Application of the ICON-ART model for CO2 fluxes estimation in the city of Zurich

Switzerland like many other countries has set ambitious goals to reach net zero CO₂ emissions by 2050. The city of Zurich has committed to an even more ambitious goal of becoming climate neutral by 2040. Recent developments in atmospheric observations and inverse modelling, including the results of the European infrastructure project ICOS, have already laid down the foundations for estimating emissions on national and continental scales. At urban scales, however, only few emission estimation studies have been conducted so far, and it is still an open question, which observational and modeling approaches are best suited and to what level of accuracy they are able to quantify the emissions of a city. The project ICOS-Cities/PAUL aims to answer these questions by evaluating different monitoring approaches in three European pilot cities. One of these cities is Zurich, where a street-level network of about 93 low-cost sensors at 60 locations is combined with a network of 21 mid-cost sensors, located mainly on roof-tops, and high-precision instruments at 3 background sites.

Our goal is to estimate the CO₂ emissions of Zurich by combining the observations from the mid-cost and high-precision instruments with the state-of-the-art atmospheric transport model ICON-ART. To this end, numerical experiments were conducted for two offline-nested domains, a European domain at ~6.6 km resolution and a second domain centered over the city of Zurich with a much higher resolution of ~0.6 km to represent the main topographic features of the urban area. Anthropogenic emission inputs were produced by merging three different inventories: the TNOGHGco inventory for areas outside Switzerland, a Swiss national inventory at 100 m resolution, and a detailed inventory of point, line and area sources produced by the city of Zurich. The biogenic fluxes of CO₂ were computed online using the Vegetation Photosynthesis and Respiration Model (VPRM) integrated into ICON-ART.

Here we present a first analysis of comparisons between model simulations and CO₂ observations inside and surrounding the city. This allows us to better understand the capabilities and weaknesses of the model at urban scales as well as to design optimal strategies for setting up an inversion framework.  A particular focus is placed on biospheric CO₂ and on how much it contributes to variability within the city in comparison with anthropogenic CO₂. The next steps will include the use of the CTDAS (The CarbonTracker Data Assimilation Shell) assimilation system in order to obtain information on CO₂ fluxes in the urban and suburban areas of Zurich and to apply the model system also to the city of Paris. During this work, we strive to develop approaches which can then be shared and applied by researchers to other cities around the world.