Probing the Heterogeneous Ozonolysis of Squalene Nanoparticles by Photoemission

The heterogeneous reaction of ozone (O3) with 200 nm squalene nanoparticles is studied using near-edge X-ray absorption fine structure (NEXAFS) and ultraviolet (UPS) and X-ray photoelectron spectroscopy (XPS). Photoelectrons are detected from free nanoparticle beams using a velocity map imaging (VMI) spectrometer capable of detecting photoelectrons with up to 40 eV of kinetic energy. Heterogeneous kinetics are quantified using changes in the UPS, XPS, and NEXAFS spectrum, yielding uptake coefficients for the decay of the double bonds in squalene of (3.1 ± 0.7) × 10–4, (2.6 ± 0.6) × 10–4, and (2.9 ± 0.7) × 10–4, respectively. When comparing these values with the uptake coefficient, (1.0 ± 0.2) × 10–3, determined by the molecular decay of squalene measured with aerosol mass spectrometry, it is found that on average 1.6 ± 0.2 double bonds are removed for each ozone-squalene reactive collision, suggesting the importance of evaporation of small molecular weight reaction products from the aerosol. From further ...