The investigation has been divided into two parts. In part one, numerical investigations of the effect of humid air with different levels of humidity on gaseous emissions of a non-premixed combustion have been investigated. This part of the investigation was a feasibility study, focused on how different levels of humidity in the intake air affects the exhaust NO emission. Part two of the investigation was verification of the numerical results with a naturally aspirated engine with natural gas as the fuel. Here, we also investigated the impact of humid air intake on engine’s particulate matter (PM) emission.For the numerical investigations, the non-premixed combustion in a single cylinder was simulated using the presumed probability density function combustion model. Simulations were performed for dry as well as humid intake air for 0%, 15%, and 30% relative humidity (RH). Numerical results have shown 65% reduction in NO emission at 15% RH, when compared with NO emission of dry intake air. With 30% RH, NO emission was reduced by 93%.For the experimental investigations, a General Motors inline 4 cylinders, naturally aspirated engine with a maximum rated horsepower (HP) of 50.8 for natural gas fuel was used. The engine was connected to a water-cycled dynamometer. NOx emission was measured by a Horiba portable emission analyzer model 250 and exhaust PM was measured using a dilution tunnel in conjunction with a cyclone with teflo filters.. The experiments were carried out at four different horse powers (HP) of approximately 5, 12.5, 25. And 37.5, and three RH of ambient (30%), 45%, and 60%. Results show for each additional 15% increase in relative humidity, there was a nearly 10% reduction in NOx emission.The PM emission increases with the addition of relative humidity, especially at low HPs. With increased HP, the PM augmentation is reduced significantly and at 37.5 HP , the ratios of PM emitted at 45% and 60% relative humidity, to the corresponding ambient baseline values ( at 30% RH), were near 2.0.