Complex terrain is a region of land that has unusual or irregular topographical features, such as mountains, valleys, or coastlines. Compared to open plains, these features drive changes in local meteorological systems, impacting air flow dynamics.¹ In the instance of heavily urbanized areas, land topography is an often-overlooked factor when considering air quality and pollution levels.² Reducing air pollution to improve population health is an increasing focus for many governments. Yet, there are unique challenges faced by cities surrounded by complex terrain that should be taken into consideration.

Solar heating of the Earth’s surface creates diurnal variations in air flow across the planet.³ As the surface heats up, the air warms and rises, meeting cooler air at higher altitudes. A convection current forms as the air cools and falls back to the surface, creating a circulation pattern.⁴ Across flat landscapes, this will be a widespread phenomenon, with multiple currents and points of vertical air mixing. Combined with horizontal advection across the land, particles will be transported away from their initial location, in a process known as atmospheric dispersion.⁵  For cities within these areas, pollution is more readily lifted and swept from population centers, acting as a natural ventilation method for ‘unhealthy’ particles.⁶ However, geographical features interact with this process and alter typical air circulation dynamics. 

Elevation is the first metric to consider when assessing air flow and particle dispersion for a given location. Valleys and basins are more sheltered from some of these dispersion effects, allowing pollutants released from transport and heavy industry to accumulate within the land feature.⁷ This stagnation can occur on flat terrain during still weather conditions, however  stagnation is a recurring feature for complex topographies. Solar radiation can somewhat abate this accumulation during the day by the formation of anabatic winds.⁸ This is a consequence of similar circulation effects as previously described, with warm mountain slopes creating a weak air flow up and out of the valley. At night however, these effects are reversed, with the denser cool air sinking down the mountain side, forming stronger katabatic winds (high density air falling down a topographical feature due to gravity).^9,10 In valleys, this can cause cool air to pool and stagnate, magnifying air pollution. During the winter especially, anabatic winds are reduced, greatly impacting air quality in urban areas within valleys.¹¹ Synoptic winds, (winds from large nearby weather systems) can increase or lessen these effects, depending on direction.¹²  

Variations in humidity and temperature give rise to additional phenomena, such as inversion effects, where the upper layer of air is warmer than that closer to the ground.  In valleys and basins this inversion acts as lid, trapping ground level air and preventing dispersion.^13,14 Katabatic winds increase the chances of this happening, as cool air sinks quickly into a valley or basin. In developed areas this can lead to dangerous levels of smog persisting over an area.^15,16 When mountainous regions have proximity to large bodies of water, a new set of challenges are created. Unfortunately, these spots are often favorable locations for ports and port cities. This increase in emission sources compounds the problem, creating a unique health concern.¹⁷