Investigation on the LIF spectrum superposition of gas-phase PAH mixtures at elevated temperatures: potential for the analysis of PAH LIF spectra in sooting flames

Laser-induced fluorescence (LIF) technique has been widely used to measure the polycyclic aromatic hydrocarbons (PAHs) in sooting flames, but the assignment of the LIF signals measured in flames to specific PAH species is hard to obtain. Investigation on the LIF spectra of gas-phase PAH mixtures could be helpful to analyze LIF spectra measured in flames for more detailed PAH information. In this study, the LIF spectra of gas-phase PAH mixtures containing naphthalene, phenanthrene, pyrene, fluoranthene were experimentally investigated in an optical cell at 673 K and 1073 K. The experimental results indicated that the integral fluorescence intensity of each PAH was in direct proportion to its mole fraction. Moreover, the LIF spectra of PAH mixtures were linearly superposed from the spectra of their components weighted using mole fractions. The component concentrations of the PAH mixtures were able to be fitted from the LIF spectra using multiple linear regression method, and most of the relative errors were less than 5%. In the light of these results from gas-phase PAHs, the LIF spectra of flame-formed PAHs measured in a laminar premixed ethylene flame were discussed. Considering the temperature effects on LIF spectra and simulated PAH concentrations in the flame, the lighter PAHs will contribute to the fluorescence signals shorter than 450 nm measured in the flame. Using the multiple linear regression method established in this study, the relative ratios of naphthalene, phenanthrene and pyrene can be fitted from the LIF spectrum of the flame. In this way, more selective information of these lighter PAHs can be obtained from the LIF spectrum measured in flames. On the other hand, fluorescence signals in the flame longer than 450 nm are possibly emitted from the PAHs with five-membered-ring structures or (and) nanoparticles and nascent soot particles, which are still ambiguous and needs to be further investigated in future works.