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A bipolar high-voltage pulse with 20-ns rising time is employed to generate diffuse dielectric-barrier-discharge plasma with very low gas temperature in both nitrogen and air using a wire-plate electrode configuration at atmospheric pressure. Both the diffuse nanosecond pulsed discharge images and the optical emission spectra are recorded successfully. The gas temperatures of the nitrogen and air discharge plasmas are determined at 400 ± 5 K and 320 ± 5 K. The effects of discharge gap distance, pulse peak voltage, and pulse repetition rate on the emission intensities of NO (A2Σ→ X2Π), OH (A2Σ→ X2Π, 0 - 0) , N2 (C3Πu → B3Πg) , and N2+(B2Σu+ → X2 Σg+, 0 - 0, 391.4 nm) in both nitrogen and air are investigated, respectively. The effects of O2 and Ar on discharge images and the emission intensities in nitrogen are obtained. It is found that the emission intensities rise with increasing pulse peak voltage, pulse repetition rate, and the concentration of argon but decrease with increasing the discharge gap distance and the concentration of oxygen. The main physicochemical processes involved are discussed.