A method characterizes gas emissions inside a monitored area that is shaped as a polygon (e.g., a quadrilateral) and bounded by first, second, third, and fourth paths. A first optical beam transmitted along the first path is reflected to propagate along the second path. A first retroreflector retroreflects the first optical beam along the second path and the first path. The first optical beam is then detected to obtain a first absorption signal. A second optical beam transmitted along the third path is reflected to propagate along the fourth path. A second retroreflector retroreflects the second optical beam along the fourth path and the third path. The second optical beam is then detected to obtain a second absorption signal. An inversion based on the first and second absorption signals determines emission characteristics of a gas species located within the monitored area.