A study of synthetic <sup>13</sup>CH<sub>4</sub> retrievals from TROPOMI and Sentinel 5/UVNS Part 1: non scattering atmosphere

Abstract

<p><strong>Abstract.</strong> Retrievals of methane isotopologues have the potential to differentiate between natural and anthropogenic methane sources types, which can provide much needed information about the current global methane budget. We investigate the feasibility of retrieving the second most abundant isotopologue of atmospheric methane (13CH4, roughly 1.1<span class="thinspace"></span>% of total atmospheric methane) from the Shortwave Infrared (SWIR) channels of the future Sentinel 5/UVNS and current Copernicus Sentinel 5 Precursor TROPOMI instruments. With the intended goal of calculating the &amp;delta;¹³C ratio, we assume that a &amp;delta;¹³C uncertainty of better than 10<span class="thinspace"></span>‰ is sufficient to differentiate between source types, which corresponds to a ¹³CH₄ uncertainty of &amp;lt;0.2<span class="thinspace"></span>ppb. Using the well established Information Content analysis techniques and assuming clear sky, non-scattering conditions, we find that the SWIR3 (2305&amp;ndash;2385<span class="thinspace"></span>nm) channel on the TROPOMI instrument can achieve a mean uncertainty of &amp;lt;1<span class="thinspace"></span>ppb, while the SWIR1 channel (1590&amp;ndash;1675<span class="thinspace"></span>nm) on the Sentinel 5 UVNS instrument can achieve &amp;lt;0.68<span class="thinspace"></span>ppb. These uncertainties combined with modest spatial and/or temporal averaging techniques can reduce &amp;delta;¹³C uncertainty to the target magnitude or better. However, we find that ¹³CH₄ retrievals are highly sensitive to errors in a priori knowledge of temperature and pressure, and accurate knowledge of these profiles are required before ¹³CH₄ retrievals can be performed on TROPOMI and future Sentinel 5/UVNS data.</p>