solid and liquid organic coats. The reactivity of BaP was enhanced at -50% relative humidity (RH) suggesting that water uptake lowers the viscosity of the SOA, hence lifting the mass transfer constraint to some degree. The kinetics for -70% RH were similar to results obtained without SOA coats, indicating that the SOA had sufficiently low viscosity and was sufficiently liquid-like that reactants could rapidly diffuse through the coat. A kinetic multi-layer model for aerosol surface and bulk chemistry was applied to simulate the kinetics, yielding estimates for the diffusion coefficients (in cm2 s(-1)) for BaP in alpha-pinene SOA of 2 x 10(-14), 8 x 10(-14) and > 1 x 10(-12) for dry (RH < 5%), 50% RH and 70% RH conditions, respectively. These results clearly indicate that slow diffusion of reactants through SOA coats under specific conditions can provide shielding from gas-phase oxidants, enabling the long-range atmospheric transport of toxic trace species, such as PAHs and persistent organic pollutants.

Global long-range transport and lung cancer risk from polycyclic aromatic hydrocarbons shielded by coatings of organic aerosol

“…Highly viscous SOA coatings limit the diffusion of PAH molecules from particle bulk to particle surface (24,25), effectively shielding PAHs from chemical degradation (24). In addition, OA is more viscous in cool/dry conditions (24,26), so the effectiveness of shielding by OA is likely higher at cool locations in the atmosphere compared with laboratory measurements, which are made at room temperature [i.e., around 296°K (18,19,25)]. Thus, temperature-and RH-dependent changes in the viscosity of OA coatings can strongly affect heterogeneous oxidation kinetics of BaP.…”

“…Therefore, heterogeneous chemical degradation of particle-bound BaP is an important loss mechanism (13)(14)(15)(16)(17)(18)(19). Laboratory measurements show that BaP adsorbed on the surface of elemental carbon, solid organic carbon, or ammonium sulfate particles reacts quickly with ozone, and its oxidation lifetime varies from several minutes to a few hours (15,18,19). However, field measurements demonstrate that BaP persists longer in the atmosphere and is therefore transported far from its sources (20).…”

“…We propose three major amendments to the currently inadequate conceptual framework for describing BaP evolution, by (i) including laboratory-observed heterogeneous oxidation of particle-bound BaP coated with OA, (ii) representing slowing, or complete shutoff of BaP oxidation in cool and/or dry conditions due to shielding by OA coatings, and (iii) including the heterogeneous oxidation products of BaP, which are assumed to remain particle-bound rather than being lost, so that they can be transported in the atmosphere and removed by deposition. Although previous measurements mostly focused on uncoated particle-surface-adsorbed BaP directly exposed to ozone, it has been recently shown that coatings of highly viscous secondary organic aerosol (SOA) material can significantly slow the oxidation of particlebound BaP adsorbed on ammonium sulfate aerosols (19). Coatings of solid organic eicosane have been shown to stop BaP oxidation Significance Polycyclic aromatic hydrocarbons (PAHs) adversely impact human health and ecosystems and are known to persist in the atmosphere.…”

“…Although gasphase BaP is rapidly degraded by oxidants such as OH radicals (10)(11)(12), most of the gaseous BaP rapidly partitions to atmospheric particulate matter due to its low volatility (12). Therefore, heterogeneous chemical degradation of particle-bound BaP is an important loss mechanism (13)(14)(15)(16)(17)(18)(19). Laboratory measurements show that BaP adsorbed on the surface of elemental carbon, solid organic carbon, or ammonium sulfate particles reacts quickly with ozone, and its oxidation lifetime varies from several minutes to a few hours (15,18,19).…”

Oxidation of a model alkane aerosol by OH radical: the emergent nature of reactive uptake

“…Comprehensive microscopic models that couple reaction and transport enable an internal view of the transformation of a particle with time as oxidation progresses. [24][25][26][27][28][29] This more detailed treatment has significantly broadened thinking about the complexity of aerosol transformations, but many of the required rate constants have not been measured. Additionally, diffusion is treated with a constant average rate initially determined from Fickian kinetics but not dynamically varied as local concentration gradients change throughout the oxidation process, leading to variances from the actual rates, and the oxidation reactions are simplified to include only general steps rather than the detailed free radical chains that are proposed to occur.…”