Considering the observation and illumination angular configuration for an improved detection and quantification of methane emissions

Abstract. A growing constellation of methane-sensitive instruments from space is making it possible to monitor methane point-source emissions from different industries and activities. Advanced plume simulation methods are becoming key to better understanding the uncertainties in the plume detection and quantification processes. This paper describes how observation and illumination angular configuration affects satellite-derived methane enhancement fields and, more critically, the calibration and uncertainty of semi-empirical emission rate estimation models, using simulated datasets. We review the mathematical expression of the retrieval and quantification methodologies to determine the uncertainty sources. We implement a method to simulate the apparent displacement of the plume when projected on the ground due to tilted illumination and observation (i.e., parallax effect). We apply this method to methane plumes generated from 3D spatial distributions of methane using the Weather and Research Forecasting Model in large-eddy simulation mode (WRF-LES). The results in the methane enhancement (ΔXCH4) maps show large spatial variations of the plumes with respect to a reference case with nadir observation and zenith illumination. In short, it suggests that the spatial distribution of the plumes is largely determined not only by the turbulence in the atmosphere but also by the acquisition's illumination and observation geometry. We tested the impact in the methane flux rate using the integrated mass enhancement (IME) method. Assuming nadir observation, these errors have a polar dependence with the solar angles that for our training dataset at 20 × 20 m2 reaches values as high as the 30 % error for high sun zenith angles orthogonal to the direction of the plume. The errors in the IME method are explained by the changing plume area with angular projection on the ground. Furthermore, it also has an impact on the probability of plume detection (PoD). The PoD is significantly reduced in the orthogonal plane to the wind direction and increases in opposite directions to the wind where the plumes are compressed. The dependence of the PoD on the angles increases with high values of U10 with differences of up to 4 times between angular configurations at high sun zenith angles. Our results illustrate the importance of considering acquisition and observation geometry when analysing plume maps with a spatial sampling of 20 m or better.