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

<p>MAMAP2D-Light is an airborne passive remote sensing imaging push-broom spectrometer developed at the Institute for Environmental Physics at the University of Bremen to determine atmospheric methane (<span class="inline-formula">CH₄</span>) and carbon dioxide (<span class="inline-formula">CO₂</span>) column anomalies in the 1.6 <span class="inline-formula">µm</span>-band to quantify point-source emissions. In its initial version, as flown in 2022 in Canada, a significant stray light level of 5.6 % of the measured signal has been observed post-campaign, causing apparent error patterns in the retrieved <span class="inline-formula">CO₂</span> and <span class="inline-formula">CH₄</span> column anomalies. 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 <span class="inline-formula">CO₂</span> and <span class="inline-formula">CH₄</span> column anomalies, as well as the derived emission rates. We successfully developed and applied a stray light correction to the instrument and investigated its impact on the <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub><mo>/</mo><msub><mi mathvariant="normal">CO</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="50pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="30509f4bc43498cd49fa5d32be8e545e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-19-871-2026-ie00001.svg" width="50pt" height="14pt" src="amt-19-871-2026-ie00001.png"/></svg:svg></span></span> proxy method, the <span class="inline-formula">CH₄</span> column, and derived point-source emissions. In nearly all cases, applying the <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub><mo>/</mo><msub><mi mathvariant="normal">CO</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="50pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="17a72e71b6b403ca9964c3b478ae2e23"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-19-871-2026-ie00002.svg" width="50pt" height="14pt" src="amt-19-871-2026-ie00002.png"/></svg:svg></span></span> proxy method reduced the stray-light-related column errors below the <span class="inline-formula">CH₄</span> column noise. The derived emission rates for the proxy-retrieval with and without stray light corrected spectra are comparable, proving the ability of the <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub><mo>/</mo><msub><mi mathvariant="normal">CO</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="50pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="a5360b4f54ba9b350c0e868cb84a78a3"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-19-871-2026-ie00003.svg" width="50pt" height="14pt" src="amt-19-871-2026-ie00003.png"/></svg:svg></span></span> proxy method to correct stray-light-related artifacts. In this paper, we additionally investigate the impact on the <span class="inline-formula">CH₄</span> total column retrieval for a high contrast scene condition under which the correction by applying the proxy method is no longer sufficient. Following the initial campaign in 2022, the post-campaign stray light characterization and analysis revealed that a significant fraction of stray light was attributed to reflective surfaces in the object plane of the spectrometer. Based on these findings, the total stray light was reduced by <span class="inline-formula">∼</span> 63 % by implementing a hardware modification from 2023 onward.</p>