<title>Comparisons of airborne CO2 DIAL measurements to high-resolution atmospheric transmission calculations</title>

Abstract

The Air Force Research Laboratory has developed and operated an airborne CO2 DIAL system for chemical detection of trace gases in the atmosphere'. This system, designated Laser Atmospheric Remote Sensing (LARS), is used for chemical detection of trace gases in the column content, topographical backscatter mode wherein detection of trace chemicals is performed by ratioing the backscattered signal strengths of combinations of transmitted CO2 laser lines absorbed by the trace chemical(s) to the backscattered signal produced by non-absorbed laser lines. Identification and quantification of trace chemical signatures sampled at multiple discrete CO2 laser frequencies is dependent upon isolation of the chemical signature from the absorption spectrum of the multi-kilometer atmospheric slant path over which measurements are made. Ambient atmospheric concentrations of C02, H20, and 03 contribute discrete line absorptions in the 9 im —11 tm spectral region in which the LARS system operates. The detailed form of the atmospheric absorption spectrum depends upon the concentration of each absorber and its variation with altitude along the slant measurement path. In addition to discrete atmospheric line absorption that must be accounted for in the DIAL measurements, a weaker continuum (smooth, slowly wavelength-varying) absorption due to water vapor must also be taken into account.