Abstract View
Aircraft Measurements of Single Particle Size and Composition Reveals Real-world Mixing State Necessary to Explain Activation Fraction during HI-SCALE
GEORGES SALIBA, David Bell, Kaitlyn J. Suski, John Shilling, Fan Mei, Gourihar Kulkarni, Adam Varble, Johannes Muelmenstaedt, Jian Wang, Jason Tomlinson, Jerome Fast, Alla Zelenyuk, Pacific Northwest National Laboratory
Abstract Number: 413
Working Group: Aerosols, Clouds and Climate
Abstract
Shallow convective clouds are ubiquitous in many regions of the world. Currently, aerosol-cloud parameterizations for convective clouds are a large source of radiative uncertainty in global climate models, highlighting the need for in-situ characterization of the size, composition, and mixing-state of activated particles and their more accurate model representation. Here we present aircraft measurements performed over the Atmospheric Radiation Measurements, Southern Great Plains climate facility in Oklahoma during the spring and summer of 2016, which were characterized by contrasting aerosol composition. These measurements quantified the properties of below-cloud aerosol, cloud droplet residuals, interstitial particles, and above-cloud aerosol of continental shallow cumuli. Organic-rich particles accounted for a larger number fraction of below-cloud particles in the summer compared to spring, consistent with higher emissions of biogenic volatile organic compounds, i.e. isoprene. We also present single-particle measurements that provide evidence of a strong influence of the size, composition, and mixing state of below-cloud aerosol on the activation fraction for shallow cumuli. The data indicate that cloud droplet residuals were larger and more hygroscopic compared to below-cloud aerosols, consistent with the efficient activation of these particles and aqueous formation of sulfate components in cloud droplets that increase the size and hygroscopicity of residuals. Using below-cloud measurements of aerosol composition, size distributions, and mixing-state, we calculated the effective cloud supersaturations between 0.06% and 0.24% (median = 0.1%). Moreover, we show that commonly used assumption of an internally mixed aerosol yielded calculated supersaturations that are significantly larger than measured, highlighting the importance of accurately describing the aerosol mixing state in models.