An apparatus for detecting the presence of polycyclic aromatic hydrocarbons in the effluent of a turbulent commercial combustion system for monitoring or controlling the composition of the effluent. The apparatus is a system for directyl selectively detecting the presence in the effluent high molecular weight polycyclic aromatic compounds having five or more rings by laser induced fluorescence. The system includes an illuminating source of select fluorescence excitation of wavelengths that induces substantial fluorescence by high molecular weight polycyclic aromatic hydrocarbons in the gas phase. Sampling structure is constructed to provide optical access to the effluent stream to sample the effluent stream at a select position where high molecular weight polycyclic aromatic compounds having five or more rings may be present in sufficient amounts that the fluorescence from the effluent at the select position at the excitation wavelength is predominantly from high molecular weight polycyclic aromatic compounds having five or more rings, when the high molecular weight polycyclic aromatic compounds are present in sufficient amounts. A detector detects the fluorescent signal from the illuminated effluent and an analyzer analyzes the signal to directly selectively determine the presence of the high molecular weight polycyclic aromatic hydrocarbons having five or more rings in the effluent.