Impact of ambient O2(a1Δg) on satellite-based laser remote sensing of O2columns using absorption lines in the 1.27 µm region

Abstract

Determination of CO2 mixing ratio columns from space using Laser Absorption Spectroscopy (LAS) requires simultaneous measurements of CO2 number density columns and knowledge of the dry atmospheric surface pressure. One approach to determining the surface pressure is to make an LAS column measurement of O2 number density in the 7857.3–7921.7 cm−1 (1.27 µm) region of the O2(1Δ) state. A complicating factor in the LAS O2 measurement is the presence of a permanent but spatially variable natural source of airglow from the O2(1Δ) state. In addition, the laser radiation can induce stimulated emission from the ambient O2(1Δ) state and also cause stimulated absorption and emission from the ground state O2 molecules as the laser beam passes through the atmosphere. Finally, the upwelling surface-reflected solar radiation is an additional source of background radiation. The effects of these additional radiation sources on the LAS measurement of O2 are examined. The surface-reflected solar radiation produces the largest background at 3 orders of magnitude more intense than the laser backscatter signal, while the airglow is of the same order of magnitude as the laser backscatter. The stimulated emission from ambient O2(a1Δg) is found to be about the same order of magnitude as the laser radiation. These effects are evaluated under noon, twilight, and midnight conditions at midlatitudes, the equator, and the pole. The stimulated emission is in the same direction and in phase with the laser signal, its contamination of the LAS O2 measurement prevents a full sunlight determination of surface pressure.