Wavelength-\nand pH-Dependent Optical Properties of\nAqueous Aerosol Particles Containing 4‑Nitrocatechol

The radiative forcing caused by atmospheric aerosol represents\none of the largest uncertainties in climate models. In part, these\nuncertainties derive from poor characterizations of the optical properties\nof light-absorbing brown carbon (BrC) containing aerosols. Here, single\nparticle cavity ring-down spectroscopy (SP-CRDS) is used to determine\nthe complex refractive index at the optical wavelength of 405 nm for\naqueous particles composed of an abundant BrC species, 4-nitrocatechol.\nMoreover, the effect of acidity on the complex refractive index of\n4-nitrocatechol is explored. UV/visible spectroscopy allows measurement\nof the wavelength-dependent (from 200 to 800 nm) imaginary refractive\nindex for bulk aqueous solutions of 4-nitrocatechol, for which the\npH is adjusted between ∼1 and 13. Applying a physically based\nrefractive index mixing rule, wavelength-dependent imaginary refractive\nindex values are estimated for the fully protonated, singly deprotonated\nand doubly deprotonated forms of 4-nitrocatechol. A Kramers–Kronig\nanalysis constrained by the 405 nm SP-CRDS and 632.8 nm elastic light\nscattering measurements gives the wavelength-dependent real refractive\nindex values. The real and imaginary refractive indices are essential\nfor computing the radiative properties of these abundant BrC chromophores\nin aerosol plumes and cloudwater.