High-accuracy Laboratory Spectroscopic Measurements of Atmospheric Gases for Monitoring Greenhouse Gases

Summary

Climate scientists are developing a new generation of spectrometers for the NASA-sponsored inventory of atmospheric greenhouse gases, most notably carbon dioxide. This inventory is critical to improving our understanding of the variations in carbon dioxide concentrations, which arise from the various sources, sinks, and atmospheric circulation patterns. These field measurements are arguably the most demanding ever attempted using absorption spectroscopy. To ensure the integrity and long-term continuity of these measurements, spectrometer operation relies on reference data of the highest metrological quality and traceability to the International System of Units (SI).

NIST is providing the spectroscopic parameters required for these spectrometers to operate at the targeted precision and accuracy.

Description

Oxygen plays a key role in remote sensing measurements because its concentration in the atmosphere is accurately known and does not change with time. The O₂ A-band absorption spectrum in the vicinity of 760 nm is particularly important to many remote-sensing applications because it is largely free of spectral interferences. An important example includes measurements of CO₂ concentrations by orbiting spacecraft, [e.g., NASA's Orbiting Carbon Observatory, (OCO)] in which absorption path lengths and CO₂ mixing ratios are to be determined with an uncertainty of 0.3 %, using the O₂ A-band spectrum as a reference. These rigorous and quantitative measurements demand that the O₂ A-band line parameters (intensity, transition frequency, pressure-induced frequency shifts and broadening, lines shape and mixing, etc.) be measured with unprecedented precision and with accuracy established by traceability to the SI.

The standard reference data provided by this research will enable new, high-precision satellite-based measurements of greenhouse gases in the Earth's atmosphere. Specifically, these reference data are required by the OCO satellite mission, which is to be launched in 2009 and which will map sources and sinks of atmospheric CO₂ over the Earth's surface, and by the Total Carbon Column Observatory Network (TCCON) an international network of ground-based high-resolution spectrometers that measure atmospheric CO₂ levels. These measurements are also critical to the following generation of satellite-borne instruments, which will rely on laser technology to measure diurnal and seasonal variations in atmospheric greenhouse.

Similarly, accurate measurements of the O₂ A-band will support meteorological observations and weather modeling, and have the potential to determine global surface pressure in remote locations where current measurements are inadequate or non-existent.

Major Accomplishments

For molecular oxygen A-band:

• Measured key parameters for P-branch
  • Uncertainties reduced twenty-fold reduction for positions, ten-fold reduced for line strengths, and five-fold for pressure shifts
• First laboratory observation of weak electric-quadrupole lines