An acoustic trap was designed and constructed to investigate, on a microsca
le, physicochemical processes relevant to the troposphere, mainly focusing
on the temperature range below 0 degreesC. Droplets ranging from 0.5 nL to
4 muL (0.1 to 2 min in diameter) were introduced into the cooled reaction c
hamber by means of a piezo-driven micro pump with a reproducibility better
than 5%. Uptake of H2O2 from the gas phase by the levitated droplet was mea
sured and calibrated by in-situ chemiluminescence. Freezing of stably posit
ioned droplets was observed and documented by means of a microscope and a v
ideo camera; this demonstrated the usefulness of the technique for simulati
on and investigation of cloud processes. Ex-situ microanalysis of sub-micro
liter droplets by use of a fiber optic luminometer was also shown to be a s
uitable means of investigating relevant physicochemical processes on a micr
o scale.