|Abstract:||Time-resolved, absolute HO$_2$ number density in diluted H$_2$-O$_2$-Ar, CH$_4$-O$_2$-Ar, and C$_2$H$_4$-O$_2$-Ar mixtures excited by a repetitive ns pulse discharge in a heated plasma flow reactor is measured by Cavity Ringdown Spectroscopy (CRDS). The experimental results are obtained at T=300-600 K and P=130 Torr, both during the discharge pulse burst and in the afterglow. The HO$_2$ number density is inferred from the CRDS data using a spectral model exhibiting good agreement with previous measurements of absolute HO$_2$ absorption cross sections. In the room-temperature H$_2$-O$_2$ mixture, as well as in CH$_4$-O$_2$ and C$_2$H$_4$-O$_2$ mixtures over the entire temperature range studied, HO$_2$ is generated only during the discharge burst and decays in the afterglow. However, in the H$_2$-O$_2$ mixture at elevated temperatures, T=400-600 K, HO$_2$ persists in the afterglow up to 10 ms after the discharge burst, comparable with the flow residence time in the reactor. Comparison with kinetic modeling shows that the sustained reactivity after the source of radicals is turned off is due to a chain propagation / hydrogen oxidation process, which dominates the radical recombination reactions. The kinetic modeling predictions are in good agreement with the relative HO$_2$ number density measured in all three mixtures, although the model unpredicts the absolute number densities in H$_2$-O$_2$ and CH$_4$-O$_2$ at T=400-600 K by up to a factor of two. Detection of the sustained low-temperature reactivity in H$_2$-O$_2$, initiated by the radical generation in the plasma, suggests that the plasma excitation may also affect kinetics of oxidation and reforming of fuels exhibiting low-temperature chemistry below hot ignition point. Development of a narrow linewidth optical parametric oscillator (OPO) for high-resolution, time-resolved HO$_2$ measurements during n-butane oxidation in a heated plasma flow reactor is underway.