Light Absorption by Brown Carbon in the Southeastern United States is pH-dependent

Light-absorbing organic material, or "brown carbon" (BrC), can significantly influence the effect that aerosols have on climate. Here, we investigate how changing pH affects the absorption spectra of water-soluble BrC from ambient particulate matter smaller than 2.5 μm collected in Athens, Georgia, in the spring and fall of 2016, including samples from nearby wildfires. We find that absorption increases 10% per pH unit from pH 2 to pH 12 with a broad, featureless tail at visible wavelengths, where the largest fractional increase is also observed. The resulting change in the spectral shape causes the absorption Ångström exponent to decrease by 0.18 per unit increase in pH. Similar behavior with humic substances suggests that they and BrC share a common link between pH and absorption, which we propose could be a consequence of conformational changes in supramolecular assemblies thought to exist in humic substances. Specifically, we hypothesize that a wider variety and larger number of absorbing charge transfer complexes are formed as functional groups in these molecules, such as carboxylic acid and phenol moieties, become deprotonated. These findings suggest that (1) the pH of ambient particulate matter samples should be measured or controlled and (2) radiative forcing by BrC aerosols could be overestimated if their pH-dependent BrC absorption is not accounted for in models.