A Modeling Framework of Atmospheric CO\({}_{2}\) in the Mediterranean Marseille Coastal City Area, France  
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1 Institut Mediterranean de Biodiversite et d'Ecologie Marine et Continental, Aix-Marseille Univ, Avignon Univ, CNRS, IRD, 13397 Marseille, France; [EMAIL_ADDRESS]  
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1 Institut Mediterranean de Biodiversite et d'Ecologie Marine et Continental, Aix-Marseille Univ, Avignon Univ, CNRS, IRD, 13397 Marseille, France; [EMAIL_ADDRESS]  
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2 Groupe de Spectrometrie Moleculaire et Atmospherique, Universite de Reims Champagne-Ardenne, 51100 Reims, France  
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3 OSU Institut PYTHEAS, Service Informatique Pytheas, 13009 Marseille, France  
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4 AtmosP, 13006 Marseille, France  
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5 Mediterranean Institute of Oceanography (MIO), Universite du Sud Toulon, 83130 Toulon, France  
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6 Earth from Space Institute, Universities Space Research Association (USRA), Washington, DC 20024, USA  
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7 Department of Atmospheric and Oceanic Science, University of Maryland, College Park, MD 20742, USA  
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8 Mediterranean Institute of Oceanography (MIO), Aix-Marseille Universite, Campus de Luminy, 13009 Marseille, France  
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10 National Oceanography Centre, Southampton SO14 3 ZH, UK  
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4 Atmospheric CO\({}_{2}\) emissions and the trend of urbanization both increase, the ability to accurately assess the CO\({}_{2}\) budget from urban environments becomes more important for effective CO\({}_{2}\) mitigation efforts. This task can be difficult for complex areas such as the urban-coastal Mediterranean region near Marseille, France, which contains the second most populous city in France as well as a broad coastline and nearby mountainous terrain. In this study, we establish a CO\({}_{2}\) modeling framework for this region for the first time using WRF-Chem and demonstrate its efficacy through comparisons against cavity-ringdown spectrometer measurements recorded at three sites: one 75 km north of the city in a forested area, one in the city center, and one at the urban/coastal border. A seasonal CO\({}_{2}\) analysis compares Summertime 2016 and Wintertime 2017, to which Springtime 2017 is also added due to its noticeably larger vegetation uptake values compared to Summertime. We find that there is a large biogenic signal, even in and around Marseille itself, though this may be a consequence of having limited fine-scale information on vegetation parameterization in the region. We further find that simulations without the urban heat island module had total CO\({}_{2}\) values 0.46 ppm closer to the measured enhancement value at the coastal Endoume site during the Summertime 2016 period than with the module turned on. This may indicate that the boundary layer on the coast is less sensitive to urban influences than it is to sea-breeze interactions, which is consistent with previous studies of the region. A back-trajectory analysis with the Lagrangian Particle Dispersion Model found 99.83% of emissions above 100 mol km\({}^{-2}\) month\({}^{-1}\) captured in Summer 2016 by the three measurement towers, providing evidence of the receptors' ability to constrain the domain. Finally, a case study showcases the model's ability to capture the rapid change in CO\({}_{2}\) when transitioning between land-breeze and sea-breeze conditions as well as the recirculation of air from the industrial Fos region towards the Marseille metroplex. In total, the presented modeling framework should open the door to future CO\({}_{2}\) investigations in the region, which can inform policymakers carrying out CO\({}_{2}\) mitigation strategies.  
carbon dioxide; regional-scale modeling; greenhouse gas; emissions quantification