A Sensitivity Study of the 4.8 micron Carbon Dioxide Absorption Band in the MWIR Spectral Range

The measurements of gas concentrations in the atmosphere are recently developed thanks
to the availability of gases absorbing spectral channels in space sensors and strictly depending on
the instrument performances. In particular, measuring the sources of carbon dioxide is of high
interest to know the distribution, both spatial and vertical, of this greenhouse gas and quantify
the natural/anthropogenic sources. The present study aims to understand the sensitivity of the
CO2 absorption band at 4.8 micron to possibly detect and measure the spatial distribution of emissions
from point sources (i.e., degassing volcanic plumes, fires, and industrial emissions). With the aim
to define the characteristics of future multispectral imaging space radiometers, the performance
of the CO2 4.8 micron absorption band was investigated. Simulations of the “Top of Atmosphere”
(TOA) radiance have been performed by using real input data to reproduce realistic scenarios on a
volcanic high elevation point source (>2 km): actual atmospheric background of CO2(~400 ppm) and
vertical atmospheric profiles of pressure, temperature, and humidity obtained from probe balloons.
The sensitivity of the channel to the CO2 concentration has been analyzed also varying surface
temperatures as environmental conditions from standard to high temperature. Furthermore, response
functions of operational imaging sensors in the middle wave infrared spectral region were used.
The channel width values of 0.15 m and 0.30 m were tested in order to find changes in the gas
concentration. Simulations provide results about the sensitivity necessary to appreciate carbon dioxide
concentration changes considering a target variation of 10 ppm in gas column concentration. Moreover,
the results show the strong dependence of at-sensor radiance on the surface temperature: radiances
sharply increase, from 1 Wm2sr1m1 (in the “standard condition”) to >1200 Wm2sr1micron1 (in the warmest case) when temperatures increase from 300 to 1000 K. The highest sensitivity has been
obtained considering the channel width equal to 0.15 micron with noise equivalent delta temperature
(NEDT) values in the range from 0.045 to 0.56 K at surface temperatures ranging from 300 to 1000 K.