Time-Dependent Density Functional Theory Investigation of the UV-Vis Spectra of Organonitrogen Chromophores in Brown Carbon

The ability of brown carbon (BrC) in aerosols to absorb solar radiation is an important but highly uncertain factor in climate forcing. The uncertainties are partially due to incomplete characterization of BrC chromophores and lack of authentic standards to confirm light absorption. Organonitrogen species are crucial components in atmospheric aerosols, but their light-absorbing properties remain to be fully characterized. To facilitate the molecular characterization of BrC chromophores, time-dependent density functional theory (TD-DFT) based computational chemistry approaches were used in this study to predict the light absorption spectra of 16 organonitrogen species, including nitroaromatics, nitro-heterocyclic compounds, organonitrates, and Maillard-type reaction products in BrC. Effects of basis sets, functionals, solvation, and pH on light absorption properties of these compounds were evaluated. Predicted absorption spectra were compared with experimental measurements. Overall, the PBE0 and B3LYP functionals tend to outperform PBE and CAM-B3LYP on the predicted absorption spectra of studied compounds. Absorbance calculated in water and methanol (bulk solvents) varies up to 2 nm (0.03 eV). Absorbance calculated in gas phase (vacuum state) blue-shifts in comparison to solvation. Absorbance of weak acids (e.g., nitrophenols) is enhanced under basic conditions, and the absorption spectra can be predicted by the fractions of conjugate acid-base species. Results from this study demonstrate that a combined use of TD-DFT predictions and experimental measurements of light absorption can allow for a rapid and reliable determination of potential chromophores in BrC when authentic standards are not available.