Exploring CCN droplet suppression with a higher sensitivity optical particle counter

Abstract

The continuous flow thermal gradient cloud condensation nuclei counter (CCNc) is the instrument currently widely used in exploring the interactions of aerosol with water vapor in the supersaturated regime. In this study we use a CCNc with an optical particle counter (OPC) modified to twice the sensitivity of the default OPC on the standard CCNc built by DMT, Inc. The standard OPC and the modified OPC-β are used to explore the droplet growth of inorganic and organic salts and the effects of cloud condensation nuclei concentrations on the final droplet sizes. We also couple the higher sensitivity OPC-β with the continuous flow streamwise thermal gradient CCNc (CFSTGC) model to estimate the mass accommodation coefficient, α. For inorganic and organic aerosol, a mass accommodation coefficient, α >> 0.2 will yield modeled droplet diameters consistent with experimental and ambient observations. However, using higher sensitivity CCN data, the final droplet diameters of CCN appear independent of aerosol hygroscopicity but strongly dependent on the CCN concentration. The final droplet diameters are suppressed by ∼ 0.2 µm for every 1000 cm-3 CCN, even at CCN concentrations less than 5000 cm-3. Our current understanding of aerosol CCN formation and droplet kinetics may therefore be dependent on the sensitivity of the optical particle counter in the CCNc. Copyright © 2018 American Association for Aerosol Research.