Monitoring CO emissions from urban districts in Mexico City using about 2 years of TROPOMI CO observations

The Tropospheric Monitoring Instrument (TROPOMI) on ESA Copernicus Sentinel-5P satellite (S5-P) monitors the total column concentration of carbon monoxide (CO) as one of its primary targets.  In this study, we present an approach to analyze the large amount of TROPOMI CO data and to estimate urban emissions on sub-city scales for metropolises like Mexico City. The results demonstrate the advance in using TROPOMI observations for monitoring regional air quality. To this end, we analyze about two years of TROPOMI CO measurements with 551 overpasses over Mexico City using tracer simulations of the regional Weather Research and Forecasting (WRF) model. Ten separate CO tracers for emissions of the districts Tula, Pachuca, Tulancingo, Ciudad de Mexico, Toluca, CDMX, Cuernavaca, Cuautla, Tlaxcala, and Puebla are used to conclude on the emissions of different urban districts. A regularized source inversion minimizes the difference with respect to a prior emission estimate. Here, the degree of freedom of the inferred sources is a powerful tool to filter on measurement information and forward model errors e.g. due to erroneous wind fields. We compare the estimated emissions with those of the national inventory ``Inventario Nacional de Emisiones de Contaminantes Criterio'' (INEM) multiplied by 0.48 to make it applicable for the years 2017 to 2019. Overall, TROPOMI confirms the total INEM CO emissions form the area but indicates clear differences in relative distribution of the emissions between the districts. For example, TROPOMI yields 0.11 Tg/yr and 0.10 Tg/yr CO emissions for the urban districts Tula and Pachuca in the North of Mexico City, which exceeds the INEM emissions of <0.008 Tg/yr. Also, for the central part of Mexico City (CDMX) the TROPOMI estimate with 0.14 Tg/yr differs significantly from the inventory with 0.25 Tg/yr. Moreover, we found that the retrieved emissions for CDMX and Ciudad de Mexico follow a clear weakly cycle with a minimum during the weekend in agreement with ground-based in situ measurements of the ``Secretaria del Medio Ambiente'' (SEDEMA) and column measurements of a Fourier Transform Spectrometer in Mexico City operated by the National Autonomous University of Mexico (UNAM). To improve further the TROPOMI CO data exploitation, our study clearly indicates the need for improvements of regional models like WRF, in particular with respect to the prediction of the local wind fields.