Institute for Atmospheric and Climate Science

The ALCATRAS experiment

Ammonium sulfate particle
Ammonium sulfate particle

Atmospherically relevant laser light-scattering of a trapped aerosol ;-).

The picture to the left shows a microscopic image of a solid ammonium sulfate particle levitated in our electrodynamic balance. You will find more information about the experiment, its scientific background, some pictures taken in the laboratory, some information about the people involved and a list of references below.

Scientific background

Why do we perform experiments on single, levitated particles in a laboratory experiment when we want to learn something about the role of aerosol particles in the atmosphere? What is the advantage of having the particles levitated rather than placing small particles on a substrate or doing experiments with bulk samples? Some answers can be found here... more »

Experimental setup

A schematic of the experimental system is shown in the figure below (click for a full-featured animation). To suspend a micron sized charged particle in air within a small region an electrodynamic balance is used. Here, a Millikan type DC field acting against the effect of gravitation is superimposed with a non uniform alternating field which has a focusing effect. The charged particle is held in space in the null point of the non uniform field. The DC field is applied between two endcap electrodes and the alternating field to the two electrodes located above and below the midplane.


The balance is placed in a three wall glass chamber, with a cooling liquid flowing through the inner walls and an isolation vacuum between the outer walls. A constant flow of an appropriate gas mixture is pumped continuously through the chamber, the total pressure can be varied between 50 and 1000 hPa. The temperature can be varied between 330 and 160 K with a stability better than 100 mK and an accuracy of +- 0.5 K. Three collinear laser beams illuminate the particle from below (HeNe @ 633 nm, Ar+ @ 488 nm, and a Tunable-Diode-Laser @ 763-787 nm)... more »

Impressions from the laboratory

How does the experiment looks like in the laboratory? We show some pictures from our lab to give you a flavor of how the experiment is build. As usual with scientific equipment it is not easy to recognize the use of the single parts, but we do provide some information on it... more »

Published results of the ALCATRAS experiment

Vapor pressures of substituted polycarboxylic acids are much lower than previously reported
Andy J. Huisman, Ulrich K. Krieger, Andreas Zuend, Claudia Marcolli, and Thomas Peter
Atmospheric Chemistry and Physics, 13, 6647-6662 (2013)
Abstract full text

... more »

People involved in the ALCATRAS experiment

Since we started designing the experiment about 10 years ago, a number of people have been involved.  more »


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