The presence of an aerosol or cloud (3) of a toxic substance is detected by producing an aerosol (17, 5) of ligands (13) to the target substance in a region of sky that contains the target substance, permitting molecules the target substance to bind to the ligand, directing light (7, 21) of a first frequency into the revised aerosol; and then inspecting (9, 25) the revised aerosol for emissions of light of a second frequency. When light of the second frequency exists an alarm (15, 31) is initiated.