Oxidation of Optically Trapped Organic Squalene Aerosol: Ultraviolet Refractive Index Changes upon Ozonolysis and Light-Induced Oxidation

Organic atmospheric aerosol particles are regularly oxidized, modifying their physical, chemical, and optical properties and affecting the radiative balance of the planet. The organic compound squalene is commonly found in organic particulate matter and regularly undergoes aging by atmospheric oxidants. Here, broadband light Mie spectroscopy is used to monitor the radius and refractive index dispersion of optically trapped squalene aerosol particles over near-ultraviolet to visible wavelengths of 0.320–0.480 μm, throughout atmospheric oxidation by ozone. Extension of broadband light Mie spectroscopy measurements to wavelengths <0.350 μm permits additional insights into the wavelength dependence of the refractive index. Exposure of a squalene particle (≈1 μm radius) to gaseous ozone (5, 38, 60 ppm) results in significant initial changes in particle radius and refractive index, consistent with the rapid loss of reactive functional groups (carbon–carbon double bond), followed by successive polymerization reactions between ozonolysis products. In contrast, exposure of squalene to gaseous oxygen in the presence of light results in an increase in the refractive index and a relatively smaller but delayed decrease in droplet radius. A waxy solid is formed after 30 and 75 min for the ozonolysis and light-induced oxidations of squalene, respectively, indicating faster solidification of squalene-containing organic aerosol in the presence of ozone.