Abstract. Cities currently covering only a very small portion ( < 3 %) of the world's land surface directly release to the atmosphere about 44 % of global energy-related CO2, but they are associated with 71–76 % of CO2 emissions from global final energy use. Although many cities have set voluntary climate plans, their CO2 emissions are not evaluated by the monitoring, reporting, and verification (MRV) procedures that play a key role for market- or policy-based mitigation actions. Here we analyze the potential of a monitoring tool that could support the development of such procedures at the city scale. It is based on an atmospheric inversion method that exploits inventory data and continuous atmospheric CO2 concentration measurements from a network of stations within and around cities to estimate city CO2 emissions. This monitoring tool is configured for the quantification of the total and sectoral CO2 emissions in the Paris metropolitan area (∼ 12 million inhabitants and 11.4 TgC emitted in 2010) during the month of January 2011. Its performances are evaluated in terms of uncertainty reduction based on observing system simulation experiments (OSSEs). They are analyzed as a function of the number of sampling sites (measuring at 25 m a.g.l.) and as a function of the network design. The instruments presently used to measure CO2 concentrations at research stations are expensive (typically ∼ EUR 50 k per sensor), which has limited the few current pilot city networks to around 10 sites. Larger theoretical networks are studied here to assess the potential benefit of hypothetical operational lower-cost sensors. The setup of our inversion system is based on a number of diagnostics and assumptions from previous city-scale inversion experiences with real data. We find that, given our assumptions underlying the configuration of the OSSEs, with 10 stations only the uncertainty for the total city CO2 emission during 1 month is significantly reduced by the inversion by ∼ 42 %. It can be further reduced by extending the network, e.g., from 10 to 70 stations, which is promising for MRV applications in the Paris metropolitan area. With 70 stations, the uncertainties in the inverted emissions are reduced significantly over those obtained using 10 stations: by 32 % for commercial and residential buildings, by 33 % for road transport, by 18 % for the production of energy by power plants, and by 31 % for total emissions. These results indicate that such a high number of stations would be likely required for the monitoring of sectoral emissions in Paris using this observation–model framework. They demonstrate some high potential that atmospheric inversions can contribute to the monitoring and/or the verification of city CO2 emissions (baseline) and CO2 emission reductions (commitments) and the advantage that could be brought by the current developments of lower-cost medium precision (LCMP) sensors.
Marseille (France) is a city on the Mediterranean coast characterized by two specific wind patterns: mistral (northwesterly wind blowing above 10 m/s) and sea/land breezes (southwesterly wind during daytime/northeasterly wind during the nighttime, blowing below 6 m/s). For the first time, this study investigates the diurnal and seasonal variability in the atmospheric boundary-layer height (ABLH) in Marseille for both wind patterns. A 532 nm aerosol lidar was installed in the urban center in the summer of 2021. The lidar installed in the winter of 2021–2022 had an additional near-infrared channel at 808 nm. The ABLH was extracted from the lidar datasets using a Haar wavelet method. For well-established mistral conditions, the ABLH reached to about 1000 m and showed a diurnal amplitude of ~650 m in winter and 740 m in summer, with a morning growth rate limited by turbulence. During sea breeze situations, the ABLH maxima were lower in both seasons (300–600 m) due to the sea’s thermal inertia. During land breeze situations, ABLH minima were estimated to be lower than 150 m. In summer, the Haar method was unable to calculate them because of unpronounced aerosol layers. While the near-infrared channel gives better results, the polarization of the green channel allows us to understand the type of aerosols and thus the origin of the air mass; a combination of the two gives complementary information.
Abstract. The Bay of Marseille (BoM, France) is impacted by the urbanized and industrialized Aix-Marseille Metropolis, which is subject to significant increases in anthropogenic emissions of CO2. A carbonate chemistry balance module has been implemented into a biogeochemical model of the planktonic food web. The resulting model, named Eco3M-CarbOx includes 22 states variables that are dispatched into 5 compartments: phytoplankton, heterotrophic bacteria, detritus, dissolved organic and inorganic matter. The model suggests that the variability of the dissolved inorganic carbon system is mainly driven by the seawater temperature dynamics. A seasonal trend is identified by the model and it shows that, during the mixed water column period, the BoM is a sink for atmospheric CO2 and a net autotroph ecosystem, while during stratified water column period, the BoM is a source of CO2 to the atmosphere and a net heterotroph ecosystem. External forcings have an important impact on the carbonate equilibrium. Wind events change seawater temperature quickly, as during upwelling, for which the BoM waters change within a few days from a source of CO2 to the atmosphere to a sink into the ocean. Moreover, the higher the wind speed is, the higher the air-sea CO2 gas exchange fluxes are. The river intrusions with nitrate and alkalinity supplies lead to a decrease in the pCO2 value, favoring the conditions of a sink of atmospheric CO2 into the BoM. The nearby highly urbanized environment of the Aix-Marseille metropolis produces strong atmospheric values of CO2, also favoring the conditions of a sink of atmospheric CO2 into the waters of the BoM.
