The UWM-FFL rack contains a custom-built laser-induced fluorescence (LIF) instrument for the detection of gas-phase formaldehyde (HCHO). Ambient air, which is sampled via a PFA teflon tube fixed on the boom of the Zeppelin, passes through a measurement cell where the formaldehyde molecules are exposed to 353 nm laser light. They absorb some of that radiation and start to emit photons (fluorescence) which are detected by a highly sensitive photomultiplier tube. Based on a calibration of the instrument the measured signal can be converted into the formaldehyde concentration in the ambient air.
For a more detailed method description, see: http://pubs.acs.org/doi/abs/10.1021/es801621f.
A description of recent improvements to the instrument is given here: http://www.sciencedirect.com/science/article/pii/S1352231011010168.