Sub-kilometer dispersion simulation of a CO tracer for an inter-Andeanurban valley

Özet

Air quality planning and management can benefit from the ability of numerical models to produce skillfulforecasts. However, obtaining these forecasts is a continuing challenge, particularly in complex terrain. Here weexamine the dispersion of traffic emissions in the Aburrá valley (located in the Colombian Andes) during anepisode of severe air pollution, using the WRF-Chem and a Lagrangian model at sub-kilometer resolution. Wefocus on the identification of areas with higher CO concentrations (the pollutant analyzed), and examine the roleof local and regional airflows in the distribution of CO inside the valley. The meteorological model performance,considering different planetary boundary layer parameterizations and grid sizes, is evaluated using availableobservations. Overall, the meteorological model performance is within recommended benchmarks for complexterrain. Model performance improves with increasing grid resolution for surface temperature and wind direction,but not for wind speed. Both dispersion models reproduce important features of the spatial and diurnal varia-bility of CO in the valley, but underestimate CO concentrations throughout the simulation period. The south andsoutheast areas of the valley present the largest CO concentrations, associated with a prevalent northerlytransport along the valley axis and reduced transport on the eastern slope. The representation of CO improveswhen the model adequately reproduces observed rainfall, due to effects of rainfall on boundary layer height andstability, which condition dispersion. Better simulation of transport processes is crucial for informing decisionson air quality management in critical areas such as urban valleys.