To enhance the performance of activated carbon (AC) for elemental mercury removal, a kind of AC was modified by nonthermal plasma (NTP) and the effects of the modification of the atmosphere (N-2, O-2, air, and HCl) were investigated by adsorption and desorption experiments. The physical and chemical properties of original and modified ACs were characterized by Brunauer-Emmett-Teller, scanning electron microscopy with energy-dispersive spectroscopy, and X-ray photoelectron spectroscopy (XPS). The results showed that better mercury removal performance of ACs was obtained after modifying by NTP in air, O-2, and HCl. The mercury removal efficiency of AC-air, AC-O-2, and AC HCl was obviously enhanced compared to the raw AC and AC-N-2 attributed to the large increase of oxygen-containing functional groups [C=O and C(O)-O C] occurring on the AC surface. AC treated in HCl could form C-Cl groups, which were beneficial to improve its oxidizing ability. In addition, stronger etching and cracking on the AC surface during NTP modification in an O, or a HCl atmosphere were found to decrease its specific surface area and micropore volume, resulting in an adverse effect on mercury removal. It was found that the desorption peaks at 290 and 340 degrees C of the adsorbed AC samples corresponded to carbonyl groups (C=O) and ester groups [C(O)-O-C], respectively, which were verified by combining the results of XPS and temperature-programmed desorption experiments.