Accurately targeting and tracking climate change mitigation efforts requires detailed bottom-up greenhouse gas (GHG) emission inventories verified by independent atmospheric measurements. While policy decisions often rely on annual national or city-scale inventories, higher-resolution data, both spatially (sub-city) and temporally (daily to hourly), though more uncertain, provide key advantages. First, they are input to inverse modelling of emission sources from atmospheric measurements, which offers a semi-independent approach to verify bottom-up estimates. Second, they can enable simulations to evaluate the impact of interventions, such as changes of policies, industrial standards or household behaviour, on emissions and atmospheric concentrations. Third, by providing localized, near real-time emissions data that can enhance the communication and tracking of climate actions, they can motivate both behavioural and policy shifts.

This study presents a straightforward method to create high-resolution carbon dioxide emission inventories for road traffic (street level) and heating (building level) using publicly available data such as OpenStreetMap. Scalable across Germany and adaptable to diverse contexts and applications, the method is demonstrated in Mannheim and Heidelberg, part of the Rhine-Neckar Metropolitan Area.

For road traffic, emissions are derived by estimating daily traffic volumes using road type, lane count, and population density, combined with speed- and fuel-specific emission factors and national vehicle fleet data. Heating emission estimates combine building data with gridded census data on heating energy sources and building age.

We validate our traffic volume against independent traffic count data and compare our emissions with existing inventories. Road traffic emissions in the Rhine-Neckar region exceeded the regional estimates of TNO (Super et al., 2021), a widely used European inventory, by 1.6%. Building heating emissions were 12% and 8% lower than inventory estimates for Mannheim and Heidelberg, respectively, primarily due to differences in emission factor assumptions.

How to cite: Ulrich, V., Block, S., Martin, M., von Elverfeldt, K., Murai von Buenau, K., Haas, P., Maiwald, R., Butz, A., and Vardag, S. N.: A scalable method for high-resolution bottom-up GHG emission inventories using open data, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-392, https://doi.org/10.5194/icuc12-392, 2025.

Urban carbon dioxide (CO₂) emissions contribute significantly to global anthropogenic greenhouse gas emissions. However, accurately monitoring emissions in individual cities remains a substantial challenge that could hinder effective local mitigation measures. In Vienna, a novel approach integrating stationary, tall-tower eddy covariance (EC) and CO₂ mole fraction measurements with mobile ground-based sampling of CO₂ concentrations was developed and tested. The mobile system consisted of a car equipped with a LI-COR 7200RS gas anaylzer and GPS device, recording location and CO₂ concentrations at 2 m height every second along a predefined route that traversed the flux footprint of the EC system (LICOR 7500DS and Gill Windmaster Pro). At the tower, continuous, parrellel measurements of turbulent fluxes and CO₂ mole fractions at 144 m above the surface were made by the said EC system and a Picarro isotope analyzer (G2131-i), respectively. The mobile measurements were averaged into 100 x 100 m grid cells and combined with the tall tower Picarro observations to derive vertical gradients in CO₂ molar concentations for each ha of the city intersected by the route. Surface fluxes were derived from these gradients using the aerodynamic resistance approach, based on Monin-Obukhov Similarity Theory (MOST) and the turbulence variables recorded by the EC system. These fluxes were compared to a downscaled CO₂ emissions inventory at both hectare-scale and zonal resolutions. Initiated in August 2024, the measurement campaigns, based on 2-hour midday surveys, have shown strong alignment between mobile measurements and inventories. This conference contribution will discuss the potential value of combining mobile and stationary measurements methods for investigating urban CO₂ fluxes. Currently, additional measurement campaigns are ongoing to assess the consistency of results across seasons. Furthermore, maps of LAI will be introduced to investigate whether vegetation explain temporal and spatial divergence between inventory and mobile measurement-derived fluxes.

How to cite: Fasano, E., Matthews, B., Meeran, K., Leitner, S., Vuolo, F., Watzinger, A., and Schume, H.: Mobile Ground-Based Measurements of CO2 Fluxes in Vienna, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-423, https://doi.org/10.5194/icuc12-423, 2025.

