Viscosity of α -pinene secondary organic material and implications for particle growth and reactivity

Abstract

Particles composed of secondary organic material (SOM) are abundant in the lower troposphere. The viscosity of these particles is a fundamental property that is presently poorly quantified yet required for accurate modeling of their formation, growth, evaporation, and environmental impacts. Using two unique techniques, namely a "bead-mobility" technique and a "poke-flow" technique, in conjunction with simulations of fluid flow, the viscosity of the water-soluble component of SOM produced by α-pinene ozonolysis is quantified for 20-to 50-μm particles at 293-295 K. The viscosity is comparable to that of honey at 90% relative humidity (RH), similar to that of peanut butter at 70% RH, and at least as viscous as bitumen at ≤30% RH, implying that the studied SOM ranges from liquid to semisolid or solid across the range of atmospheric RH. These data combined with simple calculations or previous modeling studies are used to show the following: (i) the growth of SOM by the exchange of organic molecules between gas and particle may be confined to the surface region of the particles for RH ≤ 30%; (ii) at ≤30% RH, the particle-mass concentrations of semivolatile and low-volatility organic compounds may be overpredicted by an order of magnitude if instantaneous equilibrium partitioning is assumed in the bulk of SOM particles; and (iii) the diffusivity of semireactive atmospheric oxidants such as ozone may decrease by two to five orders of magnitude for a drop in RH from 90% to 30%. These findings have possible consequences for predictions of air quality, visibility, and climate.aerosol | physical properties | secondary organic aerosol B iological sources (e.g., vegetation) and anthropogenic sources (e.g., transportation) emit copious quantities of volatile organic compounds, such as α-pinene and aromatic hydrocarbons, among others (1, 2). In the atmosphere, a complex series of chemical reactions oxidizes these volatile compounds to form semivolatile organic compounds (SVOCs) that condense to the particle phase (1, 2). This secondary organic material (SOM) constituting the particles is estimated to contribute typically 30-70% to the mass concentration of suspended submicron particles in most regions of the atmosphere (1). These particles can influence climate by scattering and absorbing solar radiation (direct climate effect) and by serving as nuclei for cloud formation (indirect climate effect), among other mechanisms (3). They can also influence air quality and health (4-6).Recently, molecular diffusion within SOM particles has become an area of intense scientific interest. Diffusion rates within particles can influence the mechanism and rates of growth of SOM particles (Fig. 1A) (7,8) and influence reactions of oxidants within the SOM particles (Fig. 1B) (9). As a result, quantitative modeling of the environmental impacts of SOM particles can depend on molecular diffusion within the particles (see, for example, Fig. 1C). Shiraiwa and Seinfeld (10) have shown that predictions of the mass concentration of SOM particles, a ke...

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

“…At cool and/or dry conditions, however, we assume that highly viscous OA effectively shields BaP from heterogeneous oxidation, similar to that observed for eicosane (a highly viscous solid organic) coatings (18) (SI Appendix, Shielding of BaP by OA in the New Modeling Formulation). This approach is consistent with highly viscous SOA observed at dry conditions (27,28), persistence of highly viscous SOA at cooler temperatures in the free troposphere (26), and also higher viscosity of complex SOA mixtures (29) that are likely more atmospherically relevant compared with laboratory-generated SOA (SI Appendix, Shielding of BaP by OA in the New Modeling Formulation). We show that including the shielding of BaP by OA results in better agreement with BaP measurements in the field, promotes stronger long-range transport, and also produces greater global incremental lung cancer risk (ILCR) estimates due to exposure to PAHs, compared with the default model that neglects this shielding by OA.…”

Gas–particle partitioning of atmospheric aerosols: interplay of physical state, non-ideal mixing and morphology

“…(b) Measured viscosity (left axis) of extracted a-pinene SOA. 36 Bulk diffusion coefficient (right axis) is obtained through the Stokes-Einstein equation. (c) Evolution of total mass concentration of initially 300 nm diameter particles at different RH under net growth conditions.…”

Role of the reaction of stabilized Criegee intermediates with peroxy radicals in particle formation and growth in air

“…81,82 The size-dependent data support the hypothesis that SOA particles from ozonolysis of trans-3-hexene are initially formed by nucleation of higher molecular weight oligomers (n Z 4) and then grow via condensation of shorter oligomers and other less oxygenated, more volatile species. 12,30,31,38,39 Phase state of SOA The phase of atmospheric particles (liquid/solid) plays a crucial role in various physical and chemical processes of particles such as growth, 49,50,52 aging, 47,48,52,53 and water uptake. 48,51 In the present study, particle impaction/ATR-FTIR measurements were performed to offer some insight into the phase/viscosity of trans-3-hexene SOA since the impaction patterns on the Ge crystal have been shown to be an indicator of phase.…”

“…We report here an expanded investigation of the ozonolysis of trans-3-hexene in which particle composition is probed as a function of size. In addition, the influence of water vapor, OH and SCI scavengers, as well as the phase state of SOA particles (which plays critical roles in various physical and chemical processes of aerosols) [47][48][49][50][51][52][53][54][55][56] are elucidated and possible mechanisms are examined in light of predictions of a box model for this system. Initial studies of the ozonolysis of the much larger and complex biogenic alkene a-cedrene are also reported, and it is shown that the mechanism of particle formation for this sesquiterpene is different from that for trans-3-hexene.…”