Shape Effects in the Infrared Spectrum of Ammonia Aerosols

Abstract

Fourier transform infrared spectroscopy has been used to study homogeneously nucleated aerosols of ammonia formed in a low-temperature absorption cell, Spectra are reported that cover the frequency range from 700 to 4000 cm-1. The observed spectra are compared with those calculated using Mie scattering theory and the discrete dipole approximation, based upon complex refractive index data taken from the literature. The results show a pronounced shape effect in the 9.4-μm band, most likely due to the aggregation of smaller particles into chains. These observations confirm earlier predictions that a range of particle shapes can result in dilution of the peak intensity of the ammonia aerosol spectrum, particularly near 9.4 μm. The implication of this in the remote sensing of ammonia aerosol in planetary atmospheres is discussed.