Kinetics of the reactive uptake of ozone on oleic acid aerosols

Abstract

Heterogeneous reactions involving particles and gas-phase species can alter important chemical and microphysical properties of aerosols, complicating modeling efforts to assess their effects on climate and human health. Organic aerosols are common in the troposphere; precursors include vegetation, the ocean surface, and various combustion processes. Ozone is an important oxidant in the troposphere (Finlayson-Pitts and Pitts, 1997), often in concentrations sufficient to cause adverse effects on human health and vegetation. Oleic acid is one of a group of organic species proposed as an important tracer species for use in source characterization of ambient aerosols (Rogge et al., 1991). However, the relative fraction of these species may change as the particle ages, and new product species are likely to be introduced. Little is known about the kinetics of organic species in atmospheric aerosols. Such knowledge is necessary for quantitative assessment of field studies as well as for use in climate models. With this motivation, we have chosen oleic acid (one of the simplest condensed phase organics with atmospheric relevance) as the first species to be used in a series of experiments aimed at investigating the dynamic evolution of the composition and size of organic particles in the presence of ozone. Our experimental setup includes an atmospheric pressure flow reactor, in which oleic acid particles of a pre-selected size are allowed to interact with ozone for a controlled time, coupled to a newly developed aerosol mass spectrometer (AMS), (Jayne et al., 2000), which monitors changes in the size distribution and composition of the aerosols. This AMS/flow reactor system permits a new approach to kinetic studies in that the depletion of the particle-phase reactant (rather than the gas-phase reactant) is monitored. In the present study we report the size dependent rate of reactive uptake of ozone by three different sizes of oleic acid particles. Depletion of oleic acid, appearance of product species, and the growth of particle size are all observed simultaneously. With interaction times from 2 to 11 seconds, a given run takes place at nearly constant ozone concentration. The figure below indicates the fraction of oleic acid remaining in a particle as a function of ozone exposure time. We are developing an uptake model which allows determination of the following fundamental physico-chemical parameters: Henry's Law solubility constant for ozone in oleic acid, H, the liquid phase diffusion coefficient for ozone in oleic acid, D, and the second order rate coefficient for ozone reacting with oleic acid, k 2. The plotted curves show a preliminary fit to the size dependent reaction data, yielding the values shown at 293°K. These parameters allow calculation of the reactive collision probability, γ, which ranges from about 10 −3 to 10 −5 (decreasing with a decrease in oleic acid concentration). Our data also show a dependence in the uptake on the purity of the oleic particles, an increase in viscosity upon reaction, and an increase in particle diameter as the interaction time increases. These effects (including those of secondary reaction of products) are currently being modeled in order to assess aerosol phase oleic acid atmospheric lifetimes.