Wavelength- and pH-Dependent Optical Properties of Aqueous Aerosol Particles Containing 4-Nitrocatechol

The radiative forcing caused by atmospheric aerosol represents one of the largest uncertainties in climate models. In part, these uncertainties derive from poor characterizations of the optical properties of light-absorbing brown carbon (BrC) containing aerosols. Here, single particle cavity ring-down spectroscopy (SP-CRDS) is used to determine the complex refractive index at the optical wavelength of 405 nm for aqueous particles composed of an abundant BrC species, 4-nitrocatechol. Moreover, the effect of acidity on the complex refractive index of 4-nitrocatechol is explored. UV/visible spectroscopy allows measurement of the wavelength-dependent (from 200 to 800 nm) imaginary refractive index for bulk aqueous solutions of 4-nitrocatechol, for which the pH is adjusted between ∼1 and 13. Applying a physically based refractive index mixing rule, wavelength-dependent imaginary refractive index values are estimated for the fully protonated, singly deprotonated and doubly deprotonated forms of 4-nitrocatechol. A Kramers-Kronig analysis constrained by the 405 nm SP-CRDS and 632.8 nm elastic light scattering measurements gives the wavelength-dependent real refractive index values. The real and imaginary refractive indices are essential for computing the radiative properties of these abundant BrC chromophores in aerosol plumes and cloudwater.