Simultaneous observation of tropospheric O3 and volcanic SO2 using a three-wavelength differential absorption lidar based on a Nd:YAG laser and a CO2 Raman cell.

Abstract

We present a three-wavelength differential absorption lidar (DIAL) method for the simultaneous retrieval of tropospheric ozone (O₃) and sulfur dioxide (SO₂). The system employs a single Nd:YAG laser and a carbon dioxide (CO₂) Raman cell to generate three ultraviolet laser wavelengths at 276, 287, and 299 nm. The number densities of O₃ and SO₂ are simultaneously retrieved by analytically solving coupled DIAL equations using the three-wavelength observation data. In contrast to conventional two-wavelength analyses that are sensitive to the choice of wavelength pairs, the proposed three-wavelength approach yields consistent retrieval results regardless of the wavelength combinations in the case where absorption cross sections are known. Spectrum factor calculations demonstrate that the three wavelengths generated by stimulated Raman scattering (SRS) in CO₂ comprise one of the most suitable wavelength sets for accurate differential absorption measurements using Nd:YAG laser harmonics. Although the system is not optimized for routine monitoring of background SO₂ at typical environmental pollution levels, it detected volcanic SO₂ at concentrations much lower than the human odor threshold. Lidar observations in Tsukuba successfully detected SO₂ plumes transported from Mt. Asama and Mt. Oyama (Miyake Island), Japan. Thus, the proposed three-wavelength DIAL method provides increased measurement capability to the existing O₃ DIAL system by extending its application to the simultaneous observation of tropospheric O₃ and advected volcanic SO₂ over the height range of 0.4-4 km without additional laser sources.