Abstract
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 µm 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 µm 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 µm and 0.30 µm 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 Wm−2sr−1µm−1 (in the “standard condition”) to >1200 Wm−2sr−1µm−1 (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 µm 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.
Highlights
The evidence of increasing Earth surface temperature in the last decades [1] is posing important questions about the causes and the influence of human activities on it
MODerate resolution atmospheric TRANsmission (MODTRAN) simulations were focused on the CO2 absorption band at 4.8 μm, in the midwave infrared (MWIR) spectral region
The at-sensor radiance sharply increases with increasing temperatures, ranging from 1 Wm−2 sr−1 μm−1 to more than 1200 Wm−2 sr−1 μm−1
Summary
The evidence of increasing Earth surface temperature in the last decades [1] is posing important questions about the causes and the influence of human activities on it. The individuation and quantification of gases point sources are pivotal to better understand the anthropogenic and natural origins. The measurements of gas concentrations taken remotely, with the identification of their spatial distribution, are developed thanks to the availability of the gases absorbing spectral channels in the sensors carried on satellite or airborne platforms. Atmospheric CO2 total columns can be measured by sensors acquiring spectra in the CO2 absorption bands at 1.6 μm and. 2.0 μm in the short-wave infrared (SWIR) spectral region, at 4 μm in the midwave infrared (MWIR), Remote Sens. The scope of the present study is to understand the capability of the absorption band at 4.8 μm to detect and measure the spatial distribution of CO2 emissions from point sources as degassing 2.0 μm in the short-wave infrared (SWIR) spectral region, at 4 μm in the midwave infrared (MWIR), Remote Sens. 2020, 12, 172; doi:10.3390/rs12010172 www.mdpi.com/journal/remotesensing fires, lava flows, and other hot features [19].
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