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.
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.
For molecular oxygen A-band: