Application of Computer-Simulated Infrared Solar Spectra to the Detection of Atmospheric Fluorocarbon-12

Abstract

Fluorocarbon-12 (CCl₂F₂) has been detected in the atmosphere by observation of infrared solar spectra in two spectral regions of the 8 to 12 µm atmospheric transmission window. The solar spectra were compared with laboratory spectra of CCl₂F₂ and also with computer synthesized spectra which were generated from the most recent Air Force Cambridge Research Laboratories line parameters listing. Water vapor lines interfere with strong Q-branch absorption features of CCl₂F₂ in the spectral region near 923 cm⁻¹. The region near 1161 cm⁻¹ is free from water vapor absorption and has the strongest Q-branch feature of CCl₂F₂. Many laboratory spectra of CCl₂F₂ were studied and this absorption feature was found to be independent of N₂ broadening pressure. No change in the half-width was found as the N₂ pressure was varied from 0 to 700 Torr. Furthermore, assuming a Lorentz line shape for this CCl₂F₂ absorption feature, an intensity of 3.8 × 10¹⁸ cm·molecule⁻¹ and a half-width of 0.026 cm⁻¹ were found. Using these parameters, synthetic solar spectra were computed which included the CCl₂F₂ absorption feature. Results of this computer generation of solar spectra indicate a ground level concentration of 110 parts per trillion of CCl²F² in our solar spectra.