Effects of Ionic Strength and Droplet Evaporation on Brown Carbon in Atmospheric Cloud and Fog Water
Author
Christopher Hennigan — University of Maryland, Baltimore County *
Category
Absorbing aerosol
Description
Multiphase processes in atmospheric cloud and fog water play a critical role in the life cycle of light-absorbing organic compounds (so called brown carbon, BrC). Secondary production in clouds is hypothesized as a significant source of BrC contributing to its relatively high abundance in the free troposphere, while loss processes initiated by direct photolysis and reactions with atmospheric oxidants in aqueous environments contribute to the ‘bleaching’ observed in the aging of primary BrC. In this work, we explore the effects of ionic strength on BrC in cloud and fog water. Bulk cloud water samples from Whiteface Mountain, NY and fog water samples from Mt. Tinajing, China are analyzed for the effects of ionic strength on BrC optical properties – including light absorption at 365 nm (Abs365), mass absorption coefficient (MAC365), and the absorption Ångström exponent (AAE). The samples span a wide range of anthropogenic pollution influences, with ionic strengths ranging from 4.3 x 10-5 to 1.6 x 10-3, and total organic carbon concentrations ranging from 2.59 mg/L to 41.1 mg/L. Evaporation of cloud and fog droplets increases the ionic strength by orders of magnitude, greatly amplifying any effects on BrC optical properties. In our prior work, we characterized the effects of pH on the optical properties of BrC in cloud water. Here, we show that ionic strength also has important effects on atmospheric BrC, with the strongest implications for changes in BrC optical properties in cloud condensation nuclei (CCN) that undergo cloud cycling.
Lead PI
Christopher Hennigan — University of Maryland, Baltimore County