In-situ measurements of CO₂ emissions play a critical role in quantifying cities’ contributions to regional and global emissions and are a key tool in the validation of city emission inventories and models. Cities are also complex environments containing a multitude of anthropogenic CO₂ emission sources such as traffic, residential heating, and industrial production. Correct sectoral attribution of urban GHG emissions is necessary to monitor emission reduction efforts, compare against emission inventories, and separate anthropogenic from biogenic emissions.

As part of the ICOS-Cities (PAUL - Pilot Application in Urban Landscapes) project, tall-tower urban eddy covariance (EC) systems were installed in a small city (Zurich, Switzerland), a moderate-sized city (Munich, Germany) and a large city (Paris, France). Use of a high-frequency multi-species gas analyser (MGA-7, MIRO Analytical, Switzerland) together with an ultrasonic anemometer (CSAT3, Campbell Scientific, USA), enabled simultaneous flux measurements of CO₂, and co-emitted species CO and NO_x. EC measurements provide gas fluxes which integrate all emission sources and sinks within the measurement footprint. By examining the ratio of these gas fluxes in combination with a spatially-resolved emission inventory within the EC footprint, one may validate or improve the emission inventory.

We present six months of EC flux ratios of CO₂/CO, CO₂/NO_x, and NO_x/CO from each of the three cities covering both growth and dormant seasons, comparing diurnal and seasonal trends in the fluxes and flux ratios, as well as direct comparison against bottom-up emission inventory ratios for each species pair. We further demonstrate that a linear-mixing model is able to generally decompose CO₂ fluxes into major sectors and anthropogenic vs biogenic emissions and discuss its performance and limitations.

How to cite: Hilland, R., Hashemi, J., Stagakis, S., Bignotti, L., Loubet, B., Lan, C., and Christen, A.: Tall-tower urban eddy covariance flux ratios of CO2, CO, and NOx in three European cities, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-468, https://doi.org/10.5194/icuc12-468, 2025.

Evaluating the efficacy of climate mitigation measures requires quantifying urban greenhouse gas (GHG) emissions. Both anthropogenic and biogenic GHG fluxes are important in urban systems, and disaggregation is necessary to understand urban GHG fluxes. In urban environments one common source of biogenic carbon dioxide (CO₂) fluxes is turfgrass. We use CO₂ fluxes measured using eddy covariance over a cemetery (less managed) and golf course (more managed) to investigate the contribution of turfgrass lawns to biogenic CO₂ fluxes in Indianapolis, IN. We assess the ability of a simple light-use efficiency model, the Vegetation Photosynthesis and Respiration Model (VPRM), commonly used to create prior fluxes necessary for determining urban carbon dioxide (CO₂) fluxes via inversion modeling, to represent daily and seasonal patterns in turfgrass CO₂ fluxes. Our results show that the existing VPRM Plant Functional Types (PFTs) cannot capture observed daily and seasonal fluxes at either location.  We then use data from these sites to create a new turfgrass PFT for the VPRM. We find that less-managed lawns like cemeteries are best represented by different parameters than heavily managed lawns like golf courses, and seasonally changing parameters best match the observed fluxes. We then use the new turfgrass PFT within the VPRM to explore daily and seasonal variability in turfgrass fluxes and their impact, integrated across the city, on urban ecosystem CO₂ fluxes. This study illustrates the importance of representing turfgrass as a unique PFT when quantifying urban GHG fluxes and the biases resulting from misrepresentation.

How to cite: Horne, J., Jin, C., Miles, N., Richardson, S., Murphy, S., Wu, K., and Davis, K.: What Is the Impact of Turfgrass on Urban Carbon Dioxide Fluxes?, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-324, https://doi.org/10.5194/icuc12-324, 2025.

