  In this work, the temporal decay of electrons produced by an atmospheric
pin-to-pin nanosecond discharge operating in the spark regime was measured via
a combination of microwave Rayleigh scattering (MRS) and laser Rayleigh
scattering (LRS). Due to the initial energy deposition of the nanosecond pulse,
a variance in local gas density occurs on the timescale of electron decay.
Thus, the assumption of a constant collisional frequency is no longer
applicable when electron number data is extracted from the MRS measurements. To
recalibrate the MRS measurements throughout the electron decay period,
temporally-resolved LRS measurements of the local gas density were performed
over the event duration. Local gas density was measured to be 30% of the
ambient level during the later stages of electron decay and recovers at about 1
ms after the discharge. A shock front traveling approximately 500 m/s was
additionally observed. Coupled with plasma volume calibration via
temporally-resolved ICCD imaging, the corrected decay curves of the electron
number and electron number density are presented with a measured peak electron
number density of 4.5*10^15 cm^-3 and decay rate of ~ 0.1-0.35*10^7 s^-1. A
hybrid MRS and LRS diagnostic technique can be applied for a broad spectrum of
atmospheric-pressure microplasmas where a variation in number gas density is
expected due to an energy deposition in the discharge.
