Reply on RC2

Abstract

Abstract. The Twin Anthropogenic Greenhouse Gas Observers (TANGO) mission, scheduled for launch in 2028, will observe CO₂, CH₄, and NO₂ emission plumes from more than 10,000 industrial facilities per year using two formation-flying CubeSats. Here, 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. 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 NO → 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.

Authors 4

1. Tobias Borsdorff corresponding Aachen Space Research Organisation Netherlands

   Space Research Organisation Netherlands

   Affiliation as printed

   Space Research Organisation Netherlands, SRON, Leiden, the Netherlands

2. Maarten Krol

   Wageningen University & Research

   Affiliation as printed

   Wageningen University & Research, WUR, Wageningen, the Netherlands

3. Pepijn J. Veefkind

   Royal Netherlands Meteorological Institute

   Affiliation as printed

   Royal Netherlands Meteorological Institute, KNMI, De Bilt, the Netherlands

4. Jochen Landgraf Aachen Space Research Organisation Netherlands

   Space Research Organisation Netherlands

   Affiliation as printed

   Space Research Organisation Netherlands, SRON, Leiden, the Netherlands