pH Dependence of the Imidazole-2-carboxaldehyde Hydration Equilibrium: Implications for Atmospheric Light Absorbance

Imidazole-2-carboxaldehyde (IC) has been identified as an aqueous "brown carbon" absorber and a possible atmospheric photosensitizer. IC exists in a pH-dependent equilibrium between its aldehyde and geminal diol form; the diol form is the dominant species in solution at pH <5. Calculated molar absorptivity coefficients are 13700 +/- 200 cm(-1) M-1 at 287 nm for the aldehyde and 7800 +/- 100 cm(-1) M-1 at 212 nm for the diol. This shift from aldehyde to diol changes the peak light absorption of the aqueous solution from 287 to 212 nm, which is beyond the actinic range and may have implications for radiative forcing. The observed pH-dependent shift in the hydration equilibrium of IC is driven by the interaction between its hydration and protonation equilibria. Calculated pK(a) values are 2.5 +/- 0.4 and 5.94 +/- 0.05 for the aldehyde and diol, respectively, and are consistent with the trend toward increasing diol as the pH decreases. The acid-base equilibrium affects both the solubility and the major species in solution depending on the protonation state of IC and may affect atmospheric light absorption, brown carbon, and photosensitization under acidic conditions. These findings indicate a need for a greater level of attention to the effect of the matrix in aqueous atmospheric systems, particularly concerning species affected by multiple equilibria.