Abstract View
Measurements of Hygroscopic and Optical Properties of Size Selected Gray Carbon Aerosols: A New Empirical Function to Estimate Radiative Forcing
TYLER CAPEK, Christian Carrico, Kyle Gorkowski, James E. Lee, Katherine Benedict, Claudio Mazzoleni, Allison Aiken, Manvendra Dubey, Michigan Technological University
Abstract Number: 340
Working Group: Aerosols, Clouds and Climate
Abstract
Water vapor can significantly affect the physical and optical properties of atmospheric aerosol leading to changes in atmospheric radiative forcing. Water can condense onto the particles, increasing their size and altering their morphology. Absorbing particles range from hydrophobic to hydrophilic, the latter leading to water uptake and enhancements in absorption and scattering. To demonstrate water uptake for internally mixed absorbing particles, we measured the optical response to humidification of ammonium sulfate (non-absorbing), nigrosin (absorbing), and two darker and lighter ammonium sulfate and nigrosin mixtures. The molar ratio of ammonium sulfate to nigrosin of the two mixtures were 1:1 and 4.7:1, respectively. We used a novel humidified cavity attenuated phase shift albedometer (H-CAPS-PMSSA) at 450 nm by Carrico, Capek et al. (Aerosol Science & Technology, 2021). This instrument measures extinction and scattering at well-controlled relative humidity values ranging from 30 ± 2% – 90% ± 3%. These measurements enable for the quantification of absorption and single scattering albedo at high relative humidity, with high precision. We found absorption enhancements due to humidification to be as high as 1.08 for pure nigrosin, and 1.15 for the mixtures. Enhancements in single scattering albedo were dependent on the amount of nigrosin within the dry aerosol. Furthermore, the enhancement at a certain relative humidity value could be empirically fit using a quadratic function, with nigrosin volume fraction as the independent variable. For each size and composition investigated, this relation was dependent on a single fitting parameter that is related to dry aerosol size and relative humidity. It is likely that this parameterization can be extended to phase separated absorbing and non-absorbing components. Our parameterization can be used to predict changes in aerosol optical properties with humidity in cloud-resolving and/or climate models to better predict their radiative forcing.