Photobleaching of Imidazole Brown Carbon in Single Levitated Aerosol Particles

Brown carbon (BrC) aerosols are key contributors to atmospheric light absorption and photochemistry, yet their optical properties and photochemical behavior are poorly understood. To explore the influence of a microdroplet environment on the photochemical decay of a representative BrC species, single particle cavity ring-down spectroscopy was used to probe individual aerosol particles containing imidazole-2-carboxaldehyde (IC) confined inside a linear electrodynamic quadrupole balance. The aerosol particles were levitated and exposed to 405 nm wavelength laser light to drive photobleaching. The imaginary component of the complex refractive index (k) provided a direct characterization of the aerosol particle light absorption and was determined for varying exposures to the photolyzing light. The determined k values decayed exponentially with exposure time, demonstrating photobleaching. A kinetic model incorporating Lorenz-Mie theory was fitted to the observed decay to obtain a photobleaching quantum yield (within the framework of the kinetic model) for IC of (9.6 ± 3.0) × 10^-5. The developed kinetic framework can be used to predict photobleaching time scales in aerosols from knowledge of these effective quantum yields. Additional photobleaching measurements on bulk IC-containing solutions showed that the photobleaching quantum yields are an order of magnitude larger in aerosol droplets, potential reasons for which are discussed.