The southern area of Italy is characterized by the presence of many active volcanic areas. In Pozzuoli (Naples, Italy), an urban area characterized by high volcanic risk, a gas emitting site is present. A high percentage of CO₂ is emitted, whose atmospheric concentration measurement is an important task in many environmental and scientific applications. In this paper, we describe the utilization over that area of a mobile infrared (IR) laser system, able to measure the CO₂ concentration along rectilinear atmospheric paths up to 1-km length, and the result of tomographic processing applied to retrieve a two-dimensional CO₂ concentration field. The laser system computes the link averaged concentration by processing the received IR laser radiation propagated along an open-air rectilinear link connecting the transmitter/receiver laser unit and a passive retroreflector device. A one-day measurement campaign has been made and 15 different atmospheric propagation links were considered moving the transmitter/receiver unit and some retroreflectors.