Bleed air is brought into aircraft cabins in order to maintain the quality of the air for passenger and crew health and comfort. The bleed air can be contaminated by oil due to oil seal leaks in the compressor which have been reported randomly and generated significant public interest. Previous studies have measured the particulate size distribution in the bleed air entering the cabin, but never distinguished the type and material of the particulate matter (PM). The particulates could be potentially hazardous oil droplets from the oil seal leaks, water droplets due the presence of fog generated by the cooling system, and so on. In this study we propose a novel technique using light scattering technology to distinguish between contaminant types. This technique uses size and complex index of refraction as the measure. Since each material has a distinct index of refraction, by determining the index of refraction, our proposed low-cost detector could distinguish the compound in the aerosol as well as determine the particle size simultaneously.