Initial Field Measurements of Atmospheric Absorption at 9–11 μm Wavelengths

Abstract

A field adapted spectrophone system employing a tuneable CO2 laser source (over wavelengths 9.2?10.8 ?m) was used to measure atmospheric gaseous and particulate absorption at an isolated desert location in the southwestern United States. Measurements were made both for ambient conditions (when aerosol particulate absorption was found to be negligible compared to that of gases) and for dusty conditions resulting from vehicular traffic. For ambient conditions the gaseous absorption coefficient was found to vary with time from expected levels upward by as much as a factor of 3. Sources which could be correlated with increased absorption are discussed. For dusty conditions the spectrophone data were compared with estimates of the absorption coefficient calculated on the basis of measured particle size distributions together with estimates of particle complex indices of refraction. Temporal variation of the absorption coefficient correlated quite closely for the two methods while the calculated values were generally higher. Sampling and calculational uncertainties are suggested as likely to be responsible for this discrepancy.