This technology uses infrared cameras with good spectral resolution and feeding their signal to a computer performing spectral analysis and differential spectral analysis. Although the measurement tools are quite different, it shares the same intellectual approach to analysing the chemical composition of distant stars through spectroscopes.
The use of IR range is related to getting good and predictable illumination of the analysed volume.
You can find here
http://www.geo.mtu.edu/~raman/papers2/PrataNatHaz08.pdf
a scientific paper cosigned by Mr
Prata and explaining different detection techniques of volcanic clouds.
(read particularly pages 4 to 9)
If you want to know more about spectral analysis, googling on "Fourier transform spectroscopy" gathers useful results.
Luc