Asian-Pacific CCN Network for Studying the Aerosol Indirect Effect / Multi-Column Continuous Flow Streamwise Thermal-Gradient CCN Chamber

Greg Roberts and V. Ramanathan (SIO)

Link to NOAA Strategic Plan: NOAA’s Mission Goal 2: Understand Climate Variability and Change to Enhance Society’s Ability to Plan and Respond

RESEARCH OBJECTIVES AND SPECIFIC PLANS TO ACHIEVE THEM

Aerosols are generally considered to exert a cooling effect at the earth’s surface by directly scattering incoming solar radiation back to space or influencing cloud properties such as enhancing cloud albedo or lifetime and reducing precipitation. Such climate impacts, known as direct and indirect effects are still poorly understood, in part because there are so few in situ observations. This project takes measurements of cloud condensation nuclei (CCN) to a new level to fill an important gap in our understanding of the climate response to aerosols. The streamwise CCN counter developed at Scripps Institution of Oceanography in 2001, and commercialized by Droplet Measurement Technology (http://www.dropletmeasurement.com) in 2004, has significantly improved the quality of CCN measurements on the ground as well as on airborne platforms. Within three years, the streamwise CCN instrument has become the de facto standard, because it is the only successful commercially available instrument of its type. The salient features of this instrument include: supersaturation is a function of flow rate and temperature gradient; continuous flow allows fast sampling (1 Hz); and simple cylindrical geometry reduces size and minimizes buoyancy effects. The use of a single column will generate CCN spectra by modifying the flow rate and/or temperature gradient to measure CCN between 0.07 and 2% supersaturation.

The use of a single column has the potential to generate CCN spectra by modifying the flow rate and temperature gradient; however, not at a time resolution sufficient for airborne measurements. Based on efforts to miniaturize the CCN instrument, the purpose of this project is to transform our single-column CCN instrument into an automated multi-column device to retrieve CCN spectra over a range of supersaturations important for aerosol-cloud interactions. Figure 1 shows the five-year evolution of the streamwise CCN instruments since its conception. By the end of the project, we will contribute to more robust CCN instrumentation and better CCN measurements to enhance our understanding of the earth climate system.

RESEARCH ACCOMPLISHMENTS

Building on earlier versions, almost every aspect of the instrument has been studied, tested and miniaturized and optimized for CCN measurements. To avoid complications originating from unexpected instrument performance (i.e., buoyancy effects and non-linear temperature profiles), we have identified the operational limits of the streamwise CCN instrument. In particular, the flow system and optical particle counter have been redesigned to improve delivery and detection of activated droplets that enhance overall instrument performance. The flow system has been designed to occupy much less space and electronics modified to maintain precise flow control. The result is an efficient design that is compact, low-power; yet robust, minimizes aerosol losses and meets expected performance. The single-column commercial instrument (Droplet Measurement Technologies) has been reduced in size and weight to a smaller package of without compromising its performance (Table 1).

Table 1. Comparison of the commercial CCN instrument and the miniature CCN instrument.

To this end, the much-reduced size of the instrument makes it far easier for field deployment (has been tested on Scripps Pier) and compatible with various aircraft, including small lightweight unmanned aerial vehicles. The miniature prototype instrument has been calibrated and its performance validated by the sharp activation curves in Figure 2. Such activation curves show the fraction of CCN to total aerosol concentrations for particles of known composition and size.

The activation curves (Figure 2a) show the activation of classified aerosol at different temperature gradients—lower temperature gradients correspond to lower supersaturations, which activate larger aerosols. The instrument’s supersaturation is determined by the diameter of the 50% activation using Köhler Theory for water-soluble salts (i.e., (NH4)2SO4). The sharpness of the change as a function of dry particle size indicates excellent performance of the instrument—even at low supersaturations. Activation curves using different salts and flow rates exhibit the same sharp curves shown in Figure 2a. Such activation curves at different flow rates and temperature gradients enable the assessment of the overall instrument’s performance (Figure 2b). A linear relationship between temperature gradient and supersaturation at different flow rates is expected; and the deviations at low temperature gradients (ΔT < 3 degrees C) represent the lower operating limit of the instrument (ca. 0.1% supersaturation at 100 cm3 min-1). The x-intercepts at 1 degree C for all experiments indicate a 0.5 degree C temperature bias between measured temperature gradient and the temperature gradient at the wetted surface inside the chamber. The commercial DMT instrument shows similar results to that of the miniature version, verifying that the miniature instrument performs as well as its larger cousin (Figure 2b). A longer column is being assembled with a length of 35 cm (twice the length of the existing miniature column). The longer column allows twice the residence time for droplet growth, which will enable direct CCN measurements to 0.05% supersaturation.

Fig 1. Development of streamwise CCN instrument from left to right: 1st prototype (2001), 2nd prototype (2002), DMT version (2004), mini-CCN (2006)

Fig. 2 a) Calibration curves for the miniature CCN. The x-axis is the size of the calibration particles and the y-axis is the fraction that activated into droplets. Each curve is for a different temperature gradient (and supersaturation). b) CCN performance at difference flows and temperature gradients. Results from the miniature CCN instrument and DMT are shown comparison