Multi-color time-resolved laser-induced incandescence for the measurement of soot and nanoparticle aerosols

Abstract

Time-resolved laser-induced incandescence (TiRe-LII) is an optical in situ diagnostics method for particle-size determination of gas-borne nanoparticles that has been established over the last decades. Recent inter-laboratory comparisons have shown that there are unresolved issues concerning measurement artifacts and noise, and discrepancies exist in reported results from similar measurement conditions. Moving from the established two-color technique (for pyrometric temperatures determination) to spectrally resolved (and multi-color) measurements reveals additional open questions. \n \nThe difficulty in LII science is that in literature, a wide range of different LII models and corresponding materials properties is available, and often, for new application cases, new models are composed from various published LII models to match the results from literature or ex situ determined quantities (e.g., particle-size distributions). In addition, signal shapes that cannot be explained by previously published LII models lead to the adjustment of the physical models and properties instead of focusing on technical issues of the LII signal acquisition. The variation in calibration, measurement and analysis procedures across the LII community motivates for the development towards standardization of LII signal acquisition and processing to increase the quality of signals that can then later be used as input for future LII model developments. \n \nThis work focuses on practical aspects of multi-color TiRe-LII as detector performance/calibration, experimental design, and signal acquisition and on the improvement of the robustness of the measurement technique using multiple detection channels. In the scope of this work, a multi-color LII device was developed along with a software solution for data acquisition and signal processing. Modular and flexible components in the setup and the analysis help to broaden the way for the application of LII on various materials systems and processes, and can build a foundation for future inter-laboratory comparisons. \n \nFor multi-color LII techniques, proper detector performance and calibration is essential to produce data that can be used as input for further processing. The detector performance is investigated for photomultiplier tubes (PMT) using pulsed and continuously operated light-emitting diodes at similar light levels as typical LII experiments. The influence of non-linear behavior during LII measurements is demonstrated for different two-color ratios and the importance of linearity as detector requirement is shown. The mathematical description for the calibration of PMTs in the context of LII is presented along with a detailed methodology for the calibration of all components within a typical LII detection system. For this purpose, the suitability of different calibration light sources is assessed and additional measurement issues that could affect the signal quality are discussed. For data acquisition and processing of LII data, a software solution was developed, following a modular approach, making it suitable for the application on various materials systems and with individual processing steps. The software is published as open-source software to allow transparency and adjustments by other researchers. In order to improve the LII signal acquisition further, a new sequential detection technique was developed that takes advantage of gated photomultiplier tubes and is capable to increase the dynamic range of the LII technique. \n \nHaving resolved many of the above-mentioned issues is an important step towards harmonization of the experimental procedure, calibration, measurement and interpretation of data.