Design of a hyperspectral Scheimpflug Raman lidar for ranging of aerosolized soot

Abstract

This thesis has investigated the potential of combining Raman spectroscopy with Sheimpflug Light Detection and Ranging (lidar) for simultaneous characterization and ranging of aerosolized soot. The first part of the project involved the implementation of conventional non-ranging Raman spectroscopy for both deposited and aerosolized soot. The second part constituted the design, modelling, and evaluation of a hyperspectral Scheimpflug Raman lidar using a Prism-Grating-Prism (PGP) design method developed by M. Aikio [1]. From the first part it was concluded that there is a clear difference between Raman spectra of deposited and aerosolized soot, motivating the use of on-line spectroscopy. This is supported by previous experiments [2]. More specifically deposited soot showed signs of increased maturation, structural order and occurrence of graphitic regions, and decreased organic content compared to aerosolized soot. Simulations in the second part showed that the design has potential to be used for simultaneous ranging and rough characterization of soot particles up to at least 50 m, albeit with lower spectral resolution than the conventional methods. The design is tailored to the short distances of a laboratory environment (<10 m), but can easily be modified to suit longer ranges. The signal strength has moreover been identified as a bottle neck for maximum detection range, both because of light distribution and atmospheric interactions. Several potential improvements have additionally been pinpointed, such as using custom prisms, choosing a detector pixel arrangement that is appropriate for the specific area of use, and adding polarizers to decrease atmospheric contributions [3]. If successfully implemented the design has potential to be used where the soot concentrations are high, such as in proximity to highways or wildfires, or in industrial exhaust flows.

Popular Abstract

Using laser light to investigate soot from afar

Burning most fuels leads to soot pollution [7] with severe negative effects on health and environment [8, 9]. This work presents a design that shows potential in detecting soot particles and examining their properties from a distance.
Soot particles has widely varying properties that are connected to how it affects the climate and our health [14]. Because of this, it is very important to be able to both detect the particles, and to see what properties they have. It is possible to study the properties using so called Raman spectroscopy, where you aim a laser at a soot sample and analyze certain parts of the light that scatters from the soot. Scientists have been able to use this to examine soot for a long time, but only for soot that has been collected to a concentrated sample. It is first in recent years that this has been successful for soot that is still in the air [2, 3]. Remarkably enough, it turns out that the structure of the particles are not the same when in the air as when collected! This means that it is very important to study soot in its airborne state. Unfortunately the experiments are limited by the tools used so far, because the soot must be located within the setup. It would of course be much more convenient to do it from a distance, even more so if you could also see how far away the soot is. Then you could for example study the soot from wildfires, heavy traffic, or perhaps even just from the ambient air? This motivates the goal of this project: to see if it is possible to design an experimental setup that can be used for Raman spectroscopy of soot and at the same time for determining the distance to the particles. The project started with replicating previous experiments on both collected and airborne soot to learn more about Raman spectroscopy of soot. It was confirmed by these experiments that there indeed was a difference, and what is even more exciting is that this was (to the knowledge of the author) the first time differences were noted when the airborne soot was examined at atmospheric pressure! An experimental setup that used a special optical concept called the Scheimpflug principle was then designed in Zemax Optic Studio, a software for optical design and simulations. The Scheimpflug principle is what makes it possible to focus on light from different distances at the same time. Judging by the simulations, the setup showed potential to be able to retrieve information on the soot properties at the same time as determining the distance to the particles. Modifying the setup slightly may however be needed to get it to work in any practical application.

• author: Öjstedt, Oskar
• supervisor: Thi Kim Cuong Le
• organization: Combustion Physics; Department of Physics
• course: PHYM01 20241
• year: 2024
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering; Physics and Astronomy
• keywords: Soot, Raman, Raman spectroscopy, Scheimpflug, lidar, hyperspectral, ranging, pollutants, particles, atmosphere.
• language: English
• id: 9164825
• date added to LUP: 2024-06-18 08:06:43
• date last changed: 2024-06-18 08:06:43