  The technique of dual modulation Faraday rotation spectroscopy has been
applied for near shot-noise limited detection of HO2 at the exit of an
atmospheric pressure flow reactor. This was achieved by combining direct
current modulation at 51 kHz of an external cavity quantum cascade laser system
with 610 Hz modulation of the magnetic field generated by a Helmholtz coil. The
DM-FRS measurement had a 1.8 times better signal-to-noise ratio than an AC-FRS
measurement acquired under identical flow reactor conditions. Harmonic
detection of the FRS signal also eliminated the substantial DC-offset
associated with electromagnetic intereference pick-up from the Helmholtz coils
that is observed in the AC-FRS spectrum. A noise equivalent angle of 4x10^(-9)
rad Hz^(-1/2) was observed for the DM-FRS measurement, and this corresponds to
a 3 sigma detection limit of 0.2 ppmv Hz^(-1/2).
