A light-weight NO 2 to NO x conversion model for quantifying NO x emissions of point sources from NO 2 satellite observations

Meier, Sandro; Koene, Erik; Krol, Maarten; Brunner, Dominik; Damm, Alexander; Kuhlmann, Gerrit

Nitrogen oxides (NO x = NO + NO 2) are air pollutants which are co-emitted with CO 2 during high-temperature combustion processes. Monitoring NO x emissions is crucial for assessing air quality and for providing proxy estimates of CO 2 emissions. Satellite observations, such as those from the TROPOspheric Monitoring Instrument (TROPOMI) on board the Sentinel-5P satellite, provide global coverage at high temporal resolution. However, satellites measure only NO 2, necessitating a conversion to NO x. Previous studies applied a constant NO 2-to-NO x conversion factor. In this paper, we develop a more realistic model for NO 2 to NO x conversion and apply it to TROPOMI data of 2020 and 2021. To achieve this, we analysed plume-resolving simulations from the MicroHH Large Eddy Simulation model with chemistry for the power plants Bełchatów (PL), Jänschwalde (DE), Matimba and Medupi (ZA), as well as a metallurgical plant in Lipetsk (RU). We used the cross-sectional flux method to calculate NO, NO 2, and NO x line densities from simulated NO and NO 2 columns and derived NO 2-to-NO x conversion factors as a function of the time since emission. Since the method of converting NO 2 to NO x presented in this paper assumes steady-state conditions as well as that the conversion factors can be modeled by a negative exponential function, we validated the conversion factors using the same MicroHH data. Finally, we applied the derived conversion factors to TROPOMI NO 2 observations of the same sources. The validation of the NO 2-to-NO x conversion factors shows that they can account for the NO x chemistry in plumes, in particular for the conversion between NO and NO 2 near the source and for the chemical loss of NO x further downstream. When applying these time-since-emission-dependent conversion factors, biases in NO x emissions estimated from TROPOMI NO 2 images are greatly reduced from between -50 and -42 % to only -9.5 to -0.5 % in comparison with reported emissions. Single-overpass estimates can be quantified with an uncertainty of 20–27 %, while annual NO x emission estimates have uncertainties in the range of 4–21 % but are highly dependent on the number of successful retrievals. Although more simulations covering a wider range of meteorological and trace gas background conditions will be needed to generalize the approach, this study marks an important step towards a global, uniform, high-resolution, and near real-time estimation of NO x emissions – especially with regard to upcoming NO 2 monitoring satellites such as Sentinel-4 and -5 and CO2M.

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Meier, Sandro / Koene, Erik / Krol, Maarten / et al: A light-weight NO2 to NOx conversion model for quantifying NOx emissions of point sources from NO2 satellite observations. 2024. Copernicus Publications.

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