On Measuring the Critical Diameter of Cloud Condensation Nuclei Using Mobility Selected Aerosol

Figures & data

FIG. 1 Schematic of measurement strategy. The two figures on the right show an example size distribution and CCN activation data for a single experiment. Vertical bars in the bottom figure represent ±2 standard deviations, computed from ∼60 independent data points.

FIG. 2 Calculated CCNc response curve (thick solid line, right ordinate, as function of mobility selected diameter) for log-normally distributed, charge-equilibrated particles transmitted through an ideal DMA followed by transit through an ideal CCN instrument. Shown are an assumed log-normal input size distribution (solid line, left ordinate, N = 900 cm⁻³, D _g = 0.14 μm, s _g = 1.33), the ratio of [+2]/[+1] (dashed line) and [+3]/[+1] (dashed-dotted line) charges at charge equilibrium (right ordinate), the assumed critical diameter for the model calculations (dotted line), the centroid sphere equivalent mobility diameter, and the mobility band-width for +1, +2, and +3 (from left to right) charged particles transmitted through the TSI long DMA column at an assumed 10:2 sheath-to-monodisperse flow ratio (gray intervals).

FIG. 3 Panels (a, b): Measured CCNc response (circles) for DEHS+OH experiment at S = 0.64%. Vertical bars represent ±2 standard deviations, computed from ∼60 independent data points. Gray histogram, panel a: normalized spectral density of the input size distribution. Gray histogram, panel b: same as figure a, but the mode diameter is shifted by ∼0.1 μm toward smaller sizes. Dashed and solid lines, panels a, b: Calculated CCNc response curves for various assumed D _c (see text). Panels c, d: Normalized χ²-statistic as a function of assumed values of D _c . The minimum value (dotted line) is used to determine the best-fit CCNc response curve.

FIG. 4 Measured CCNc response (circles) for DEHS+OH experiment; (a) S = 0.75%, (b) S = 0.64%, (c) S = 0.53%, (d) S = 0.33%. Vertical bars represent ±2 standard deviations, computed from ∼60 independent data points. Gray histogram: normalized spectral density of the input size distribution. Solid line: best-fit CCNc response curve. The critical diameter yielding the best fit to the observation is indicated by the dotted line; (a) D _c = 0.16 μm, (b) D _c = 0.19 μm, (c) D _c = 0.23 μm, (d) D _c = 0.32 μm.

TABLE 1 Comparison between critical diameters inferred from the inversion and those inferred from a sigmoidal fit (Snider et al. 2006)

	Critical Diameter (μ m)
Supersaturation (%)	Inversion	Sigmoidal fit
0.75	0.162	0.121
0.64	0.192	0.160
0.53	0.230	0.217
0.33	0.317	0.311

Snider , J. R. , Petters , M. D. , Wechsler , P. and Liu , P. S. K. 2006 . Supersaturation in the Wyoming CCN Instrument . J. Atmos. Ocean. Technol. , 23 ( 10 ) : 1323 – 1339 .

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