Numerous cities are developing ambitious climate action plans, reducing fossil fuel CO₂ emissions and increasing carbon removals through urban greening. Monitoring their progress towards carbon neutrality requires novel observational and modelling approaches. A major challenge is the accurate attribution between the anthropogenic CO₂ emissions and the biogenic CO₂ fluxes. Widely applied methods in natural ecosystems, such as source partitioning using eddy covariance observations or ecosystem model simulations, are hampered by the extreme heterogeneity of the urban environment. Within ICOS-Cities project, intensive field ecophysiological and meteorological observations were implemented in several green areas of Zurich, Munich, and Paris. These included continuous tree sap-flow, soil temperature/moisture, and local meteorology observations, as well as regular field campaigns of tree leaf area index and chamber gas exchange measurements at soils, lawns and tree leaves. This study presents a new CO₂ assimilation and respiration model, which is calibrated, forced and constrained by the available in-situ observations, simulating hourly tree and lawn CO₂ fluxes, representative of the observation areas. The photosynthetic CO₂ assimilation model is constrained based on sap-flow estimated stomatal conductance for trees, and an empirical estimation of stomatal conductance based on vapour pressure deficit and soil moisture for lawns. Plant and soil respiration is modelled based on calibrated versions of the Arrhenius equation, considering also soil moisture in the case of soil respiration. The model is evaluated in an urban forest in Paris using eddy covariance data. The simulated fluxes are compared between locations and cities to assess the carbon sequestration potential of the urban green areas and identify the effects of the local environment and management practices on the urban biospheric processes. The first results indicate that old parks with reduced impervious land cover show high carbon sequestration and are less vulnerable to summer drought.

How to cite: Stagakis, S., Bignotti, L., Li, J., Emberger, S., Loubet, B., Fortineau, A., Chen, J., Mauder, M., Buchmann, N., and Kalberer, M.: Assessing the biogenic CO2 fluxes in urban green areas using an observation-constrained modelling approach, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-407, https://doi.org/10.5194/icuc12-407, 2025.

To date, photosynthesis by plant communities remains a widely applicable and feasible carbon sequestration method that helps offset some unavoidable greenhouse gas emissions. However, most research on this nature-based climate solution focuses on natural green spaces rather than urban green infrastructure. Further, large-scale and longitudinal studies are lacking, especially outside temperate regions. This research aims to study how environmental stress caused by the urban environment affects the carbon sequestration capacity of urban green spaces, contributing to a more thorough estimation of the carbon budget in the inventory reports and enlightening better design and management plans for enhancing the capacity, as a path to carbon neutral in the climate action plans. The research utilizes the classic CASA model to calculate Singapore's net primary production (NPP) with cloud-free mosaic Sentinel data from 2021 to 2024 to face cloud coverage challenges. Propensity score matching based on canopy height and soil type matches urban green spaces within 800 meters from their boundaries with green spaces further away with less intervention, and the research calculates their difference in NPP, reflecting the anthropogenic impact. The results show that the duration of shading by buildings, the density of surrounding residential units, restaurants, food centers, and power plants, proximity to roads and waterbodies, percentage of surrounding impervious land, mean land surface temperature around, and traffic flow can explain 68% of the difference between actual and potential carbon sequestration. The duration of shading is the most evident anthropogenic factor, which decreases urban green infrastructure's carbon sequestration capacity, followed by surrounding land surface temperature. The research provides quantitative evidence of anthropogenic impacts on urban NPP in tropical cities with causal inference. Combined with further microclimate and plant species studies, the research would offer detailed planning and design strategies to relieve anthropogenic stress and increase carbon sequestration for urban green spaces.

How to cite: Wu, Y. and Biljecki, F.: Unraveling the Anthropogenic Impact and Environmental Stress on Net Primary Production of Urban Green Spaces in Singapore, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-711, https://doi.org/10.5194/icuc12-711, 2025.

