A better understanding of the chemical evolution of organic aerosol (OA) in the atmosphere is crucial for our predictive understanding of air quality, atmospheric chemistry, health and climate. However, it is challenging to accurately predict the chemical processing of OA by multiphase oxidation reactions involving OH, in part, due to the impact of ambient temperature and humidity variations on reaction kinetics and particle phase state. We present new experimental studies of the reactive uptake coefficients (γ) of OH by thin films of single and binary OA surrogate species including glucose, glucose/hexanetriol mixtures, triacontane, squalane, and oleic acid. γ is measured by employing a chemical ionization mass spectrometer coupled to a coated-wall flow-tube reactor. Experimental results show that γ depends significantly on substrate temperature, generally decreasing by a factor of 2~5 when temperatures decrease from 20 °C to -60 °C. In this temperature regime, the amorphous phase state of the organic surrogate species can vary from liquid to solid. The substrate phase was verified by conducting qualitative film poking experiments. Temperature directly impacts the OH uptake process by affecting desorption and chemical reactivity, and indirectly by modulating substrate viscosity which in turn renders molecular diffusion. Applying resistor model analysis, the different contributions to the overall uptake reactivity can be examined separately. The reactive diffusive length dL is a key parameter that couples reaction rate and OH diffusivity thereby determining the OH concentration profile in the substrate’s near-surface layer. To account for subsurface layer OH reactions that result in an increase in measured pseudo first order surface reaction rate, a dimensionless scaling factor is introduced into the resistor model as a function of dL. Based on steady state assumption, our estimated dL of OH radical in both squalane and oleic acid is around 1 nm at 20 °C. This further allows estimation of the temperature dependent OH diffusion coefficients in the organic films. To better understand the sensitivity of surface reaction rate to temperature, a model is set up that relates the reaction rate with desorption rate, free Gibbs enthalpy (ΔG) and bond dissociation energy (BDE). By applying literature BDE and ΔG values for H abstraction, the adsorption energy Eads and pre-exponential factor of the desorption rate can be explored.

Impact of temperature on the reactive uptake kinetics of OH by amorphous organic aerosol surrogates

2019 Fall National Meeting

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