Investigation of the Christiansen effect in the mid-infrared region for airborne particles.

During measurements of open-path Fourier transform infrared spectra, airborne dust may be present in the infrared beam. We have investigated the feasibility of identifying and quantifying the airborne particulate matter from spectra measured in this way. Although the results showed that analysis of the particulate matter was not able to be performed from these spectra, insight into the size and wavelength dependence of the Christiansen effect at wavelengths where the particles absorb strongly was obtained. Airborne particles larger than or equal to the wavelength of the incident radiation give rise to asymmetrical features in the spectrum caused by the Christiansen effect. However, the transmittance at wavelengths where the refractive index of the particles equals that of the atmosphere never reaches 1.0 because of absorption by the particles. As the particle size becomes much smaller than the wavelength of the incident radiation, the Christiansen effect becomes less pronounced and eventually is not exhibited.