  We report on time-resolved measurements of electron number density by
continuous-wave laser absorption in a low-energy nanosecond-scale
laser-produced spark in atmospheric pressure air. Laser absorption is a result
of free-free and bound-free electron excitation, with the absorption
coefficient modeled and evaluated using estimates of the time-variation in
electron temperature and probe laser absorption path length. Plasma electron
number densities are determined to be as high as $n_\text{e}=7\times10^{19}$
cm$^{-3}$, and decay to $1/e$ of their peak values over a period of about 50 ns
following plasma formation using a 20 mJ, 10 ns pulse width frequency-doubled
Nd:YAG laser. The measured plasma densities at later times are shown to be in
reasonable agreement with Stark broadening measurements of the
3s[$^5S{^o}$]-3p[$^5P$] electronic transition in atomic oxygen at 777 nm. This
study provides support for the use of such continuous wave laser absorption for
time resolved electron density measurements in low energy spark discharges in
air, provided that an estimate of the electron temperature and laser path
length can be made by accompanying diagnostics.
