Carbon Dioxide Retrieval by Employing the MWIR Absorption Band at 4.8 µm: Model Sensitivity Studies and Aircraft Data Exploitation

Measuring atmospheric concentrations of the carbon dioxide from space represents a challenge, due to the high interest in knowing the distribution of this greenhouse gas and quantifying the natural/anthropogenic sources. The aim of the present study is to understand the capability of the absorption band at 4.8 μm to detect and measure the CO₂ emissions from different HTEs (High Temperature Events) like degassing plumes from active volcanic sources, fires and industrial emissions. The performance of this channel was investigated by using the MODTRAN (MODerate resolution atmospheric TRANsmission) radiative transfer model. The TOA (Top Of Atmosphere) radiance simulations have been operated with realistic input data to reproduce realistic scenarios: vertical atmospheric profiles of pressure, temperature and humidity are obtained from probe balloons. Several simulations were carried out in order to analyze the sensitivity of the channel to CO₂ concentrations, considering different surface temperatures as boundary conditions. The simulations show the impact of increasing/decreasing CO₂ concentrations on the radiance at the sensor; high surface temperatures (i.e. on lava flows or fires) provide better changes to detect CO₂ emissions from local sources. Moreover, the FWHM (Full Width Half Maximum) of the sensor is a crucial parameter to detect changes in the gas concentration: two values of this parameter (0.15 and 0.30 µm) are considered in the present study. Finally, the simulations supply preliminary indications on the NEDT (Noise Equivalent Delta Temperature) necessary to appreciate carbon dioxide variations and then retrieving its concentration from a space sensor. With the aim to test the channel at 4.8 µm, data acquired by the multispectral MASTER (Modis ASTER) airborne simulator were considered; gas emissions in the Pu'o'O vent plume (Hawaii) and a wildfire occurred in Santa Barbara area (California) were analysed as test cases.