Size-resolved Scanning Flow CCN Analysis (SFCA): A method for fast measurements of CCN spectra

The Continuous-Flow Streamwise Thermal-Gradient Cloud Condensation Nuclei Counter¹ is a commercially-available instrument that is widely used for laboratory and field measurements of cloud condensation nuclei (CCN). Typically, the instrument is operated at a constant flow rate, and supersaturation is adjusted by changing the column streamwise temperature difference, which is slow (20-40 seconds, and sometimes more) and may introduce particle volatilization biases². We present a new mode of operation, termed "Scanning Flow CCN Analysis" (SFCA), in which the flow rate in the growth chamber is changed over time, while maintaining a constant temperature gradient. This causes supersaturation to continuously change, allowing the rapid and continuous measurement of supersaturation spectra over timescales relevant for airborne measurements (10-60 seconds and potentially even less) without being affected from shifts in activation kinetics and aerosol composition. We present results of size-resolved CCN spectra obtained using SFCA, coupled with an electrostatic classifier to demonstrate the application of the technique to ambient measurements of urban aerosol.

¹Roberts, G. and A. Nenes. Aerosol Sci. Technol., 39, 2005

²Asa-Awuku, A., et al., Atmos. Chem. Phys., 9, 2009.