MExico city’s Regional Carbon Impacts – MERCI-CO2

Mexico City (MC) is the home of 21.2M people, 19% of the country's population. The MC urban area has intense emissions of pollutants and greenhouse gases, which accumulate in the overlying air-shed due to the location of the city in a high-altitude basin surrounded by mountains. Local and national authorities have engaged into aggressive emission reduction strategies, e.g. relocation of heavy industry and vehicle-free days. The purpose of MERCI-CO2 is to develop the research needed to set up atmospheric CO2 measurements that will enable to verify the effectiveness of CO2 emission reductions taken by the city authorities, represented in the project by the Ministry of Environment of MC’s government (SEDEMA).
To do so, we propose to measure atmospheric CO2 concentrations gradients within and around the megacity, attribute them to CO2 emissions from the city, and combine them with a 3D atmospheric transport model and emission inventories to reduce the uncertainty on CO2 emissions in support of reduction policies taken by regional authorities.
This research project will do the design and lay the foundation of a future city scale CO2 monitoring network to be operated by the regional air-quality network, partner of the project. Given the huge extent of MC, sampling the atmosphere to best capture its CO2 emissions is a scientific challenge, which requires several stations. The proposed solution to overcome the costs of high-precision CO2 instruments is to deploy an array of 10 novel low-cost medium precision (LCMP) CO2 sensors identified and already tested at LSCE, with two existing high-precision CO2 instruments. The performances of LCMP will be tested in real-conditions against the precision instruments. The locations of the LCMP will be selected after analysis of existing air quality observations and gridded CO2 emissions fields based on energy use statistics available from the MC authorities. The new CO2 stations will be co-located with existing air quality stations measuring CO and NOx concentrations, two pollutants co-emitted with CO2 by the combustion of fuels, with different ratios depending on the emitting sector considered. Combining CO2, CO and NOx observations will help us to identify the contribution of different emission sectors (traffic, residential, industry) to total CO2 emissions.
CO2 concentrations measured in the boundary layer above the city with in-situ sensors will be complemented by total air column CO2 observations made with ground-based remote sensing spectrometers. These measurements will characterize large-scale CO2 gradients between up and downwind of MC, bringing an independent check on whole city emissions, whereas the in-situ sensors provide information of spatial/sectorial details within MC area. Both measurements will be compared to simulations from a high-resolution 3D model prescribed with city-scale CO2 emission maps. First, comparison of modeled CO2 with observed data will help to identify key mesoscale transport processes and possible emission hot-spots. This will allow us to understand under which conditions the transport model performs well enough to be used in an inverse model, i.e. to retrieve CO2 emissions from measured CO2 gradients. Secondly, the inversion of selected CO2 gradients in an analytical Bayesian inversion framework will allow an independent estimate of MC emissions that will be compared to the existing official inventory based on energy use and fuel statistics. The differential CO2 column measurements of instruments located up and downwind of the city will provide a top-down verification of the overall sources at city scale.
The research proposed in MERCI-CO2 will deliver a reduction of uncertainties on the MC CO2 emissions that will be taken to operations by Mexican partners after the lifetime of the project. This project benefits from the existing pollutant measurement infrastructure in Mexico City and from the strong interest and involvement of Mexican partners.