The New York Metro Area (pop. 20M) is the most populous urban area in the United States of America (USA) and the largest urban source of CO₂ and methane (CH₄) in the US. Our urban core observatory in Manhattan and background tower sites in rural areas around the city allow us to study the source characteristics of various greenhouse gas and related air pollutant fluxes in the city. Using our observations, an atmospheric transport model (HRRR-STILT), and various prior emissions estimates, we have quantified the city-scale emission rate of methane and carbon monoxide (CO) and during multiple winter-to-spring transitions (Jan-May 2019-2024) and year-round since May 2023. We find that the observed city-scale methane fluxes from NYC are 2-3 times higher than expected from inventories. The fraction of natural gas in mixed methane source areas is often quantified by comparing the observed atmospheric ethane (C₂H₆):CH₄ ratio to that reported in the pipeline. However, this approach assumes no change in the C₂H₆:CH₄ ratio during the natural gas combustion process. We observed depleted ethane (lower C₂H₆:CH₄ ratios) in combustion plumes with high CO. In the laboratory, we sampled the stack exhaust of a natural gas boiler and found badly operated boilers can release CH₄, C₂H₆ and CO during the incomplete combustion of natural gas, but with C₂H₆ depleted relative to the C₂H₆:CH₄ ratio of the incoming pipeline. Previous studies using ethane:methane would have underestimated the natural gas contribution of methane emission if they assumed an unchanged C₂H₆:CH₄ ratio. We continue to characterize street, rooftop and point source methane emissions within and around the city using this new understanding of C₂H₆:CH₄:CO ratios to understand the source characteristics of methane emitted from pre- and post-meter natural gas, wastewater treatment plants and landfills.

How to cite: Commane, R., Hallward-Driemeier, A., Schiferl, L., and Zhao, Y.: Methane sources in cities re-interpretated using new laboratory results, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-320, https://doi.org/10.5194/icuc12-320, 2025.

Reducing emissions of greenhouse gases (GHGs) from urban areas – currently accounting for 70% of the global GHG emissions – are an integral part of the solution to meet net zero targets (IPCC, 2022). We have developed an observational framework to determine long-term trends in GHG emissions from the City of Edinburgh, Scotland, which has a 2030 net-zero (CO₂e) target. This is part of the GHG Emissions Monitoring network to Inform Net-zero Initiatives +Edinburgh (GEMINI+Edinburgh) which extends GEMINI-UK (Kurganskiy et al., 2025).

To determine city-wide net emission estimates of CO₂ and methane, we use column concentrations of CO₂ and methane retrieved from six EM27/SUN Fourier transform solar absorption spectrometers (“EM27”, Bruker GmbH, Germany) deployed in a ring around Edinburgh with 5 – 8 km separation. The spectrometers use the sun as their open-path source to measure solar radiation (4000 - 12000 cm^-1, 0.5 cm^-1 spectral resolution) to retrieve column abundances of CO₂, methane, CO, O₂, and H₂O. We apply an upwind-downwind mass balance approach to the data collected from the spectrometers to estimate city-wide net emissions. Spatial variations in near-surface wind fields are captured by eight automatic weather stations (Vaisala WXT530) and sonic anemometers (Gill WindSonic 75) co-located with each EM27 and on additional tall buildings.

We have developed methods to ensure that GEMINI+Edinburgh delivers long-term (O10 year) measurements and reliable GHG emission trends. As delivered, the EM27 is not weatherproof and not designed for unsupervised operation. We demonstrate the performance of a new weatherproof enclosure (Karn Scientific Ltd., Edinburgh) using co-located (un)enclosed EM27s. We employ a data processing workflow to enable long-term automated operation of the network, including an instrument meta data system, data transfer, processing, diagnostics plots, and archiving. We present preliminary data from the observation network since spring 2025 and propose strategies to communicate and evaluate Edinburgh’s net-zero progress.

How to cite: Morrison, W., Woodwark, J., Finch, D., and Palmer, P.: A ground-based remote sensing measurement network designed to infer net emission of CO2 and methane from the City of Edinburgh , 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-780, https://doi.org/10.5194/icuc12-780, 2025.

