AAAR 34th Annual Conference
October 12 - October 16, 2015
Hyatt Regency
Minneapolis, Minnesota, USA
Abstract View
Unraveling the Development of Supersaturation under Dynamic Flow Operation of CCN Counters
JACK J. LIN, Sara Purdue, Tomi Raatikainen, Athanasios Nenes, Georgia Institute of Technology
Abstract Number: 202 Working Group: Instrumentation and Methods
Abstract The Droplet Measurement Technologies Continuous Flow Streamwise Thermal Gradient Cloud Condensation Nuclei (CCN) Counter$^1 is widely used to measure CCN concentrations and study aerosol hygroscopicity. When operated in Scanning Flow CCN Analysis (SFCA) mode, the instrument is capable of scanning across a wide range of supersaturations in a short amount of time$^2. Recent work has shown that pressure field transients that develop inside the CCN growth chamber can significantly affect instrument supersaturation. These effects arise from the flow rate changes and the resulting compressive heating and expansive cooling$^3 occurring during an SFCA flow cycle. Work to date has not systematically explored the effect of these pressure transients on the supersaturation fields for a variety of flow scan forms. Pressure effects on supersaturation, if taken advantage of, can considerably augment the ability to measure CCN spectra.
In this work, we present a comprehensive characterization of the supersaturation cycles that develop during SFCA operation for a wide range of operating conditions, including pressure, temperature gradient, scan time, and flow rate waveform. Supersaturation was characterized by flowing ammonium sulfate calibration aerosol into a DMT CCN-100 unit operating under SFCA mode. For precise flow control, a specialized hardware module consisting of an Arduino microcontroller coupled to a modified mass flow controller was developed and attached to the CCN unit. The new module can be easily programmed to follow any arbitrary flow scan profile and waveform. Here we present results using triangular and sinusoid SFCA cycles. The use of the new hardware module reduces the uncertainty in supersaturation compared with traditional SFCA operation. For both waveforms, the dynamic supersaturation range was 0.2-0.9% at 900 mb and 0.1-0.8% at 600 mb for flow cycles ranging between 30 and 60 seconds. The results of the calibrations are interpreted with a comprehensive model$^3 and an optimum sampling and SFCA flow cycle strategy is developed for ground-based or aircraft instrument deployments.
1. Lance at al., Aerosol Sci. Tech., 2006.
2. Moore and Nenes, Aerosol Sci. Tech., 2009.
3. Raatikainen et al., Aerosol Sci. Tech., 2014.