TANGO CO ₂ and NO ₂ observations: synergistic usage to improve emission quantification and characterize atmospheric chemistry

Abstract. The Twin Anthropogenic Greenhouse Gas Observers (TANGO) mission, scheduled for launch in 2028, will observe carbon dioxide (CO2), methane (CH4), and nitrogen dioxide (NO2) emission plumes from more than 10 000 industrial facilities per year using two formation-flying CubeSats. In general, NO2 plume structures exhibit substantially lower random noise than the corresponding CO2 features, motivating a synergistic exploitation of both species for improved emission quantification and for enhanced characterization of atmospheric chemistry within plumes. Using large-eddy simulations in combination with the integrated mass enhancement (IME) method, we assess NO2-based masking of CO2 plumes for emission rates in the range 2.0–12.5 Mt yr−1. This yields CO2 emission estimates with precisions between 18.5 % and 3.4 %, depending on the emission strength, and corresponding absolute biases that decrease from 15.3 % to 2.4 %. As an alternative approach, we analyze the observed CO2 / NO2 ratio. By fitting an empirical model to measurement simulations of this ratio and subsequently reconstructing the CO2 plume from NO2 observations, we obtain a substantial reduction in the apparent noise of the reconstructed CO2 plume. For the inferred emission rates, however, the precision remains largely unchanged, with the masking approach consistently showing lower absolute biases than the reconstruction approach across all emission strengths. Consequently, despite reduced errors in individual pixel-level observations, plume reconstruction does not enhance the precision of CO2 emission estimates, because it converts originally uncorrelated pixel noise into spatially correlated errors. Neglecting these spatial error correlations leads to a severe underestimation of the retrieval uncertainty. A key advantage of the empirical CO2 / NO2 ratio model is its ability to characterize plume chemistry. Here CO2 serves as non-decaying reference tracer. We demonstrate that an effective timescale for the nitric oxide (NO) to NO2 conversion in emission plumes can be inferred for sources with CO2 emissions >5.0 Mt yr−1. Application of the method to Environmental Mapping and Analysis Program (EnMAP) observations demonstrates its practical utility, confirming its applicability to real satellite data.