| Summary: | 碩士 === 國立臺灣大學 === 職業醫學與工業衛生研究所 === 94 === Objective: To investigate the feasibility of retrieving aerosol size distribution information from OP-FTIR measurements.
Method: We first obtained the complex refractive index of water, ammonium nitrate and ammonium sulfate from published literatures. The extinction spectra were then simulated with various size distribution (geometric mean ranged from 2 to 10μm; geometric standard deviation ranged from 1.1 to 2.5) based on the Mie theory. An optimization algorithm was developed to retrieve the geometric mean and standard deviation of the aerosols size distribution from the spectra, assuming the complex refractive index is known. We also added 1%, 4%, 7% and 10% noise levels to the simulated spectra for sensitivity analysis. We further collected the extinction spectra with a FTIR system for water aerosols in a controlled environment to verify the simulation study.
Results: For all three compounds, when the geometric standard deviation was the same, the reconstruction results for larger particles were better than for the smaller particles. The sensitivity analysis results showed that when the noise level was less than 4% and geometric mean was large (>3.5μm), the fit was good with the R2 value between the input and reconstructed spectra ( ) greater than 0.9. The differences between the input and retrieved geometric mean and geometric standard deviation were less than 10% under the same condition. When the noise level was equal to 10%, the reconstruction results for a narrow aerosol size distribution were better than for a wide distribution. In the FTIR-aerosol experiments, two unique extinction spectra of water aerosols were collected. The ranged from 0.8 to 0.9 and the geometric mean (geometric standard deviation) ranged from 1 (1.7) to 4 (2) μm. Similar to the findings in the simulation study, the reconstructed spectra fit better to the input spectra for larger particles than for the small particles.
Conclusion: The reconstruction algorithm performs reasonable well for particles larger than 3.5μm. It is also less sensitive to the spectral noise for the noise levels up to 10%. Our study results demonstrate that it is feasible to retrieve the aerosol size distribution from the OP-FTIR measurements.
Key words: particulate matter, OP-FTIR, extinction spectrum
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