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.
Zhang et al. (Mon,) studied this question.