Long-term simulation and evaluation of cosmogenic Beryllium-7: insights into atmospheric transport and deposition processes

Cosmogenic beryllium-7 (⁷Be) is a valuable natural radiotracer for constraining atmospheric transport, vertical exchange, and aerosol removal processes in chemical transport models. We present a comprehensive implementation of ⁷Be in the Danish Eulerian Hemispheric Model (DEHM), a regional chemical transport model, and present model output for 2000-2025 evaluated against long-term ⁷Be observations over Europe.

The ⁷Be production rates are prescribed from the CRAC:7Be (CRAC:Be) model, which includes the main geophysical controls on atmospheric cosmogenic ⁷Be production. Three-dimensional production fields are mapped to the DEHM grid with 75×75 km resolution horizontally and 29 vertical layers from surface to 100 hPa, and coupled with the advection–diffusion, boundary-layer mixing, and wet/dry deposition schemes in DEHM to simulate near-surface ⁷Be concentrations. Meteorological fields driving DEHM are simulated using the Weather Research and Forecasting (WRF) model based on ERA5 reanalysis data. ⁷Be is treated as particle-bound accumulation-mode aerosol in all the model processes and with a representative particle diameter of 0.33 μm for dry deposition.

The model is evaluated against observations from 18 European sampling sites spanning contrasting climatic and dynamical regimes (mid- and high-latitude stations), with heterogeneous sampling strategies (daily to weekly/monthly). Model performance is assessed using temporal Pearson correlation (r), root mean square error (RMSE) and mean absolute percentage error (MAPE).

DEHM reproduces observed ⁷Be temporal variability with strongly site-dependent skill. Mid-latitude stations, particularly in central Europe, show high temporal correlations and lower bias, indicating that synoptic-scale transport and mixing are reasonably represented at 75 km resolution. In contrast, northern and southern Europe stations show larger MAPE and more pronounced regional mean biases. These discrepancies likely reflect challenges in representing wet scavenging, boundary-layer dynamics, and sub-grid processes including orographic precipitation, coastal effects, and convective activity. The larger errors over southern Europe may also be partly linked to episodic coarse-mode aerosol conditions (e.g., dust outbreaks), which are not explicitly resolved in the accumulation-mode ⁷Be aerosol size representation.

To better understand these discrepancies, we stratify model performance by precipitation regime to constrain wet scavenging and by season to assess boundary layer mixing and stratosphere-troposphere exchange. We quantify wet versus dry deposition contributions on temporal variability and assess resolution/representativeness effects at coastal and complex-terrain sites. Sensitivity tests of different particle size assumptions on Saharan dust events to assess the role of size-dependent removal processes. These analyses advance our understanding on ⁷Be cycling mechanisms and demonstrate the value of cosmogenic tracers for evaluating transport and deposition processes in regional models.