A cavity ring down system is optimized to precisely measure trace gases or particles in an air sample by using time sampling detection and multiple-sample averaging resulting in a high signal-to-noise ratio. In one embodiment, a cavity ring down system is programmed to measure the rise time and the fall time of the light level in an optical cavity. The cavity ring down system is programmed to integrate a plurality of sample portions during a rise time and a plurality of sample portions during a fall time (in alternate intervals) to obtain a time constant with no sample present and a time constant with sample present. The measurements are used to calculate trace gases in the air sample.