A pilot-scale study of sodium sulfide (Na 2 S) aqueous solutions for NO 2 absorption was conducted to obtain field data for process design and system operations. Considered parameters include Na 2 S 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 NO 2 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 NO x scrubbing system. The absorption of NO 2 is not sensitive to the inlet NO 2 concentration. However, as expected, the NO 2 absorption increases with the increase of L/G ratio. An L/G greater than 2.5 is essential to a practical NO 2 scrubbing application. The pilot test suggested that the scrubbing solution is with Na 2 S concentration greater than 0.0015 M at pH 9. The solution effectively removed more than 90% of the incoming NO 2 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% NO 2 absorption. 2011 American Institute of Chemical Engineers Environ Prog, 30: 632-639, 2011