Application of the NARCM model to high‐resolution aerosol simulations: Case study of Mexico City basin during the Investigación sobre Materia Particulada y Deterioro Atmosférico‐Aerosol and Visibility Research measurements campaign

The Northern Aerosol Regional Climate Model (NARCM) is used to study the visibility and three‐dimensional (3‐D) evolution of aerosol distributions within the Mexico City basin. NARCM simulates transport, diffusion, deposition, and size distributions of sulphur aerosol particles in the region. The model assumes only simple sulphur oxidation, not taking explicitly into account the urban air chemistry. Rather, it focuses on detailed aerosol microphysics and 3‐D optical properties. The simulation performance is compared with upper air and ground‐based observations for the following specific days of intensive measurement: 2, 4, and 14 March 1997. Time series at Mexico City airport shows lower values of visibility in the morning due to a shallow mixed layer and higher values in the afternoon following the evolution of the mixed layer depth. Modeled visibility shows large dependence on cardinal direction and size distribution of particles. It is found that better resolution of particle size leads to better representation of coagulation processes and to realistic size distributions which show a characteristic accumulation mode around 0.3 μm. As a result, visibility simulations are closer to those observed at the airport location. Comparing visibility is a stringent test for the model because it requires an accurate representation of 3‐D meteorological fields together with a realistic aerosol simulation.