Environmental Science and Engineering Seminar

Light absorbing organic compounds, collectively called brown carbon (BrC), make an important yet highly uncertain contribution to aerosol effects on climate. Multiphase atmospheric processes, including those in clouds, play a key role in the life cycle of BrC as they can facilitate production or loss, depending on the conditions. For example, the evaporation of droplets and deliquesced particles can accelerate secondary BrC formation and may serve as a significant pathway for its production. Further, the acidity (or pH) of atmospheric particles and clouds affects the optical properties of BrC and rates of BrC loss through chemical reaction or photolysis. Given the wide variability of pH in the atmosphere (pH in particles and clouds ranges from -1 to 8), the optical properties of BrC and its bleaching behavior are expected to vary significantly, and the link between pH and BrC is a major uncertainty in attempts to constrain its climate forcing effects. In this work, we characterize the pH-dependence of BrC optical properties – including light absorption at 365 nm (Abs365), mass absorption coefficient (MAC365), and the absorption Ångström exponent (AAE) – in bulk cloud water sampled from the summit of Whiteface Mountain, NY. We also examine BrC formation in cloud droplets and ambient particles undergoing drying. Overall, we do not find evidence for BrC formation in evaporating cloud water or ambient particles sampled in the eastern U.S. during summer. The results provide guidance for laboratory experiments investigating multiphase processes. The results also show that, in addition to secondary BrC production, photobleaching, and the altitudinal distribution, the climate forcing of BrC is quite strongly affected by its pH-dependent absorption.