The DAnish Lagrangian Model (DALM) (Andersen et al., 2024; Andersen, 2023) is a three-dimensional, high-resolution, Lagrangian particle dispersion model, first and foremost developed to compute air pollution concentrations over Denmark with a 1 km × 1 km resolution. It builds upon the capabilities of its predecessor, the Urban Background Model (UBM), and is able to compute hourly values of a set of health-related chemical components.

The model calculates trajectories of tens of millions of fictitious particles, each representing an ensemble of real atmospheric particles. These trajectories are determined by the local mean wind field and by random motion to simulate turbulent dispersion. This Lagrangian particle approach is very flexible since it easily allows for the inclusion of additional chemical components and the treatment of each particle independently from all the other particles. Furthermore, Lagrangian particle models exhibit less numerical diffusion compared to other popular model types and are well suited at handling emissions from point sources since they retain sub-grid information.

The model is coupled to the long-range chemical transport model, DEHM, for chemical boundary conditions and the WRF model for meteorology. For emissions, high-resolution data is provided together with the option to apply vertical profiles. The model includes dry deposition of some gases and the removal of nitrogen dioxide (NO₂) through chemical reactions with hydroxyl (OH) radical and ozone (O₃) and a relatively simple equilibrium mechanism for the photochemical reactions between nitrogen monoxide (NO), NO_2, and O₃. For examples of model results, see Figure 1 to 3 below as well as this 2D animation.

There is continual work to improve DALM and validate the model against measurements. In the future, DALM is planned to be integrated into the DEHM/UBM/AirGIS human exposure modelling system, with the goal of improving its accuracy.

References

• Andersen, C., M. Ketzel, O. Hertel, J. H. Christensen, J. Brandt: "The Danish Lagrangian Model (DALM): Development of a new local-scale high-resolution air pollution model". Environ. Model. Softw. 176, 106010 (2024). DOI: doi.org/10.1016/j.envsoft.2024.106010.
• Andersen, C.: "High-Resolution Modeling of Air Pollution in Denmark: With the DAnish Lagrangian Model (DALM)". Ph.d.-afhandling (2023), Aarhus Universitet.

Figure 1. Map of the monthly averaged modeled NO_x surface concentration for July 2017.

Figure 2. Modeled versus observed monthly averaged NO_x surface concentrations for 2015 to 2018. The dashed reference lines show a 2:1 and a 1:2 relationship between modeled and observed concentrations while the solid reference line shows a 1:1 relationship.

Figure 3. Monthly averaged NO_x surface concentrations for 2015 to 2018 at the rural monitoring station RISOE for DALM (red) and observations (green).