Sensitivity Study and Intercomparison of CO2 Absorption Bands in SWIR Region

Abstract

The short-wave infra-red (SWIR) region is sensitive to Carbon dioxide (CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> ) gas and this region is used for detection of CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> source and sink. Generally, three channels of CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> absorption have been used for this purpose: 1570, 1602 and 2009 nm. Out of all the three bands, 2009 nm band has been widely used by various researchers, followed by 1602 nm band. CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> and water vapor (WV) absorption coexist in the same spectral locations, which makes retrieval of atmospheric CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> concentration quite challenging. The main objective of the study is to compare the sensitivity of these three CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> absorption bands and to understand the effect of WV on it. In order to model and decouple the WV and CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> absorptions, we have used MODTRAN (MODerate spectral resolution atmospheric TRANsmittance) 5.3 radiative transfer model simulations by varying WV and CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> levels in SWIR channels. Through the statistical analysis, we found band 2009 nm is strongly affected by WV absorption and 1570 nm band is partially affected, while 1602 nm band is quite clear and unambiguous with respect to WV absorption. Moreover, band 1602 nm shows narrow width, sharp absorption and less sensitivity to the WV concentration compare to other bands. Therefore, 1602 nm band is also suitable for differential absorption methods to retrieve CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> concentration.