This parameter allows extra line broadening to be introduced into the calculation of the synthetic spectrum. The value is in km s. This quantity enters into the calculation of the line profile function, which is used by the line opacity and emissivity. The line profile function is a Voigt profile, which resembles a Gaussian in the Doppler core. The turbulent velocity then enters into the `sigma' of the Gaussian. The Gaussian 'sigma' in velocity units is where is the ion thermal speed.