An inter-comparison of bottom-up and satellite-derived emissions for trace gases over agricultural regions in Europe

Trace gases, such as ammonia (NH₃) and nitrogen oxides (NO_x), serve as key precursors for secondary inorganic aerosols and play an important role in air quality and the nitrogen cycle. The primary anthropogenic sources of NH₃ and NO_x in Europe are agriculture, industry and transportation. Additionally, soil NO_x emissions contribute significantly to the total NO_x budget. Anthropogenic emission reporting is based on “bottom-up” emission inventories, which provide gridded NH₃ and NO_x emissions, and are widely used for air quality modelling and effective policymaking.

However, bottom-up emission inventories rely on statistical information, activity data and emission factors, resulting to uncertainties and significant time lags in data availability. Therefore, in addition to bottom-up emission inventories, “top-down” methods have been developed using various inverse modelling techniques. These techniques make use of satellite data, which provide independent information on emissions and can be used to evaluate bottom-up emission inventories and help to identify potential unknown or unreported sources.

In this study we use the Daily Emissions Constrained by Satellite Observations (DECSO) inversion algorithm combined with Cross-track Infrared Sounder (CrIS) and TROPOspheric Monitoring Instrument (TROPOMI) satellite data to derive NH₃ and NO_x emissions on a spatial resolution of 0.1^o x 0.1^o. DECSO is coupled to Eulerian regional offline CHIMERE CTM. Our study focuses on BENELUX and Po-Valley, two regions with high agricultural emissions, as part of the Agricultural Atmospheric Emissions (AGATE) ESA project.

We validate the DECSO NH₃ and NO_x emissions by comparing them directly against bottom-up inventories, such as Copernicus Atmosphere Monitoring Service (CAMS) and Emissions Database for Global Atmospheric Research (EDGAR). Satellite-derived emissions are consistent with bottom-up inventories regarding the magnitude of country-region totals.

To further validate the DECSO emissions we also perform forward simulations using CHIMERE CTM with DECSO and bottom-up emission inventories. The focus of the validation here is on aerosol precursors, such as nitrogen dioxide (NO₂) and NH₃, and compare the model results against in-situ observations and independent satellite data. The comparison against observations shows that the model simulations using the DECSO NH₃ and NO_x emissions perform similarly to simulations using bottom-up inventories providing further confidence on the quality of satellite-derived emissions.