We present a theoretical study on the atmospheric plasmas driven by nanosecond-pulse voltages without dielectric barrier covered on the electrodes by using PIC-MCC simulations. The numeric simulation results show that only one discharge event can be observed when the pulsed voltage is applied on the bare electrodes

which displays very different discharge pattern from the case that the electrodes are covered by dielectric barriers. The discharge current density almost reaches the peak value at the moment when the pulse voltage starts to decrease. During the voltage increasing phase

the sheath field keeps increasing and the sheath thickness becomes smaller

and a high energy tail of electron energy distribution function can always be observed

simultaneously

a plenty of electrons with very larger value are also produced. The space charges in the sheath region significantly enhance the electric field. The eventual extinguishment of the discharge occurs in the decreasing phase of applied voltage

but still possesses very strong electric fields in the sheath region. From the simulation data

the discharge evolution and underpinning physics of atmospheric plasmas driven by nanosecond pulsed voltage can be further understood.