Thermal inversion.
A thermal inversion forms when a warm layer of air traps a cooler layer of air close to Earth's surface. The cold air ends up at the surface when the mountains block the morning sunlight from warming the floor of the valley or particulate pollution blocks sunlight from reaching the surface.
The increased aerosols in the urban atmosphere reduce the incoming solar radiation and increase the atmospheric longwave radiation in the urban areas. The response of the surface to the change of absorbed radiation is strong at night and weak during the day.
The warm air above cooler air acts like a lid, suppressing vertical mixing and trapping the cooler air at the surface. As pollutants from vehicles, fireplaces, and industry are emitted into the air, the inversion traps these pollutants near the ground, leading to poor air quality.
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