The ionization current generated by two-color laser interaction with different gas atoms can produce strong terahertz (THz) emissions. The ionization potential of atoms determines the ionization rate. Thus, THz emission from different atoms varies. Particle-in-cell simulations are conducted to investigate the THz emission from He, Ne, Ar, and N. The THz emissions as a function of the laser field are different because the ionization rate and electron speed depend on the laser field and ionization potential.