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Two-dimensional angular light-scattering in aqueous NaCl single aerosol particles during deliquescence and efflorescence

Christian Braun and Ulrich K. Krieger

Open Access Open Access

Abstract

We present a new method to analyze two–dimensional angular light–scattering patterns of single aerosol particles by image processing. A pattern distortion parameter can be calculated to determine the solid–to–liquid partitioning in micron sized composite particles similar to using temporal light–scattering intensity fluctuations. We use the scattering patterns during deliquescence of a NaCl crystal to prove the feasibility of the method. In addition we show that even fast processes like the efflorescence from a supersaturated solution droplet can be analyzed where temporal fluctuation analysis fails. We find that efflorescence cannot be described as a time reversed deliquescence. There is indication that during efflorescence a solid shell grows at the surface of the liquid droplet which finally collapses due to mechanical stress.

©2001 Optical Society of America

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Supplementary Material (2)

Media 1: MOV (2251 KB)     
Media 2: MOV (2006 KB)     

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Figures (7)
Fig. 1.
Fig. 1. Schematic plot of the electrodynamic balance employed for the aerosol droplet storage. Scattered laser light of two wavelengthes is monitored by two CCD-sensors, one in the optical near field for position information fed to the U DC-feedback loop for the vertical stabilisation of the particle, the other in the optical far field for pattern distortion parameter and Mie scattering analysis. An additional photo multiplier tube is used to measure the temporal light-scattering fluctuations.

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Fig. 2.
Fig. 2. Scheme for calculating the pattern distortion parameter from an individual 2D scattering pattern: liquid particle on left panel, solid particle on right panel. The concentric circles seen on both images are not an effect of the scattering of the particle but are due to scattering of the monochromatic light by the CCD’s protective glass cover (thickness: 750 µm).

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Fig. 3.
Fig. 3. Raw data of a deliquescence experiment (left) and an efflorescence experiment (right). For each experiment the DC voltage U compensating the gravitational force (panel a1 and a2), the radius r determined from the fringe pattern on CCD2 (panels b1 and b2), and the temporal fluctuation data (panels c1 and c2, full circles) and the pattern distortion parameter Δ (red crosses) are plotted versus time. Relative humidity in the deliquescence experiment: RH=75%±2%, RH is increasing with a nominal rate of 2.5·10-3%/s. For the efflorescence experiment: RH=37%±2%. RH is decreasing with a nominal rate of 9·10-3%/s. Note that the time scale is stretched by about a factor of 10 for the efflorescence experiment. As a guide to the eye, the vertical dotted lines indicate the end of the deliquescence process and the start of the efflorescence process, respectively.

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Fig. 4.
Fig. 4. (2.2 MB) Movie of the deliquescence.

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Fig. 5.
Fig. 5. (1.96 MB) Movie of the efflorescence.

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Fig. 6.
Fig. 6. Calculated median of Δ from 27 deliquescence (panel a) and 22 efflorescence processes (panel b) of the same particle as a function of ̂. For deliquescence: ̂=(U-U liquid(deliq)/(U liquid(deliq)-U solid), and for efflorescence: ̂=(U liquid(effl) - U)/(U liquid(effl)-U solid). U is the DC voltage to compensate for gravitational force, all other voltage values are constant and taken from Fig. 3. Error bands are between the second and the fourth quintile, red line see text.

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Fig. 7.
Fig. 7. Median of Δ as a function of solid-to-liquid mass partitioning. Left panel for the deliquescence, right panel for efflorescence. Error bands are between the second and the fourth quintile.

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