Scanning Flow CCN Analysis-A Method for Fast Measurements of CCN Spectra

The Continuous-Flow Streamwise Thermal-Gradient Cloud Condensation Nuclei Counter 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 and may introduce particle volatilization biases. 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 (similar to 10-60 seconds and potentially even less) without being affected by shifts in activation kinetics and aerosol composition. SFCA has been evaluated with both calibration and ambient aerosol with excellent results.