A pilot-scale study of sodium sulfide (Na2S) aqueous solutions for NO2 absorption was conducted to obtain field data for process design and system operations. Considered parameters include Na2S concentrations in the scrubbing solutions, gas mass flow rate, liquid mass flow rate, liquid/gas ratio (L/G), pH, and gas residence time. The effects of these parameters on NO2 absorption in a packed tower were investigated thoroughly. The height of transfer unit of the system was deducted as a function of the tested parameters that can be used for designing a NOx scrubbing system. The absorption of NO2 is not sensitive to the inlet NO2 concentration. However, as expected, the NO2 absorption increases with the increase of L/G ratio. An L/G greater than 2.5 is essential to a practical NO2 scrubbing application. The pilot test suggested that the scrubbing solution is with Na2S concentration greater than 0.0015 M at pH 9. The solution effectively removed more than 90% of the incoming NO2 in the five-foot height packed bed scrubber. The study also indicates that a gas residence time of 4.5 s is essential to achieve more than 80% NO2 absorption. © 2011 American Institute of Chemical Engineers Environ Prog, 2011.