  A novel methodology for measuring gas flow from small orifices or nozzles
into vacuum is presented. It utilizes a high-intensity femtosecond laser pulse
to create a plasma within the gas plume produced by the nozzle, which is imaged
by a microscope. Calibration of the imaging system allows for the extraction of
absolute number densities. We show detection down to helium densities of
$4\times10^{16}$~cm$^{-3}$ with a spatial resolution of a few micrometer. The
technique is used to characterize the gas flow from a convergent-nozzle aerosol
injector [Struct.\ Dyn.~2, 041717 (2015)] as used in single-particle
diffractive imaging experiments at free-electron laser sources. Based on the
measured gas-density profile we estimate the scattering background signal under
typical operating conditions of single-particle imaging experiments and
estimate that fewer than 50 photons per shot can be expected on the detector.
