Investigating atmospheric mechanisms behind the long-range transport of fire aerosols: from the regional free atmosphere to the local boundary layer in a narrow valley

Regional fires result in large emissions of pollutants that can be transported through the atmosphere and impact air quality in remote regions. Studying regional fire aerosol transport is essential to understanding and forecasting air pollution events in different urban centers worldwide. However, there is a lack of research analyzing atmospheric patterns and mechanisms during regional fire aerosol transport events in tropical regions with complex topography. We investigate atmospheric conditions during regional fire aerosol transport events in the Aburrá Valley, a mountainous and highly urbanized region in the Colombian Andes. We combine observational and modeling approaches, including reanalysis data, high-resolution meteorological simulations, remote sensing information, and ground-based stations. Initially, regional transport events were selected from information from a Black Carbon monitor and verified using back-trajectories and hotspots data from MODIS. Besides, we performed two-month (February and March) 1-km resolution meteorological simulations with WRF model simulations for five years (2020-2024) to identify characteristic mesoscale patterns during aerosol transport events. Subsequently, we used information from a wind profiler radar, a microwave radiometer, and a series of sonic anemometers to study the incidence of polluted air masses in the local boundary layer. Our results show a notable reduction in precipitation both at a regional scale and in the path of air mass trajectories during regional transport events. Anomalous northwesterly regional winds are characteristic at low levels, and east-southeasterly winds dominate at mid-levels. At a local scale, these regional winds are channeled from the north and katabatic winds are intensified during night and early morning, leading to vertical wind shear within the boundary layer. These conditions favor the generation of mechanical turbulence during the night, enhancing the mixing of external pollutants towards the valley surface. Our study advances the understanding of mechanisms related to the impact of regional fire events on Aburrá Valley’s air quality, which can become more frequent under climate change conditions. Besides, these results contribute to the improvement of forecasting systems in the region, which is essential for comprehensive air quality management.