The article presents issues related to the assessment of concentrations of harmful substances in the exhaust gas cloud behind a compression-ignition passenger vehicle. The introduction discusses the subject of optical measurement methods and refers to Lambert Beer's law describing the phenomenon of absorption of electromagnetic radiation when passing through a partially absorbing and scattering medium. The vehicle used during the tests was characterized by a four-cylinder drive unit with a maximum power of 110 kW at 4000 rpm and a maximum torque of 320 Nm at 2000 rpm. The vehicle was type-approved in accordance with the Euro 4 standard. Exhaust gas dispersion tests behind the vehicle were carried out both in stationary conditions (a specially prepared laboratory stand) and in real operating conditions. PEMS testing equipment was used for this type of measurements. During the measurements, concentrations of harmful exhaust gas compounds were analyzed in relation to the distance of the measuring probe from the exhaust system. In stationary conditions, the influence of the engine speed on the dispersion of pollutants was also studied. The tests carried out show that the concentrations obtained behind a moving vehicle significantly decrease with the distance of the measuring probe, and their dispersion in most cases is much smaller than in the case of stationary tests. This is the basis for recognizing that thanks to this, it is possible to analyze the concentrations obtained and conduct tests using the emission gate.