Spectroscopic Analysis of the Complex Refractive Indices for Imine Brown Carbon Aerosol Particles

Brown carbon (BrC) contributes substantially to light absorption by atmospheric aerosols, which represents a key uncertainty in estimates of aerosol radiative forcing. Among its diverse constituents, imine BrC formed from aqueous reactions between glyoxal and nitrogen-containing species is of widespread interest. We report sensitive measurements of particle size-resolved optical cross sections, associated retrievals of complex refractive indices (<i>m</i> = <i>n</i> + <i>ik</i>), and effective density measurements for imine BrC aerosols generated from aqueous solutions containing glyoxal and nitrogen species across a wide pH range. An Aerodynamic Aerosol Classifier was coupled with cavity ring-down and photoacoustic spectroscopy to measure extinction and absorption cross sections at the short-visible wavelength of 405 nm, enabling retrievals of <i>n</i> and <i>k</i> with high precision and accuracy while eliminating multiply charged particle artifacts that impair more common mobility-based approaches. Detectable light absorption was observed under basic conditions (pH ≈ 9.5) only, yielding values of <i>k</i> in the range 0.0016-0.0018 and <i>n</i> of ∼1.50, demonstrating that imine BrC may contribute modest absorption at short-visible wavelengths. Meanwhile, aerosols generated from aqueous solutions under acidic and near-neutral conditions were nonabsorbing (<i>k</i> < 10^-4). Comparison with bulk UV/vis absorption spectra indicates that aerosolization and drying enhanced chromophore formation under basic conditions, likely through the supersaturation of dissolved reacting solutes in drying aqueous aerosol droplets. Measured values for effective particle density were consistent with the formation of partially oligomerized glyoxal hydrates.