Aerosols are ubiquitous and there is tremendous interest in characterizing their physical form and light-scattering behavior. For example, understanding climate change is perhaps the foremost scientific challenge for the world's society, a major component of which is to quantify how aerosols contribute to the solar radiative forcing of the atmosphere. According to the 2013 Intergovernmental Panel on Climate Change (IPCC), the estimated aerosol forcing is comparable to all other factors, including greenhouse gasses. Yet, the uncertainty in aerosol forcing is nearly as large as the forcing value itself, making their influence on climate the least understood of effects. What is missing in this regard are accurate in situ observations of the real atmospheric particles, and with such knowledge, the aerosol component of climate models could be improved. The development of new computational and experimental techniques to enable these observations is the primary objective of this research project.
Traditional microscopy may seem ideal for particle characterization, especially for large particles, but it often gives an inaccurate picture of the true particle-morphology due to the inherent need to collect the particles. In such cases, particles can fragment, aggregate, or become distorted; all of which result in an inaccurate picture of the aerosolized morphology. In some cases, this morphology may be completely destroyed as with liquid and frozen particles. Thus, only an in situ imaging technique will suffice and electromagnetic scattering offers this capability. The way a particle scatters incident light into other directions constitutes the scattering pattern and this pattern depends on the particle's morphology, composition, and orientation. As such, proper interpretation of a measured pattern can, in principle, be useful to infer the desired physical characteristics of an unknown particle. The problem is that no general unambiguous relationship between a measured pattern and a particle's characteristics exists, a difficulty that is known as the inverse problem; this problem has been partly solved by us [here].
A large part of our research effort has been aimed at overcoming this inverse problem using digital holography. The technique can provide an unambiguous image of a particle while retaining all of the in situ capabilities of conventional electromagnetic scattering. Fundamentally, holography is a two-step process: First, a particle is illuminated with coherent light and the intensity pattern resulting from the interference of this light with that scattered by the particle is recorded. This pattern constitutes the hologram, from which an image of the particle is reconstructed via a Fourier transform. Traditionally, holograms are recorded with photographic film due to the film’s high resolution, which is required to capture the finest fringes in the interference pattern. The subsequent chemical development of the film is costly and time consuming, and this greatly limits the practical utility of the technique. For this reason optoelectronic sensors, such as charged coupled device (CCD) detectors, are now widely used to record holograms. The resulting digital hologram can then be computationally processed, rather than chemically, to reconstruct an image of the particle. Holography is not only useful for imaging. Our group has recently developed a new theory that relates optical observables to a particle's hologram; see [here] for more information. These include the scattering pattern and total cross section, which are important to quantify a particle's role in radiative forcing.
Figure 1: An early experimental apparatus we built to capture holographic images of free-flowing aerosol particles. Details of this experiment can be found in this [paper].
Shown above in Fig. 1 is an early experimental apparatus we built to capture holographic images of free-flowing aerosol particles. The arrangement involves two subsystems: aerosol-particle sensing and hologram recording. An aerosol stream is delivered via a nozzle to the measurement volume where an optical trigger, consisting of crossed (red) diode-laser beams, senses the presence of a particle. These lasers have different wavelengths of 635 and 670 nm and intersect near the nozzle outlet. When a flowing particle enters this intersection, it scatters both wavelengths of light simultaneously. That light is then received by two photomultiplier (PMT) modules, each sensitive to only one of the two wavelengths. A series of signal-analysis units determines if the signals produced by the PMT modules are coincident. If so, this indicates the presence of a particle at the trigger laser-beam intersection and a fire signal is sent to a pulsed (green) laser, which forms the hologram. An example hologram is shown in the inset. Several examples of sand aerosol particles are shown in Fig. 2 below with further information in this [paper].
Figure 2: Saharan and Tunisian sand particles from the experimental arrangement in Fig. 1. Images (a) and (b) show the aerosol particles' holograms and the corresponding reconstructed images are presented in (c) and (d).