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Improvement of Static Thermal Diffusion Chamber for More Accurate Measurement of Atmospheric Cloud Condensation Nuclei

by Will H. Cantrell II, Department of Physics and Geophysical Institute

Reliable and accurate data are crucial to an understanding of the climate system. Unless the current conditions are accurately quantified, the climate system cannot be understood and the impacts of anthropogenic activity cannot be assessed. This research has focused on improving the techniques for the measurement of cloud condensation nuclei (CCN), a parameter important for both the global and arctic climate system. An increase in CCN could cause some clouds to brighten, reflecting more solar radiation away from Earth. The degree of brightening in these clouds is sensitive to the cloud droplet concentration, which depends upon the CCN available.

To measure CCN, aerosol must be subjected to a supersaturated environment, which is commonly done with a thermal diffusion chamber. Two wetted plates separated by a small distance are held at different temperatures. Because of the nonlinear dependence of the saturation vapor pressure on temperature, the air between the two plates will be slightly supersaturated. The supersaturation can be calculated if the distance between the two plates and the respective temperatures are known.

Determining the supersaturation can present a number of difficulties. The saturation vapor pressure is very sensitive to temperature. The temperature difference between the plates must be known to an accuracy of at least 0.2° for the supersaturation to be accurate to within 0.05%. An error of 0.05% could translate into an error of several hundred in the measured CCN concentration.

The temperature difference in our CCN counter, the DH-1, is established by cooling the bottom plate. In the original design, the temperature difference was measured with a thermocouple inserted into the top and bottom plate. This introduced a substantial, systematic error in the calculated supersaturations because there is a temperature gradient within the bottom steel plate. There is as much as 1°C difference in temperature between the surface of the plate and the point at which the temperature was measured. A 1°C error in temperature propagates into a 0.3% error in the supersaturation.

To eliminate these errors, the temperature must be measured at the surface of the plate. This can be done with thin film RTDs. These devices have thicknesses of less than 1 mm, and can be affixed to the surface of the plate without disturbing the air flow or optical sensors.

To evaluate a CCN counter, aerosol of a known chemical composition is generated; then a subset of a specific size, which has an associated critical supersaturation (Sc), is selected for further analysis. Below Sc, the aerosol will not be CCN; above it, they will. This subset of aerosol is monitored by a particle counter and by the CCN counter. The particle counter records the aerosol number concentration. The CCN counter subjects the sample to a specified supersaturation and records the number of aerosol that become CCN. If the supersaturation is above Sc, the CCN counter and the particle counter should report the same concentration; if the supersaturation in the CCN counter is below Sc, the CCN counter should report a concentration of zero.

Figure 1 shows results from a test using the DH-1. The data from the DH-1 are referenced to the results predicted by the Köhler theory, using the chemical composition of the test aerosol and the measured number distribution.

Figure 1
Figure 1

If the uncorrected supersaturations are used, the DH-1 CCN counter under- predicts the number of CCN by a factor of two at the highest supersaturations. Using the corrected supersaturations, the DH-1 comes much closer to the reference curve. Though still not perfect, the DH-1 is a better instrument for monitoring the current CCN concentrations and for assessing potential anthropogenic impacts upon the climate system.

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