Using Nanoparticle X-ray Spectroscopy to Probe the Formation of Reactive Chemical Gradients in Diffusion-Limited Aerosols

Journal Article · 21 June 2019 · Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, and General Theory

DOI: https://doi.org/10.1021/acs.jpca.9b04507 · OSTI ID:1608272

Jacobs, Michael I.  ^[1]; Xu, Bo  ^[2]; Kostko, Oleg  ^[2]; Wiegel, Aaron A. ^[2]; Houle, Frances A.  ^[2]; Ahmed, Musahid  ^[2]; Wilson, Kevin R.  ^[2]

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1. Univ. of California, Berkeley, CA (United States); Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
2. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)

For aerosol particles that exist in highly viscous, diffusion-limited states, steep chemical gradients are expected to form during photochemical aging in the atmosphere. Under these conditions, species at the aerosol surface are more rapidly transformed than molecules residing in the particle interior. To examine the formation and evolution of chemical gradients at aerosol interfaces, the heterogeneous reaction of hydroxyl radicals (OH) on ~200 nm particles of pure squalane (a branched, liquid hydrocarbon) and octacosane (a linear, solid hydrocarbon) and binary mixtures of the two are used to understand how diffusion limitations and phase separation impact the particle reactivity. Aerosol mass spectrometry is used to measure the effective heterogeneous OH uptake coefficient (γ_eff) and oxidation kinetics in the bulk, which are compared with the elemental composition of the surface obtained using X-ray photoemission. When diffusion rates are fast relative to the reaction frequency, as is the case for squalane and low-viscosity squalane–octacosane mixtures, the reaction is efficient (γ_eff ~ 0.3) and only limited by the arrival of OH to the interface. However, for cases, where the diffusion rates are slower than reaction rates, as in pure octacosane and higher-viscosity squalane–octacosane mixtures, the heterogeneous reaction occurs in a mixing-limited regime and is ~10× slower (γ_eff ~ 0.03). This is in contrast to carbon and oxygen K edge X-ray absorption measurements that show that the octacosane interface is oxidized much more rapidly than that of pure squalane particles. The O/C ratio of the surface (estimated to be the top 6–8 nm of the interface) is measured to change with rate constants of (3.0 ± 0.9) × 10^–13 and (8.6 ± 1.2) × 10^–13 cm³ molecule^–1 s^–1 for squalane and octacosane particles, respectively. The differences in surface oxidation rates are analyzed using a previously published reaction-diffusion model, which suggests that a 1–2 nm highly oxidized crust forms on octacosane particles, whereas in pure squalane, the reaction products are homogeneously mixed within the aerosol. This work illustrates how diffusion limitations can form particles with highly oxidized surfaces even at relatively low oxidant exposures, which is in turn expected to influence their microphysics in the atmosphere.

• Research Organization: Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences, and Biosciences Division; National Science Foundation (NSF)
• Grant/Contract Number: AC02-05CH11231
• OSTI ID: 1608272
• Journal Information: Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, and General Theory, Vol. 123, Issue 28; ISSN 1089-5639
• Publisher: American Chemical SocietyCopyright Statement
• Country of Publication: United States
• Language: English