Two identical Fourier transform infrared (FT-IR) field monitoring syst
ems have been used to investigate differences in apparent plume concen
trations as a function of height. The two systems also have been used
to monitor the same plume simultaneously at the same height to estimat
e the precision that can be expected. The data collected suggest that
IR beam height may play a significant role in the measured concentrati
ons of plume constituents.