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Diesel engines are known for their reliability and fuel economy when compared to their gasoline counterparts. They are the engine of choice for application in marine vessels but are also known contributors of regulated emissions, especially oxides of nitrogen (NOx). NOx emissions have both negative health and environmental effects, which are exacerbated in port regions where marine activity is high. Recent emissions regulations have dramatically reduced engine out emissions for on-road and off-road engines by requiring new technologies but mainly apply to those newly produced. Marine diesel engines have useful lives of over 20 years. The focus of this research was on the reduction of NO x emissions produced from older in use engines. A combined oxidation and wet scrubber system was designed, constructed, and tested to determine its NOx absorption capability. The byproduct of this process was a concentrated, stored NOx in the form of weak nitric acid. A method for the onboard reduction of the nitric acid was examined. Laboratory versions of the systems were demonstrated and a data set collected. The oxidation system showed NOx conversion efficiencies between 51 and 85%. Overall NOx absorption, in the wet scrubber ranged from 16 to 60%. The acid destruction system demonstrated NOx reduction capabilities of over 98%. The experimental system was demonstrated using a 298 kW engine. Marine engines such as those used in tugs and ferries are often larger than the tested engine by a factor of five or more. Therefore, the experimental data set was utilized along with 1-D Simulink® and dimensional analysis models and guidelines from literature to appropriately size a scaled system that could be applied to an in use marine engine. The sizes of all major system components were determined for a marine engine previously retrofitted with an SCR system. It was estimated that the scaled wet scrubber system could reduce in use NOx by 55% or just over 15 tons per year. A cost analysis was performed. The combined capital and operating cost of the system for a five-year period was estimated at {dollar}725,287. The cost per reduced ton of NOx was found to be {dollar}9,444/ton. This value was lower than those set by assistance programs in Texas and California, which had the limits of {dollar}15,000 and {dollar}16,000 per ton of NOx, respectively. It was determined that such a NOx reduction system was feasible and within the acceptable ranges for NOx reduction and cost of support programs.