Remote sensing 101 - Cloned

TRUE Initiative’s “Remote sensing 101” explains how roadside remote sensing systems use absorption spectroscopy and a light source/detector setup to measure pollutant concentrations in exhaust plumes from passing in-use vehicles.
The page notes that each measurement lasts less than one second and lists pollutants that can be estimated (e.g., NO, NO2, CO, HC, CO2, SO2, NH3), with particulate matter measured indirectly using plume opacity.