Abstract. Most of the global fossil fuel CO2 emissions arise from urbanized and industrialized areas. Bottom-up inventories quantify them but with large uncertainties. In 2010–2011, the first atmospheric in situ CO2 measurement network for Paris, the capital of France, began operating with the aim of monitoring the regional atmospheric impact of the emissions coming from this megacity. Five stations sampled air along a northeast–southwest axis that corresponds to the direction of the dominant winds. Two stations are classified as rural (Traînou – TRN; Montgé-en-Goële – MON), two are peri-urban (Gonesse – GON; Gif-sur-Yvette – GIF) and one is urban (EIF, located on top of the Eiffel Tower). In this study, we analyze the diurnal, synoptic and seasonal variability of the in situ CO2 measurements over nearly 1 year (8 August 2010–13 July 2011). We compare these datasets with remote CO2 measurements made at Mace Head (MHD) on the Atlantic coast of Ireland and support our analysis with atmospheric boundary layer height (ABLH) observations made in the center of Paris and with both modeled and observed meteorological fields. The average hourly CO2 diurnal cycles observed at the regional stations are mostly driven by the CO2 biospheric cycle, the ABLH cycle and the proximity to urban CO2 emissions. Differences of several µmol mol−1 (ppm) can be observed from one regional site to the other. The more the site is surrounded by urban sources (mostly residential and commercial heating, and traffic), the more the CO2 concentration is elevated, as is the associated variability which reflects the variability of the urban sources. Furthermore, two sites with inlets high above ground level (EIF and TRN) show a phase shift of the CO2 diurnal cycle of a few hours compared to lower sites due to a strong coupling with the boundary layer diurnal cycle. As a consequence, the existence of a CO2 vertical gradient above Paris can be inferred, whose amplitude depends on the time of the day and on the season, ranging from a few tenths of ppm during daytime to several ppm during nighttime. The CO2 seasonal cycle inferred from monthly means at our regional sites is driven by the biospheric and anthropogenic CO2 flux seasonal cycles, the ABLH seasonal cycle and also synoptic variations. Enhancements of several ppm are observed at peri-urban stations compared to rural ones, mostly from the influence of urban emissions that are in the footprint of the peri-urban station. The seasonal cycle observed at the urban station (EIF) is specific and very sensitive to the ABLH cycle. At both the diurnal and the seasonal scales, noticeable differences of several ppm are observed between the measurements made at regional rural stations and the remote measurements made at MHD, that are shown not to define background concentrations appropriately for quantifying the regional (∼ 100 km) atmospheric impact of urban CO2 emissions. For wind speeds less than 3 m s−1, the accumulation of local CO2 emissions in the urban atmosphere forms a dome of several tens of ppm at the peri-urban stations, mostly under the influence of relatively local emissions including those from the Charles de Gaulle (CDG) Airport facility and from aircraft in flight. When wind speed increases, ventilation transforms the CO2 dome into a plume. Higher CO2 background concentrations of several ppm are advected from the remote Benelux–Ruhr and London regions, impacting concentrations at the five stations of the network even at wind speeds higher than 9 m s−1. For wind speeds ranging between 3 and 8 m s−1, the impact of Paris emissions can be detected in the peri-urban stations when they are downwind of the city, while the rural stations often seem disconnected from the city emission plume. As a conclusion, our study highlights a high sensitivity of the stations to wind speed and direction, to their distance from the city, but also to the ABLH cycle depending on their elevation. We learn some lessons regarding the design of an urban CO2 network: (1) careful attention should be paid to properly setting regional (∼ 100 km) background sites that will be representative of the different wind sectors; (2) the downwind stations should be positioned as symmetrically as possible in relation to the city center, at the peri-urban/rural border; (3) the stations should be installed at ventilated sites (away from strong local sources) and the air inlet set up above the building or biospheric canopy layer, whichever is the highest; and (4) high-resolution wind information should be available with the CO2 measurements.
