A Newly Designed and Constructed Instrument for Coupled Infrared Extinction and Size Distribution Measurements of Aerosols

Abstract

Although atmospheric particles are often non-spherical, Mie theory is commonly used to acquire aerosol optical depth, composition, and transport information from satellite retrievals. In the infrared (IR) region, the radiative effects of aerosols, usually modeled with Mie theory, are subtracted from satellite spectral data to determine key atmospheric and oceanic properties. To gain a better understanding of the infrared radiative effects of aerosols and the methods used to model them, an instrument has been designed to simultaneously measure infrared extinction spectra and particle size distributions obtained from a scanning mobility particle sizer (SMPS) and an aerodynamic particle sizer (APS). Infrared extinction spectra are simulated with Mie theory using the measured particle size distributions and available literature optical constants. As a result, the errors associated with using Mie theory to model the infrared extinction of mineral dust aerosol can be quantitatively examined. Initial results for this instrument are presented here. For ammonium sulfate, the Mie theory simulation is in good agreement with our measured extinction spectrum. This is in accordance with the nearly spherical shape of ammonium sulfate particles. However, for illite, an abundant clay mineral, there is poor agreement between the experimental spectrum and the Mie simulation. This result is attributed to particle shape effects.

This material is based upon work supported by the National Science Foundation under Grant No. ATM-0425989. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. The authors would like to thank Professor Scot Martin for providing us with the Mie simulation code and Professor Kuo-Ho Yang for his continued work on this code. PKH would like to thank Dr. Charles Brock for helpful thoughts and discussions on size distributions and aerosol measurements.