Jay R. Odum

Formation of Organic Aerosols from the Oxidation of Biogenic Hydrocarbons

AbstractMeasurements of aerosol formation during thephotooxidation of α-pinene, β-pinene,d-3-carene, d-limonene, ocimene, linalool, terpinene-4-ol, andtrans-caryophyllene were conducted in anoutdoor smog chamber. Daylight experiments in thepresence of

Some observations on times to equilibrium for semivolatile polycyclic aromatic hydrocarbons

{\text{NO}}_x

Mechanistic and Kinetic Studies of the Photodegradation of Benz[a]anthracene in the Presence of Methoxyphenols.

and dark experiments withelevated ozone concentrations were performed. Theevolution of the aerosol was simulated by theapplication of a gas/particle absorption model inconnection with a chemical reaction mechanism. Thefractional aerosol yield is shown to be a function ofthe organic aerosol mass concentration andtemperature. Ozone and, for selected hydrocarbons, theNO3 reaction of the compounds were found torepresent efficient routes to the formation ofcondensable products. For initial hydrocarbon mixingratios of about 100 ppb, the fractional aerosol yieldsfrom daylight runs have been estimated to be ∼5%for open-chain hydrocarbons, such as ocimene andlinalool, 5–25% for monounsaturated cyclicmonoterpenes, such as α-pinene, d-3-carene, orterpinene-4-ol, and ∼40% for a cyclic monoterpenewith two double bonds like d-limonene. For the onlysesquiterpene investigated, trans-caryophyllene, afractional aerosol yield of close to 100% wasobserved. The majority of the compounds studied showedan even higher aerosol yield during dark experimentsin the presence of ozone.

Gas/Particle Partitioning and Secondary Organic Aerosol Yields

Abstract A dynamic model is developed for gas-particle absorptive partitioning of semi-volatile organic aerosols. The model is applied to simulate a pair of m-xylene/NOx outdoor smog chamber experiments. In the presence of an inorganic seed aerosol a threshold for aerosol formation is predicted. An examination of characteristic times suggests conditions where an assumption of instantaneous gas-particle equilibrium is justified. Semi-volatile products that are second-generation, rather than first-generation, products of a parent hydrocarbon cause a delay in aerosol formation due to the delayed rate at which the second-generation products are formed. The gas-particle accommodation coefficient is the principal transport parameter and is estimated to have a value between 1.0 and 0.1 for the m-xylene aerosol.

The Atmospheric Aerosol-Forming Potential of Whole Gasoline Vapor

The relative time scales at which semivolatile gas and particle PAH approach equilibrium under both moderate and cool temperatures were investigated. Combustion particles were added directly from a diesel car and a wood stove to a 190 m^3 outdoor Teflon film chamber. The rate of migration of a gas-phase semivolatile PAH to combustion particles at a warm outdoor temperature was explored by volatilizing solid deuterated pyrene (d10-py) in a hot injector (200 °C) into the rural background air in the chamber atmosphere. After 2 h, diesel exhaust was added to the chamber. Results show an initial rapid migration of d10-py from the gas to the particle phase in an attempt to re-establish equilibrium. The relative closeness to equilibrium was monitored by calculating the equilibrium constant Kp overtime as measured by PAH_(part)/(PAH_(gas) x TSP). As gas-phase PAH concentrations changed in the chamber, due to wall losses, particle off-gassing occurred so that Kp was reasonably constant over time. Under cool outdoor conditions (-1 to -4 °C), PAH loss from the particle phase could not keep up with gas-phase PAH wall losses, and the system departed from equilibrium. Kinetic simulations suggested that tens of hours would be required to reestablish 90% of equilibrium concentrations for compounds like phenanthrene and pyrene once they had departed from equilibrium in the particle phase by 34 and 18%, respectively.

Aromatics, Reformulated Gasoline, and Atmospheric Organic Aerosol Formation

A radial diffusion model was developed to describe the dynamic mass transfer of semivolatile organics into and out of combustion aerosols. The model combustion aerosol consists of a solid carbon core that is surrounded by a viscous, liquid-like, organic layer. Diffusion takes place only within the organic layer and is controlled by mass transfer at the particle surface. Modeling of semi volatiles requires the tuning of two separate parameters: a diffusion coefficient (D) and a surface mass transfer coefficient (α). Preliminary testing of the model on the uptake of deuterated pyrene by diesel exhaust aerosol at a temperature of 25 °C suggests that diffusion coefficients for PAH are of the order of 10^(-15) cm^2/s and that surface mass transfer coefficients for pyrene are of the order of 10^(-10) cm/s.

Gas/particle partitioning of semivolatile organic compounds to model inorganic, organic, and ambient smog aerosols

Kinetic studies were employed to assess an empirical rate law describing the rate of photodegradation of polycyclic aromatic hydrocarbons (PAH) in the presence of substituted methoxyphenols. A solution of benz[a]anthracene (BaA) and vanillin in toluene was chosen as the model system. Further experiments using structure activity relationships were applied to the model system to investigate the mechanism for BaA photodegradation. Data from these experiments suggest that the rate-determining step in the mechanism is hydrogen abstraction of the phenolic hydrogen from vanillin