We propose to design, build and test a multi-color (red, green, blue) particle optical extinction monitor suitable for use in either land or airborne applications. The monitor will also contain a fourth measurement cell to allow for real-time subtraction of interferences caused by gas phase interferents such as nitrogen dioxide. The instrument will fit into a rack-mountable box that less than 18" high (10U). Its time response will be less than 2 seconds and its precision better than 1 inverse megameter in 1 second. The accuracy of the measurements will be within ?5% of the values obtained using measurements of polystyrene latex spheres.