Humidity Effects on Thermal Atmospheric Transmissions: Study of Potential Effects of Small Hygroscopic Aerosol Particles in the Longwave Infrared Region

This study investigates small aerosol particles as a source of an imaging phenomenon observed in thermal remote sensing data. The phenomenon is characterized by degraded atmospheric transmissions in the thermal infrared while high transmissions (clear conditions) are observed in the visible wave-length region. This atmospheric anomaly has been linked to conditions of high environmental humidity. A hypothesis attributes the cause of this phenomenon to small hygroscopic particles (under the 200 nm diameter) which weakly scatter in the visible region, but may have (for high particle concentrations) sufficient absorption effects in the Longwave Infrared (LWIR). We describe an experiment to test this hypothesis. The method takes a simple, but novel approach of using a suite of cameras to image an aerosol stream from a Harvard Ultrafine Concentrated Ambient Particle System (HUCAPS). Used primarily for toxicology studies of environmental aerosols, the HUCAPS has the ability to control and vary properties of humidity, temperature, particle size distribution, and number density of aerosol particle stream concentrated by this system. This gives a unique opportunity to image a controlled and well characterized plume of very fine aerosol particles and determine if any significant optical effects can be observed in the LWIR region.