Three-dimensional density field reconstruction of vehicle exhaust plumes using 3D gas schlieren imaging sensor system for remote emission sensing applications

Emission measurement of on-road vehicles in traffic is an important step for air pollution control and, thus, the reduction of negative effects on public health. Remote emission sensing (RES) is a state-of-the-art technology to detect high emitters by monitoring thousands of vehicles in traffic continuously. State-of-the-art (SOTA) RES systems use optical techniques to measure the ratio of specific pollutants to CO2 in vehicle exhaust plumes in order to determine emission factors. Highly accurate SOTA systems use laser absorption spectroscopy for measurement of the pollutant ratio in vehicle exhaust plumes. To obtain the absolute concentration of the single pollutants in the exhaust plume, the absorption path length must be known. In this work we present a 3D gas schlieren imaging sensor (GSIS) system which allows the geometrical reconstruction of 3D density fields of vehicle exhaust plumes in RES applications. Thus, it allows us to obtain the vehicle exhaust plume size and thereby enables estimation of the absorption path length from any direction. Furthermore, it is possible to determine where the laser intersects with the exhaust plume and, thus, to assess if the measurement is valid. The 3D-GSIS system consists of an array of low-cost digital cameras operating in the range of visible light. By means of advanced image processing and tomographic reconstruction techniques, the 3D displacement and density fields of vehicle exhaust plumes can be reconstructed. For validation, we characterized the 3D-GSIS system in the lab using hot air and CO2 plumes. Moreover, the 3D density fields of on-road passing vehicles are estimated and reconstructed using the 3D-GSIS system. The 3D-GSIS system is to be combined with an advanced RES system to measure the direct concentration of pollutants in vehicle exhaust plumes in the future.