Applying open-path Fourier transform infrared spectroscopy for measuring aerosols

Area of Science:

• Atmospheric Science
• Spectroscopy
• Aerosol Science

Background:

• Aerosols significantly impact climate and air quality.
• Accurate measurement of aerosol size distribution is crucial for atmospheric modeling.
• Open-Path Fourier Transform Infrared Spectroscopy (OP-FTIR) offers potential for in-situ aerosol characterization.

Purpose of the Study:

• To evaluate the feasibility of using OP-FTIR for aerosol measurement.
• To develop and validate an optimization procedure for retrieving aerosol size distribution parameters.
• To assess the sensitivity of OP-FTIR measurements to noise and aerosol mixing state.

Main Methods:

• Simulations using Mie theory to generate extinction spectra for various aerosols.
• Development of an optimization algorithm to retrieve geometric mean (GM) and geometric standard deviation (GSD) of aerosol size distributions.
• Experimental validation using water aerosols generated in a controlled chamber and measured with a modified FTIR.

Main Results:

• The optimization procedure successfully reconstructed size distributions for homogeneous/internally mixed aerosols with known refractive indices.
• Results remained robust up to 10% noise levels after applying censoring criteria.
• Externally mixed aerosols showed increased sensitivity to system noise, though most ambient aerosols are internally mixed.

Conclusions:

• OP-FTIR is a feasible technique for measuring aerosol size distributions.
• The method is robust for internally mixed aerosols and can tolerate moderate noise.
• OP-FTIR can serve as an effective tool for initial aerosol screening and identifying regions for detailed measurement campaigns.