Our Los Alamos electrodynamic balance facility is now operational. This allows us to perform high precision spectrally resolved optical and microphysical measurements of small particles (50 nm to 100 μm) in a carefully controlled environment. Our balance is an extension of the classical Milliken oil drop experiment. The mass of the particle is measured by the dc electric field necessary to balance the gravitational force. Light scattering and Mie theory allow a precise determination of particle size and refractive index simultaneously as shown for a polystyrene particle the figure below.

Left panel shows our electrodynamic balance and the right panel shows measured Mie profile for a 6 micron polystyrene bead of known refractive index, the dotted line uses this to fit the measured scattering and reproduces the size exactly.

Our balance, with its isolated particle environment,is ideally suited for water uptake and nucleation studies and is equipped with relative humidity and temperature controls. We have also developed particle modulation and lock-in detection to measure optical absorption by a single aerosol particle using photo-acoustic spectroscopy. We have begun experiments on levitated spherical glassy carbon (0.1-10 mm) particles. We plan to measure water uptake by a variety of soot substrates, with simultaneous measurements of changes in optical properties. This will allow us to validate the mixing state models for black carbon. Mixtures of black carbon with salt and sulfates will also be studied later in the project.