Urban XCO$_2$ Gradients From a Dense Network of Solar Absorption Spectrometers and OCO‐3 Over Mexico City

Satellite measurements of urban CO$_2$ plumes offer a global approach to track CO$_2$ emissions for large cities. To examine and to quantify the feasibility of space-based monitoring, an intensive measurement campaign (MERCI-CO$_2$) using seven solar-tracking Fourier transform infrared (FTIR) spectrometers has been conducted over the Mexico City Metropolitan Area (MCMA) to monitor urban emissions and to evaluate Snapshot Area Map (SAM) observations from the NASA's Orbiting Carbon Observatory-3 (OCO-3) mission. Once adjusted for their respective averaging kernels, we diagnosed a positive difference between OCO-3 and FTIR column measurements (1.06 ppm). Thanks to this unprecedented amount of column observations over a large city, we demonstrate that XCO$_2$ gradients within OCO-3 SAMs align with the inter-calibrated FTIR measurements (mean bias of 0.3 ppm), confirming the potential to track CO$_2$ emissions from space over large metropolitan areas. XCO$_2$ urban-rural differences across the FTIR network, show a strong correlation with observed gradients, with Pearson's correlation coefficients (R) around 0.92. The correlation is significantly lower when considering intra-urban gradients, where R drop to around 0.24. ... mehrSimulated XCO$_2$ enhancements (ΔXCO$_2$) based on X-STILT for both FTIR and OCO-3 show relatively high correlations (R is around 0.6) using high-resolution footprints and two gridded inventories. Spatial correlations with OCO-3 improve when aggregating satellite retrievals at coarser resolutions (10 km). Our study demonstrates the capabilities of detecting urban gradients by FTIR network and OCO-3 SAM observations over MCMA, a promising result to evaluate the evolution of MCMA's emissions over the coming decade.