Abstract. In 2015, the Greenhouse gas Laser Imaging Tomography Experiment (GreenLITE™) measurement system was deployed for a long-duration experiment in the center of Paris, France. The system measures near-surface atmospheric CO2 concentrations integrated along 30 horizontal chords ranging in length from 2.3 to 5.2 km and covering an area of 25 km2 over the complex urban environment. In this study, we use this observing system together with six conventional in situ point measurements and the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) and two urban canopy schemes (Urban Canopy Model – UCM; Building Effect Parameterization – BEP) at a horizontal resolution of 1 km to analyze the temporal and spatial variations in CO2 concentrations within the city of Paris and its vicinity for the 1-year period spanning December 2015 to November 2016. Such an analysis aims at supporting the development of CO2 atmospheric inversion systems at the city scale. Results show that both urban canopy schemes in the WRF-Chem model are capable of reproducing the seasonal cycle and most of the synoptic variations in the atmospheric CO2 point measurements over the suburban areas as well as the general corresponding spatial differences in CO2 concentration that span the urban area. However, within the city, there are larger discrepancies between the observations and the model results with very distinct features during winter and summer. During winter, the GreenLITE™ measurements clearly demonstrate that one urban canopy scheme (BEP) provides a much better description of temporal variations and horizontal differences in CO2 concentrations than the other (UCM) does. During summer, much larger CO2 horizontal differences are indicated by the GreenLITE™ system than both the in situ measurements and the model results, with systematic east–west variations.
La glace peut jouer un role important dans la composltlon de la phase gazeuse atmospherique : (1) lorsqu'elle se forme, elle peut incorporer des gaz traces par condensation simultanee de gaz et de vapeur d'eau (co-condensation) ou par solidification rapide de gouttelettes surfondues contenant des gaz dissous (givrage) ; et (2) apres la precipitation, des echanges de gaz traces peuvent se produire entre le manteau neigeux et la troposphere. De plus, la surface de la glace peut catalyser des reactions chimiques heterogenes entre des gaz traces inertes en phase gazeuse. Par exemple dans la stratosphere polaire hivernale, de telles reactions heterogenes entre des composes chlores (HCI, .. . ) se produisent a la surface de particules de glace constituant les nuages stratospheriques polaires (PSCs) et sont a l'origine de la destruction de l'ozone. Dans ce travail, nous avons etudie l'incorporation de gaz traces HCI et HBr lors des deux mecanismes de formation de la glace atmospherique (co-condensation et givrage), ces especes ayant ete choisies pour leur interet atmospherique et pour des raisons de faisabilite experimentale a l'ESRF. de Grenoble. Nous avons tout d'abord developpe un dispositif de spectroscopie infrarouge qui nous a permis de fabriquer des filins de glace cristalline stables a 190 K, temperature de la stratosphere polaire hivernale, et d'etudier les interactions entre HCI et la glace par condensation d'un melange gazeux HCI/H20 a 190 K. Nos resultats semblent montrer qu'HCI est incorpore de facon homogene dans la glace par un processus de solvatation ionique qui entraine une deformation du reseau cristallin du solide. Cela suggere que la glace constituant les PSCs serait constituee de nombreux defauts de volume et de surface. Entre autres, ces defauts pourraient jouer un role non negligeable dans la reactivite de la glace catalysant les reactions heterogenes conduisant a la destruction de l'ozone stratospherique polaire. De plus, nous avons etudie l'environnement d'HBr dans la glace formee par givrage de solutions aqueuses d'HBr par EXAFS a l'ESRF. Nos resultats sont preliminaires et suggerent qu'a environ -20°C, une part importante d'HBr degaze. La partie restant dans la phase solide semble etre incorporee de facon homogene dans la glace sous forme d'une solution solide sursaturee. Ainsi le devenir des gaz traces pieges dans le manteau neigeux serait con1role par le cyle de fonte/regel se produisant lors du metamorphisme de la neige plutot que par des mecanismes de diffusion en phase solide, trop lents pour permettre le degazage de ces especes.
