The method described uses differential absorption of a narrow-band tunable UV laser for detn. of trace gas concns. over a path length of 12.8 km, at near-ground level. A XeCl excimer laser serves as a ref. beam to compensate for turbulence-induced intensity losses. After suitable statistical averaging the method is capable of detecting differential absorptions as low as 10-4, allowing simultaneous detection of SO2, CH2O, and OH radicals at l .apprx.308 nm.