Broadband Light Scattering to Characterize the Absorption Spectra of Brown Carbon Chromophores

Aerosol particles that contain brown carbon (BrC) chromophores will absorb solar radiation, with important implications for their effects in the environment. Although UV/vis spectroscopy of bulk BrC samples can be used to characterize absorption spectra, these methods can be of limited use when applied to aerosol particles due to high chromophore concentrations and the formation of metastable supersaturated or supercooled states in atmospheric particles. Here, we report a method to characterize the absorption spectra of levitated aqueous particles in a linear-quadrupole electrodynamic balance using broadband light scattering, allowing the wavelength dependence of the imaginary component of the refractive index, k, to be determined from a single spectrum. We show that a non-absorbing particle is an effective reference for the illuminating light and provides a reliable indication of the intensity variation across the measured wavelength range. This allows spectra from sample particles containing light-absorbing components to be referenced and normalized in a way that decouples the processed spectrum from any artifacts introduced by the LED spectrum and optical setup. Using this approach, we explore the pH-dependent absorption spectra of 4-nitrocatechol particles under a range of relative humidity (RH) conditions. We demonstrate sensitivity to k in the range 0.0001 to 0.01 and compare our results to predictions using bulk UV/vis data. This method allows the characterization of light absorption at much higher chromophore concentrations than traditional UV/vis measurements, enabling a more direct comparison to atmospheric aerosol particles.