Methane (CH₄) is a potent greenhouse gas with a significantly higher global warming potential than carbon dioxide over shorter timescales. Urban areas are key contributors to CH₄ emissions, driven by industrial activities, landfill sites, wastewater treatment plants, and other anthropogenic and biogenic sources. However, the lack of ground-based methane monitoring limits our understanding of its spatial distribution and seasonal variability in Indian cities. This study leverages satellite-based remote sensing to assess methane concentration variations across small, medium, and large cities of Gujarat for the year 2024 using data from the TROpospheric Monitoring Instrument (TROPOMI).

By analysing monthly and seasonal CH₄ profiles, we explore the spatial and temporal patterns of methane emissions and investigate their correlation with urbanization levels, emission sources, and meteorological factors. The study provides an inter-comparison of methane levels among cities of varying sizes, offering insights into the role of urban growth, industrialization, and climatic influences in shaping methane distribution. The findings highlight key emission hotspots and temporal trends, which are crucial for policymakers and regulatory agencies in designing targeted methane mitigation strategies.

Understanding these variations is critical for improving air quality management and aligning with climate action policies. The outcomes of this research can support the development and enhancement of city-specific air quality action plans, contributing to more effective methane reduction efforts in urban environments.

Keywords: Methane; Greenhouse gases; Satellite Data; Air Pollution; Mitigation

How to cite: Gadekar, K. and Kandya, A.: Assessing the Methane Concentration Variation over the Small, Medium and Big Cites of Gujarat, India Using Satellite Data, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-710, https://doi.org/10.5194/icuc12-710, 2025.

Cities are hotspots for anthropogenic greenhouse gas (GHG) emissions and, therefore, play an essential role in mitigating climate change. It is crucial to accurately monitor urban GHG emissions to track the reduction targets that many cities have set. The ICOS Cities project aims to pioneer GHG measurement methodologies. Munich, along with Zurich and Paris, serves as one of the pilot cities for this project. Here, we present key results of the ICOS Cities project for Munich.

We developed a multi-scale sensor network in Munich, including five solar-tracking spectrometers (MUCCnet) for total column CO₂, CH₄, and CO measurements, 20 mid-cost CO₂ sensor systems, 99 low-cost CO₂ sensors, and 30 low-cost air quality sensor systems on rooftops and streetlamps. A mobile measurement unit, consisting of an e-bike carrying a high-precision instrument, is used to find emission hotspots in the city. In addition, CO₂ and CH₄ satellites specifically target Munich to enable a comparative analysis with MUCCnet.

To achieve a high-resolution and accurate emission estimate for Munich, we also developed (i) an emission inventory with high spatial and temporal resolution (100m, hourly) for the road traffic, heating, public power, and human respiration sectors; (ii) a microscale transport model (10m, hourly) based on GRAMM/GRAL simulations; and (iii) a high-resolution biosphere model (10m, hourly) based on VPRM using a self-developed vegetation land cover (1m) and Sentinel-2 satellite vegetation indices.

Combining these fundamental components, we performed inverse modeling to assess the emissions in Munich for more than five years. We compared the performances of inverse modeling algorithms using different background approaches, transport models, and prior emissions. Changes due to interventions and policies, e.g., COVID lockdown and energy transition, can be observed in the emission assessments. We present measurement-based emission trends determined by our observations and modeling tools, and provide recommendations on monitoring strategies for other cities.

How to cite: Chen, J., Stauber, J., Li, J., Aigner, P., Kühbacher, D., Makowski, M., Luther, A., Wenzel, A., Tang, H., Hinderer, J., Dietrich, F., and Asam, C.: Observational and Modeling Tools for Monitoring Urban Greenhouse Gas Emissions: Results of the ICOS Pilot City Munich, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-721, https://doi.org/10.5194/icuc12-721, 2025.

