CiTYCHEM

The CiTYCHEM extension uses the functionality of the EPISODE urban dispersion model to run simulations on an Eulerian grid as main grid with an embedded regular receptor grid which samples concentrations from Gaussian sub-grid dispersion calculations. The model solves the time-dependent advection/-diffusion equation on the Eulerian 3-D grid including complex chemical transformation and deposition processes. Finite difference method numerical are applied to integrate the solution forward in time. The combined EPISODE-CityChem (EPISODE with CiTYCHEM extension) model offers a suite of pre-processing utilities for generation of input data such as pollutant emissions, regional background concentrations, and meteorology.

The user can choose from numerical schemes with varying degree of accuracy for solving the advection/-diffusion processes. Depending on the chosen chemistry options, EPISODE-CityChem can be run either as a tracer model or as a chemistry model. Running as tracer model implies that all air pollutants are only dispersed as passive tracers, whereas running as chemistry model implies chemical transformation driven by photo-oxidation using either a detailed atmospheric chemistry mechanism or simple photo-stationary steady-state between ozone, nitrogen oxide and nitrogen dioxide.

Model Features

CiTYCHEM offers a series of features which are distinct from other dispersion models typically used for urban air quality studies:

• Calculates time-varying concentrations of air pollutants on a 3-D main grid with coarse resolution (≤ 1 km x 1 km) and simultaneously local ground air concentrations with high resolution (100 m x 100 m) as well as in selected locations (monitoring stations).
• Reads meteorological fields from model output of various meteorological models. Utilities are provided to produce a diagnostic wind field based on meteorological observations.
• Reads time-varying 3-D concentrations of compounds at the lateral and vertical boundaries from regional air quality models as initial and boundary concentrations.
• All physical processes of the model can be switched on/off for debugging or sensitivity testing.
• Runs on Linux computers in single processor mode.
• Provides concentration output and other diagnostic output in form netCDF files, enabling quick conversion into ArcGIS shapes.

License

The CiTYCHEM extension was developed at Helmholtz-Zentrum hereon GmbH. EPISODE is an Eulerian dispersion model developed at the Norwegian Institute for Air Research (NILU) appropriate for air quality studies at the local scale (Hamer et al., 2020). The EPISODE model and the CiTYCHEM extension are open source code subject to the Reciprocal Public License ("RPL") Version 1.5.

Documentation

• The model description of the EPISODE-CityChem model is published in Geoscientific Model Development.
• Find detailed guidance and information on installation and usage of CiTYCHEM in the User's Guide.

Publication

The application and performance of the software was demonstrated in several publications:

• CiTYCHEM 1: published in Atmospheric Chemistry and Physics (Ramacher et al. 2019), evaluate the urban population exposure in three Baltic Sea harbour cities.
• CiTYCHEM 2: published in IJERPH (Ramacher and Karl, 2020), combine CiTYCHEM with a dynamics exposure tool to improve population exposure assessment in urban transport environments.
• CiTYCHEM 3: published in Toxics (Karl et al., 2023), investigate ultrafine particles (UFP) in a Mediterranean harbour city.
• CiTYCHEM 4: The forecast system urbanAQF is documented in the publication by Karl et al. (2024) in the International Journal of Digital Earth.

Key Applications

• urban Air Quality Forecast for Hamburg displays maps of the forecasted air quality index and pollution concentrations for the city of Hamburg with a spatial resolution of 100 meter.
• Population exposure studies for air pollution and health on city scale.
• Simulation the atmospheric dispersion of pollutants from ships under movement (moving point source).
• Urban aerosols: secondary formation of aerosol particles and aerosol composition changes in urban areas.

Download

The software package can be downloaded from Zenodo.

Contact

Should you have any specific inquiries regarding CiTYCHEM, please do not hesitate to reach out to Matthias Karl (matthias.karl 'at' hereon.de) or to Martin Ramacher (martin.ramacher 'at' hereon.de).