Revisiting the airshed concept in the tropics for assimilative capacity characterization.

Air pollution has become a serious problem in developing countries due to industrial growth, poor understanding of the atmospheric circulation, weakly planned land use, and weakly enforced air pollution regulation. As part of this, the location of industrial emission clusters frequently does not account for air mass transport, pollutant dispersion, and topographic considerations. This leads to increased regional air pollution; furthermore, regulation remains inadequately focused on urban planning challenges and environmental impact assessments to allow future emissions. Simpler regulatory methodologies usually do not consider regional atmospheric characteristics governed by meteorology, topography, and current pollution load. Ignoring the impact of complex meteorological and topographic features on industrial cluster location leads to an incomplete framework for air quality governance and management. There are two conceptual approaches to regional air pollution management from physical and regulatory / governance perspectives. First, the airshed is defined as a geographic area (also known as an air quality control region), where the air pollutant emissions impact a common group of receptors independent of the administrative / jurisdictional boundaries. Second, the air basin concept refers to a volume of atmosphere in a region within the boundary layer where an airflow phenomenon occurs with similar meteorological and topographic characteristics. The application of this concept in tropical regions is further complicated due to their unique characteristics: complex airflow dynamics, including ventilation, stagnation, recirculation, vertical mixing within the tropical atmospheric boundary layer (ABL), and deep moist convection phenomena. For this reasons, air basins are fundamentally dynamic, with spatial and seasonal variability that can be addressed using the assimilative capacity concept, i.e., the allowable pollutant load under acceptable ambient air concentrations (emissions limits). We argue that the air basin and airshed definitions are currently poorly founded on meteorological and physical region characteristics, particularly in the tropics. In addition, assimilative capacity daily and seasonal variations are usually ignored, and air ventilation maps are not considered as tools for environmental licensing and emissions permitting assessment. Framing air basins is essential to properly estimating assimilative capacity and environmental and land use policies, potentially applicable to tropics. Moreover, an integral definition of an air basin applied to the tropics would cement the basis for the development of appropriate air quality regulations, approval of emission permits, interjurisdictional coordination for mitigation of air pollutants in regions. These concepts will be discussed along with a case example in an inter-Andean in Colombia.