  We study the temporal evolution of the Na I D1 line profiles in the M3.9
flare SOL2014-06-11T21:03 UT, using high spectral resolution observations
obtained with the IBIS instrument on the Dunn Solar Telescope combined with
radiative hydrodynamic simulations. Our results show a significant increase in
line core and wing intensities during the flare. The analysis of the line
profiles from the flare ribbons reveal that the Na I D1 line has a central
reversal with excess emission in the blue wing (blue asymmetry). We combine
RADYN and RH simulations to synthesise Na I D1 line profiles of the flaring
atmosphere and find good agreement with the observations. Heating with a beam
of electrons modifies the radiation field in the flaring atmosphere and excites
electrons from the ground state $\mathrm{3s~^2S}$ to the first excited state
$\mathrm{3p~^2P}$, which in turn modifies relative population of the two
states. The change in temperature and the population density of the energy
states make the sodium line profile revert from absorption into emission.
Analysis of the simulated spectra also reveals that the Na I D1 flare profile
asymmetries are produced by the velocity gradients generated %and opacity
effects in the lower solar atmosphere.
