title: 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.
authors.affiliation: Department of Chemistry, University of British Columbia, Vancouver, BC, Canada V6T 1Z1.
authors.forename: Lindsay
authors.initials: L
authors.lastname: Renbaum-Wolff
authors.forename: James W
authors.initials: JW
authors.lastname: Grayson
authors.forename: Adam P
authors.initials: AP
authors.lastname: Bateman
authors.forename: Mikinori
authors.initials: M
authors.lastname: Kuwata
authors.forename: Mathieu
authors.initials: M
authors.lastname: Sellier
authors.forename: Benjamin J
authors.initials: BJ
authors.lastname: Murray
authors.forename: John E
authors.initials: JE
authors.lastname: Shilling
authors.forename: Scot T
authors.initials: ST
authors.lastname: Martin
authors.forename: Allan K
authors.initials: AK
authors.lastname: Bertram
mesh_terms: D000336:Aerosols; D000393:Air Pollutants; D001272:Atmosphere; D000081008:Bicyclic Monoterpenes; D002980:Climate; D004784:Environmental Monitoring / Q000379:methods; D005740:Gases; D039821:Monoterpenes / Q000737:chemistry*; D009584:Nitrogen / Q000737:chemistry; D010100:Oxygen / Q000737:chemistry; D010126:Ozone / Q000737:chemistry; D010316:Particle Size; D012995:Solubility; D013696:Temperature; D014783:Viscosity; D055549:Volatile Organic Compounds; D014835:Volatilization; D014867:Water / Q000737:chemistry
chemical_list: D000336:Aerosols; D000393:Air Pollutants; D000081008:Bicyclic Monoterpenes; D005740:Gases; D039821:Monoterpenes; D055549:Volatile Organic Compounds; D014867:Water; D010126:Ozone; C005451:alpha-pinene; D009584:Nitrogen; D010100:Oxygen
keywords: aerosol; physical properties; secondary organic aerosol