Recent improvements in atmospheric trace gas monitoring using mid-infrared tunable diode lasers

摘要

Abstract: This paper discusses recent advances in our techniques for monitoring atmospheric trace gases using lead salt liquid nitrogen cooled diode lasers. Our approach employs an optical system with all reflective optics. Our closed path systems rely on a proprietary astigmatic multipass cell to achieve long optical path lengths in a low volume sampling cell. We have also developed open path systems which we have used for remote sensing of automobile and aircraft engine exhaust. Our data acquisition method uses rapid frequency sweeping followed by nonlinear least squares analysis of the retrieved spectrum. Recent advances include an emphasis on multi-laser multi-species detection systems, such as simultaneously monitoring the nitric oxide, carbon monoxide and carbon dioxide concentrations in automobile exhaust. Other advances are focused on achieving improved detection sensitivity. In support of this goal, we have demonstrated astigmatic multipass cells with very long optical paths, we have improved the nonlinear least squares spectral fitting routines allowing them to fit complex multi-peak spectra and we have introduced photolytic modulation as a method to discriminate spectra of photolytically active species from background absorption and optical interference fringes. These techniques are being applied to the monitoring of a wide variety of atmospheric molecules including CH$-4$/, CO, CO$-2$/, N$-2$/O, NO, NO$-2$/, HONO, HNO$-3$/, O$-3$/ and HOCl. !21