Retrieving the translational diffusion coefficient of water from experiments on single levitated aerosol droplets.

The time-dependent growth and shrinkage of aqueous aerosol particles trapped in an electrodynamic balance exposed to changes in relative humidity (RH) depend on the translational diffusion coefficient of water (DH2O). Resonances in the Mie scattering patterns of the illuminated micrometre-sized droplets are used to follow the compositional evolution through stepwise changes in RH. Under conditions where the diffusion of water molecules becomes sufficiently slow, e.g. in the highly viscous or even glassy regime, the concentration and temperature dependent values of DH2O can be determined iteratively by comparing the observed shifts in the Mie resonant wavelengths with predicted shifts from a diffusion model of a multi-layered sphere. It is shown that condensation and evaporation of water vapour from or to highly viscous or glassy droplets follow different kinetic regimes, a result that is consistent with previous studies of adsorption and desorption on glassy surfaces.

A laser induced fluorescence-based instrument for in-situ measurements of atmospheric formaldehyde.

Direct, in situ detection of gas phase formaldehyde (HCHO) via laser induced fluorescence in a White-type multipass cell is demonstrated with a (3sigma) limit of detection of approximately 0.051 parts per billion by volume in a 1 s sampling time. Calibration is performed in two ways: using permeation tubes and with air bubbled through an aqueous solution of HCHO. The concentration of HCHO output from the bubbler is measured by cavity ring-down spectroscopy. Measurement of ambient HCHO is carried out at the University of Wisconsin, Madison for a period of several days.

Laser-induced phosphorescence for the in situ detection of glyoxal at part per trillion mixing ratios.

Glyoxal is a molecule of emerging importance to the atmospheric chemistry community because of its role in aerosol formation and utility as an indicator for oxidative chemistry. We describe the Madison laser-induced phosphorescence (LIP) instrument, an instrument based on LIP for direct, in situ measurement of gas-phase glyoxal with a S/N = 3 limit of detection (LOD) of 18 ppt(v)/min, with planned upgrades to reduce the LOD to 5 ppt(v)/min. By employing this technique, we have built an instrument with exceptional in situ limits of detection, tremendous selectivity, and the considerable advantage of direct, fast measurements that requires neither derivatization nor ex situ analysis. The instrument is equally well-suited for laboratory and field measurements. It was deployed for the first time to the BEARPEX 2007 field campaign in Georgetown, CA, producing nearly one month of continuous data with mixing ratios ranging from 20 to 250 ppt(v) glyoxal. To the authors' knowledge, this represents the first use of LIP for a field measurement.