An discharge sustained by an electromagnetic surface wave is investigated in order to develop an efficient nitrogen atom source. We take advantage of the flexibility of surface-wave discharges (SWDs) in terms of operating frequency to examine the influence of the field frequency (f = 13.56, 40.68, 440 and 2450 MHz) on the nitrogen atom concentration in the discharge spatial afterglow. The effects of absorbed power (up to 200 W), pressure p (1 to 8 Torr) and discharge tube diameter (4.5 and 15 mm) are also considered. The N atom concentration is determined through emission spectroscopy from the afterglow following validation by NO titration. We find that: (i) the N atom concentration increases with but saturates past a certain power, the value of which decreases with increasing f, (ii) the N atom saturation concentration is at 2450 MHz but only at ; (iii) the vibrational `temperature' of the state in the discharge varies in the same way as the N atom concentration with respect to f and ; (iv) whatever f, saturation of the N atom concentration occurs at a threshold value -600 K of the molecule rotational `temperature' in the discharge, suggesting that too high a gas temperature is the limiting factor of the N atom yield (v) the N atom concentration increases with increasing p and decreasing .