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.