Cities are hotspots for fossil fuel CO₂ (ffCO₂) emissions, and independent top-down validation of ffCO₂ emissions is essential to guide their reduction. However, the lack of direct ffCO₂ flux observations and the heterogeneous distribution of ffCO₂ and non-fossil CO₂ (nfCO₂) sources and sinks in urban areas impede the partitioning of net CO₂ flux measurements. While radiocarbon (¹⁴CO₂) is an ideal tracer for ffCO₂, the logistics and costs associated with laboratory analysis of air samples prevent continuous measurements. Proxy-based ffCO₂ estimates from continuous measurements of co-emitted species such as CO, on the other hand, are subject to larger uncertainties, as the proxy:ffCO₂ emission ratio varies depending on the combustion process. Consequently, flux partitioning often relies on bottom-up information and/or results from inverse modeling. 

Relaxed eddy accumulation (REA) measurements of ¹⁴CO₂ and co-emitted species such as CO, conducted within the ICOS Cities project on urban tall-towers in the cities of Zurich, Paris and Munich, have enabled us to independently estimate ffCO₂ fluxes and proxy:ffCO₂ flux ratios for ~250 individual hours. The measurements show significant nfCO₂ contributions as well as highly variable CO:ffCO₂ flux ratios. We will analyze temporal and spatial patterns, examine the proxy:ffCO₂ flux ratio of dominant sectors, and compare the mean atmospheric ratios between the three cities. Continuous ffCO₂ estimates are obtained by combining the calibrated proxy:ffCO₂ ratios with continuous proxy measurements. We will discuss the uncertainties of this approach with respect to the advantages of continuous CO₂ estimates.  

Although the combination of ¹⁴CO₂ and co-emitted species observations with the REA method presents a sophisticated approach that challenges the limits of current analytical capabilities, it provides a unique opportunity to experimentally verify the CO₂ flux composition for both bottom-up and inversion model-based urban CO₂ emissions.

How to cite: Kunz, A.-K., Hammer, S., Bignotti, L., Borchardt, L., Della Coletta, J., Emmenegger, L., Eritt, M., Gutiérrez, X., Hilland, R., Holst, C., Jordan, A., Kneißl, R., Lan, C., Legendre, V., Loubet, B., Preunkert, S., Ramonet, M., Stagakis, S., and Christen, A.: Towards Estimating Continuous Fossil Fuel (ff) CO2 Emissions in Urban Areas: 14CO2-Calibrated Proxy:ffCO2 Flux Ratios From Relaxed Eddy Accumulation Measurements, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-426, https://doi.org/10.5194/icuc12-426, 2025.

Global carbon dioxide (CO₂) concentrations continue to rise at an accelerating rate, prompting universal efforts to mitigate emissions. Urban areas play a significant role in this increase, as their substantial anthropogenic sources often surpass the capacity of natural sinks. Urban emissions vary in response to the need for the combustion of fossil fuels for building heating or cooling, vehicular traffic, and commercial/industrial activities. The emissions also vary spatially depending on urban land cover (LC) and land use (LU), generating particular emission patterns across the city. The surface exchange of CO₂ has been monitored since 2016 in the city center of Heraklion, Greece using the Eddy Covariance (EC) approach. During this period, two major incidents occurred that altered the transportation behavior of the residents and the LC/LU of the city center. In January 2018, the municipality of Heraklion imposed traffic regulations and started a long reconstruction project, close to the flux tower’s location. In addition, COVID-19 spread mitigation measures implemented by the Greek government in 2020, changed the commuting patterns of citizens. Results show a reduction in CO₂ emissions throughout these eight years likely caused by the shift in land use, vehicle traffic, and commuting patterns. The reduction amounts to ~ 35% when comparing the monthly medians of June 2017 and 2022. The period during the COVID-19 restrictions exhibits an even larger reduction, showing sudden drops in measured CO₂ fluxes.

How to cite: Politakos, K., Stagakis, S., Roth, M., and Chrysoulakis, N.: Dynamic changes in urban form and function affect Carbon Dioxide Fluxes in a Mediterranean city., 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-311, https://doi.org/10.5194/icuc12-311, 2025.

