Rapid scan dynamic humidity particle spectrometer

Description

The Continuous-Flow Streamwise Thermal-Gradient CCN counter (CFSTGC), which has been commercialized by Droplet Measurement Technologies (Roberts and Nenes, AST, 2005; Lance et al., AST, 2006), has proven to be reliable, robust, and relatively simple to operate for ground-based and airborne measurements. The supersaturation profile in the CFSTGC depends on pressure, flow rate, the streamwise temperature gradient, and to a lesser extent, the inlet temperature. Currently, the instrument is operated at a constant flow rate, and its supersaturation is adjusted by changing the column temperature gradient in the streamwise direction. Although a well-established and robust approach, this mode of operation provides CCN measurements at one supersaturation at a time. Switching supersaturations is slow, requiring 20–40 seconds for column temperatures to stabilize during a supersaturation change (occasionally even longer periods). Data collected during transient periods are often discarded, which is a problem if CCN measurements are done in rapidly changing environments (such as those encountered in aircraft studies).

A new mode of operation, called “Scanning Flow CCN Analysis”, or SFCA (Moore and Nenes, AST, 2009), addresses these issues. SFCA involves changing the flow rate in the instrument over time, while maintaining a constant temperature gradient; this causes supersaturation to continuously change, allowing the measurement of a supersaturation spectrum (i.e., CCN concentrations between 0.1 and 1.0% supersaturation) over a flow cycle, which can be as short as 10 seconds (and without loss of data continuity). Another important aspect of SFCA is the minimization of CCN measurement biases from volatilization effects that could occur under the high temperature gradients typically associated with the conventional operation of the instrument. SFCA combines the simplicity and robustness of the DMT CCN counter with the dynamical range and temporal resolution of a CCN spectrometer. We present demonstrations of a prototype SFCA unit deployed in a number of airborne campaigns (CalNex, GRIP, IFEX) aboard the CIRPAS Twin Otter, NOAA WB-P3, and NASA DC-8 platforms. Apart from the important scientific results and successful data collected during these campaigns, we present “lessons” learned and discuss the ongoing efforts to improve the resolution and dynamic range of the technique.