We will 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 13" high (7U). Its time response will be less than 2 seconds and its precision (1 sigma) 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. It will provide user access through serial and/or USB port connections as well as over the Internet. A working unit will be delivered to NASA langley Research Center.