There is tremendous interest in the measurement of nanoparticles, whether intentionally produced as a commercial product, generated unintentionally as the by-product of a process (such as soot from combustion), or occurring naturally. Laser-induced incandescence (LII), which was first applied to measure the concentration of soot in flames, has been shown to be suitable for application to a much wider range of refractory and metallic nanoparticles suspended in gas.

The soot particles, like many other nanoparticles, are made up of spherical primary particles that aggregate into structures whose size distribution can be described by fractal-based theories. There is increasing interest in developing the LII technique to determine the size of the nanoparticles as well as their concentration. Initially the mean primary particle diameter was estimated from LII intensity decays, but interest has now shifted to determining the distributions of the primary particle diameter and the effect of fractal aggregate size on the measured cooling rates.

This paper presents new physical models for characterizing fractal structured aggregated nanoparticles, based on interpretation of the nanoscale heat transfer by conduction for particles that have been rapidly superheated by a pulsed laser. Conventionally, LII models were developed for isolated single spherical primary particles in which the effect of particle aggregation on the conduction heat loss rate was neglected. It was also shown that the ability to extract meaningful results about the distribution of the primary particle diameter distribution is dependent on the surrounding gas temperature as well as the degree of aggregation.

LII involves rapidly heating nanoparticles with a pulsed laser beam. The particles are typically heated to temperatures in the 3000–4500 K range in a few nanoseconds. After the laser pulse, the particles cool to ambient temperatures in a time of the order of one microsecond. For a large fraction of this period, the cooling is primarily due to conduction heat transfer to the surrounding gas.

For isolated spherical primary particles of a uniform diameter, this cooling should occur as a single exponential decay of the particle temperature. However, as stated above, in reality there is a distribution of the diameters of primary particles, and these particles form fractal aggregate structures, also with a distribution in the size of the aggregates. Both aggregation and the distribution of the diameter of the primary particles cause the decay of particle temperature to deviate from a single exponential.

Our paper found that LII can be used to determine the primary particle diameter only under certain conditions. Outside the range of its applicability, the particle temperature decay curve in LII alone cannot differentiate a cluster of small particles from a single large particle.

Our group became involved in the development of LII techniques about 10 years ago and pioneered a method of determining soot concentration based on 2-color pyrometry to determine soot temperature, and absolute LII intensity determination to infer soot concentration.

Most early LII measurements consisted of measuring a single LII wavelength and inferring soot morphology, temperature, and soot optical and thermodynamic parameters from this single intensity decay. By routinely measuring soot temperature directly we were more clearly able to see the limitations in the predictions of existing models of the LII heating and cooling cycle.

As a result our group became very active in developing nanoscale heating and cooling models of the LII process and, in particular, the effect of aggregation of the soot primary particles on the cooling process. This was a direct outcome of our experimental LII work.

  Are there any social or political implications for your research?

Our research has profound implications for monitoring, in real time, the concentration and morphology of particulates emitted from various combustion devices such as power plants and vehicles. Epidemiological studies suggest that the morphology of fine and ultrafine particles is linked to their causation of respiratory, cardiac, and other diseases. Our research is potentially able to provide the information required to identify the particles that are likely to contribute to such adverse health effects.

Similarly, our research may be applied to characterize and monitor black carbon nanoparticles in the atmosphere. Black carbon has been recently recognized as a serious contributor to global warming, with a forcing factor estimated to be second only to CO₂.