Abstract Continuous monitoring of the atmospheric boundary layer (ABL) depth ( z i ) is important for investigations of trace gases with near‐surface sources. The aim of this study is to examine the temporal variability of z i on both diurnal and seasonal time scales over a full year (2011) and relate these changes to the atmospheric 222 Rn concentrations ( C Rn ) measured near the top of a 200 m tower at a rural site (Trainou) in France. Continuous z i estimates were made using a combination of lidar and hourly four‐height carbon dioxide (CO 2 ) profile measurements. Over the diurnal cycle, the 180 m C Rn reached a maximum in the late morning as the growing ABL passed through the inlet height (180 m) transporting upward high C Rn air from the nocturnal boundary layer. During late afternoon, a minimum in the C Rn occurred mainly due to ABL‐mixing. We argue that ABL dilution occurs in two stages: first, during the rapid morning growth into the residual layer, and second, during afternoon with the free atmosphere when z i has reached its quasi‐stationary height (around 750 m in winter or 1700 m in summer). An anticorrelation ( R 2 of −0.49) was found while performing a linear regression analysis between the daily z i growth rates and the corresponding changes in the C Rn illustrating the ABL‐dilution effect. We also analyzed the numerical proportions of the time within a season when z i remained lower than the inlet height and found a clear seasonal variability for the nighttime measurements with higher number of cases with shallow z i (<200 m) in winter (67.3%) than in summer (33.9%) and spring (54.5%). Thus, this pilot study helps delineate the impact of z i on C Rn at the site mainly for different regimes of ABL, in particular, during the times when the z i is above the measurement height. It is suggested that when the z i is well below the inlet height, measurements are most possibly indicative of the residual layer 222 Rn, an important issue that should be considered in the mass budget approach.
Abstract. Fire is regarded as an essential climate variable, emitting greenhouse gases in the combustion process. Current global assessments of fire emissions traditionally rely on coarse remotely sensed burned-area data, along with biome-specific combustion completeness and emission factors (EFs). However, large uncertainties persist regarding burned areas, biomass affected, and emission factors. Recent increases in resolution have improved previous estimates of burned areas and aboveground biomass while increasing the information content used to derive emission factors, complemented by airborne sensors deployed in the tropics. To date, temperate forests, characterized by a lower fire incidence and stricter aerial surveillance restrictions near wildfires, have received less attention. In this study, we leveraged the distinctive fire season of 2022, which impacted western European temperate forests, to investigate fire emissions monitored by the atmospheric tower network. We examined the role of soil smoldering combustion responsible for higher carbon emissions, locally reported by firefighters but not accounted for in temperate fire emission budgets. We assessed the CO/CO2 ratio released by major fires in the Mediterranean, Atlantic pine, and Atlantic temperate forests of France. Our findings revealed low modified combustion efficiency (MCE) for the two Atlantic temperate regions, supporting the assumption of heavy smoldering combustion. This type of combustion was associated with specific fire characteristics, such as long-lasting thermal fire signals, and affected ecosystems encompassing needle leaf species, peatlands, and superficial lignite deposits in the soils. Thanks to high-resolution data (approximately 10 m) on burned areas, tree biomass, peatlands, and soil organic matter (SOM), we proposed a revised combustion emission framework consistent with the observed MCEs. Our estimates revealed that 6.15 Mt CO2 (±2.65) was emitted, with belowground stock accounting for 51.75 % (±16.05). Additionally, we calculated a total emission of 1.14 Mt CO (±0.61), with 84.85 % (±3.75) originating from belowground combustion. As a result, the carbon emissions from the 2022 fires in France amounted to 7.95 MtCO2-eq (±3.62). These values exceed by 2-fold the Global Fire Assimilation System (GFAS) estimates for the country, reaching 4.18 MtCO2-eq (CO and CO2). Fires represent 1.97 % (±0.89) of the country's annual carbon footprint, corresponding to a reduction of 30 % in the forest carbon sink this year. Consequently, we conclude that current European fire emission estimates should be revised to account for soil combustion in temperate forests. We also recommend the use of atmospheric mixing ratios as an effective monitoring system of prolonged soil fires that have the potential to re-ignite in the following weeks.
Abstract. Measurements of the mole fraction of the CO2 and its isotopes were performed in Paris during the MEGAPOLI winter campaign (January–February 2010). Radiocarbon (14CO2) measurements were used to identify the relative contributions of 77% CO2 from fossil fuel consumption (CO2ff from liquid and gas combustion) and 23% from biospheric CO2 (CO2 from the use of biofuels and from human and plant respiration: CO2bio). These percentages correspond to average mole fractions of 26.4 ppm and 8.2 ppm for CO2ff and CO2bio, respectively. The 13CO2 analysis indicated that gas and liquid fuel contributed 70% and 30%, respectively, of the CO2 emission from fossil fuel use. Continuous measurements of CO and NOx and the ratios CO/CO2ff and NOx/CO2ff derived from radiocarbon measurements during four days make it possible to estimate the fossil fuel CO2 contribution over the entire campaign. The ratios CO/CO2ff and NOx/CO2ff are functions of air mass origin and exhibited daily ranges of 7.9 to 14.5 ppb ppm−1 and 1.1 to 4.3 ppb ppm−1, respectively. These ratios are consistent with different emission inventories given the uncertainties of the different approaches. By using both tracers to derive the fossil fuel CO2, we observed similar diurnal cycles with two maxima during rush hour traffic.
