MAMAP – a new spectrometer system for column-averaged methane and carbon dioxide observations from aircraft: instrument description and performance analysis

Abstract

Carbon dioxide (CO₂) and Methane (CH₄) are the two most important anthropogenic greenhouse gases. CH₄ is furthermore one of the most potent present and future contributors to global warming because of its large global warming potential (GWP). Our knowledge of CH₄ and CO₂ source strengths is based primarily on bottom-up scaling of sparse in-situ local point measurements of emissions and up-scaling of emission factor estimates or top-down modeling incorporating data from surface networks and more recently also by incorporating data from low spatial resolution satellite observations for CH₄. There is a need to measure and retrieve the dry columns of CO₂ and CH₄ having high spatial resolution and spatial coverage. In order to fill this gap a new passive airborne 2-channel grating spectrometer instrument for remote sensing of small scale and mesoscale column-averaged CH₄ and CO₂ observations has been developed. This Methane Airborne MAPper (MAMAP) instrument measures reflected and scattered solar radiation in the short wave infrared (SWIR) and near-infrared (NIR) parts of the electro-magnetic spectrum at moderate spectral resolution. The SWIR channel yields measurements of atmospheric absorption bands of CH₄ and CO₂ in the spectral range between 1.59 and 1.69 μm at a spectral resolution of 0.82 nm. The NIR channel around 0.76 μm measures the atmospheric O₂-A-band absorption with a resolution of 0.46 nm. MAMAP has been designed for flexible operation aboard a variety of airborne platforms. The instrument design and the performance of the SWIR channel, together with some results from on-ground and in-flight engineering tests are presented. The SWIR channel performance has been analyzed using a retrieval algorithm applied to the nadir measured spectra. Dry air column-averaged mole fractions are obtained from SWIR data only by dividing the retrieved CH₄ columns by the simultaneously retrieved CO₂ columns for dry air column CH₄ (<i>X</i>CH₄) and vice versa for dry air column CO₂ (<i>X</i>CO₂). The signal-to-noise ratio (SNR) of the SWIR channel is approximately 1000 for integration times (<i>t</i>_int) in the range of 0.6–0.8 s for scenes with surface spectral reflectances (SSR)/albedo of around 0.18. At these integration times the ground scene size is about 23 &times; 33 m² for an aircraft altitude of 1 km and a ground speed of 200 km/h. For these scenes the actual <i>X</i>CH₄ or <i>X</i>CO₂ dry air column retrieval precisions are typically about 1% (1 σ). Elevated levels of CH₄ have been retrieved above a CH₄ emitting landfill. Similarly the plume of CO₂ from coal-fired power plants can be well detected and tracked. The measurements by the MAMAP sensor could enable estimates of anthropogenic, biogenic and geological emissions of localized intense CH₄ and CO₂ sources such as anthropogenic fugitive CH₄ emissions from oil and gas industry, coal mining, disposal of organic waste, CO₂ emissions from coal-fired power plants, steel production or geologic CH₄ and CO₂ emissions from seepage and volcanoes. Appropriate analysis of the measurements of MAMAP potentially also yields natural CH₄ emissions from less intense but extensive sources such as wetlands.