Radiative Constraints on VIIRS Nighttime Detectability of Lower-Tropospheric Methane

Abstract. This study investigates the feasibility of detecting nighttime methane (CH₄) plumes in the lower troposphere using operational thermal infrared (TIR) imagery from the VIIRS sensor M12 midwave IR (MWIR) channel (3.7 µm), through a comprehensive radiative transfer sensitivity analysis using MODTRAN6 anchored to a real atmospheric profile. Simulations indicate that the atmosphere in the M12 band is optically thick (τ_eff ≈ 6.81) at background methane concentrations, and the detectable signal is therefore driven by the plume's thermal emission (path radiance) rather than surface absorption. We find that while plumes confined to the shallow nocturnal boundary layer produce a negligible signal regardless of concentration, plumes extending into the deeper residual layer yield substantially larger brightness temperature perturbations. At the native pixel scale, theoretical simulations indicate that detection (SNR ≥ 2) is probable only under an extreme super-emitter scenario at a 9× baseline concentration (~800 % enhancement). Incorporating observationally constrained surface emissivity retrievals from the Combined ASTER and MODIS Emissivity over Land (CAMEL) dataset at 3.6 µm into the uncertainty budget propagated a surface-derived brightness temperature uncertainty of σ_emis = 0.217 K, exceeding the instrument noise-equivalent temperature difference (NEΔT) and establishing an irreducible noise floor independent of window size. Thus, surface emissivity heterogeneity, rather than instrument noise, is the dominant limitation on detectability, and spatial averaging provides only limited benefit in mixed-covered scenes.