Tall-tower eddy-covariance (EC) measurements are useful to quantify the dynamics of net CO₂ emissions of large urban areas. With increasing measurement height, however, the change in CO₂ storage below the sensors (storage flux) and the divergence of horizontal advection may affect the accuracy of EC-based emission estimates more strongly. Such problems are expected if atmospheric layers become decoupled at night or if thermally induced circulations such as land-lake breezes develop over complex land cover or topography. For consistent comparisons with models and emission inventories, it is important to assess these effects. In the ICOS-Cities project, a tall-tower EC system has collected data in the city of Zurich, Switzerland, for more than two years. Furthermore, a network of mid-cost CO₂ concentration sensors was installed in and around the city – mainly on rooftops in combination with wind and temperature sensors. This study investigates the combined use of the tall-tower EC and the rooftop CO₂ and weather observations for an improved direct estimation of the net CO₂ emissions. The storage flux is estimated by dividing the control volume into two layers characterized by a CO₂ concentration change derived from the rooftop network and the tall-tower observation, respectively. The rooftop network is also used in a proof of concept for estimating the along-wind CO₂ concentration gradient and horizontal advection effects. Median diurnal cycles of the turbulent CO₂ flux show a pronounced morning peak but surprisingly low values during the evening rush hour if the wind originates from the direction of the city center and, at greater distance, lake Zurich. Considering the wind field and differences in potential temperature, CO₂ concentration, and specific humidity between the tall-tower and low-altitude sites, we investigate circulation patterns and vertical decoupling. The significance of the estimated storage flux and horizontal advection effects will also be discussed.

How to cite: Sigmund, A., Stagakis, S., Hilland, R., Brunner, D., Christen, A., Feigenwinter, C., Vogt, R., Emmenegger, L., and Kalberer, M.: Direct measurements of net CO2 emissions using a tall-tower eddy-covariance system and a rooftop CO2 sensor network in the city of Zurich, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-713, https://doi.org/10.5194/icuc12-713, 2025.

Urban environments play a critical role for greenhouse gas (GHG) monitoring as they are responsible for more than 70% of global carbon emissions and their role is expected to further increase with continued urbanization. At the same time, many political decisions are implemented on a city-level and many cities have their own targets for carbon neutrality. Yet, tracking changes in urban emissions is challenging because GHG are a complex mix of natural and anthropogenic fluxes with strong spatio-temporal variability.

Eddy covariance (EC) measurements of carbon dioxide (CO2) fluxes have become a widely applied method in urban areas in recent years. However, multi-annual studies capable of tracking long-term trends and inter-annual variabilities remain scarce. Here, we present a decade of CO2-flux measurements from the Urban Climate Observatory (UCO) Berlin site TU Campus Charlottenburg and a seven-year time-series from the ICOS-site Rothenburgstrasse (DE-BeR). We use footprint modeling, meteorological and high-resolution data on urban form and function to attribute the observed fluxes to local sources and sinks of CO2. The flux observations are used to validate first results from a bottom-up modelling study to quantify surface exchanges of CO2 for the whole city of Berlin at high spatial resolution. The model includes the main anthropogenic and biological components of the carbon cycle in the urban system, such as road transportation, building energy consumption, human respiration, and Net Ecosystem Exchange of CO2 for an holistic approach.

The observations show a slight gradual reduction of CO2 emissions over the analysed time. Further, observational and modelling results show a significant reduction of CO2 emissions during the COVID-19 pandemic in comparison to previous years, with a maximum decrease of up to 25% in traffic emissions at the city scale. Our data and findings support city-scale quantification of GHG emissions, as well as future studies focussing on, e.g., inter-city comparisons.

How to cite: Meier, F., Anjos, M., and Fenner, D.: Long-term observations and bottom-up modelling of carbon dioxide fluxes in Berlin, Germany, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-463, https://doi.org/10.5194/icuc12-463, 2025.