Isotopic
CO2 measurements have
been identified as an important component of NASA’s Earth Science
Enterprise’s Carbon Cycle Initiative as part of its program in global
climate change. The isotopic
composition of atmospheric CO2, and especially its 13CO2/
12CO2 ratio, is an established tool for understanding
the details of the global carbon cycle, since this ratio can distinguish
between oceanic and terrestrial biospheric sinks of CO2.
Figure. Time series showing the relationship between atmospheric CO2 and its isotopes in the marine boundary layer, made by NOAA CMDL and UC INSTARR. Greater seasonal variation is seen for the northern high latitudes driven by the terrestrial biosphere.
Measuring 13CO2/CO2
is not easy, since a precision of better than 1 per mil is required (better than
0.1 per mil desired) to distinguish trends, seasonal, diurnal, and latitudinal
variations. This capability has
never been demonstrated for in-flight atmospheric measurements, but would have
an enormous impact on global carbon science.
Figure. Synthetic spectral region where ALIS will make 13CO2/12CO2 isotope measurement demonstration, and schematic of laser isotope spectrometer optical head. The calibration standard gas channel “breathes” with the external pressure changes, and both cal gas and measurement channel are isothermal.
