Photo-Thermal Interferometry (PTI) Particulate Absorption Monitor

Understanding the role that aerosols play in atmospheric radiative forcing is currently the major focus of the Department of Energy (DOE) Atmospheric Sciences Program- Aerosol particles affect the radiative balance of the earth directly, by scattering and absorbing solar and terrestrial radiation, and indirectly, by acting as cloud condensation nuclei- It is now recognized that the atmospheric loading of aerosols generated through human activities, especially combustion, can exert an influence on the earth’s radiation budget comparable in magnitude with greenhouse gases- This proposal addresses the specific issue of accurate measurement of aerosol absorption- We propose to design, build and test an optical absorption monitor based on the use of photothermal interferometry monitor- In this technique, absorbing particles are heated by a modulated excitation laser producing a small temperature change in the surrounding air- This temporary change in temperature is recorded as a change in the refractive index as measured by an interferometer- A major advantage of this technique is that the modulation frequency of the excitation laser can be varied so as to avoid acoustic noise, an issue for instruments deployed on aircraft and ground-based vehicles-