Impact of stray light on greenhouse gas concentration retrievals and emission estimates as observed with the passive airborne remote sensing imager MAMAP2D-Light

MAMAP2D-Light is an airborne passive remote sensing push-broom spectrometer developed at the Institute for Environmental Physics at the University of Bremen to measure atmospheric methane (CH4) and carbon dioxide (CO2) column anomalies to quantify point-source emissions in the 1.6 μm-band. In its initial version, as flown in 2022, a significant stray light level of ~4 % of the measured signal has been observed, causing apparent error patterns in the retrieved CO2 and CH4 column anomalies. In this paper, we report the successful application of a stray light correction developed for the instrument. Measurement data collected during an airborne campaign in 2022 in Canada offer the unique opportunity to investigate the end-to-end impact of stray light and its correction on the retrieved CO2 and CH4 column anomalies, as well as the retrieved emission rates. Stray light caused apparent error patterns in the retrieved column anomaly maps. In nearly all cases, applying the CH4/CO2 proxy method reduced the stray-light-related column errors below the column noise, leading to comparable final emission rate estimates for proxy-only and stray-light-corrected data. In this paper, we additionally investigate the special scene contrast conditions under which the correction by applying the proxy method is no longer sufficient. Following the initial campaign in 2022, the stray light was reduced by ~ 75 % by the implementation of a hardware modification from 2023 onward.