Imagine a pollutant as an unwanted guest in a family house. The pollutant will eventually overstay its welcome. Certain processes act like the “enforcers” of the family and remove the pollutant from the house. This happens frequently in the atmosphere!
There are three distinct processes that remove pollution from the atmosphere. The three processes are: gravitational settling, dry deposition, and precipitation scavenging.
Gravitational settling is where particulates come down to the surface by gravity. This process removes most particulates that are greater than 0.1 micrometers. The larger particles will be removed quickly. However, particles on the smaller end could remain suspended by turbulence. When the turbulence dies down the smaller particles will eventually settle down.
Dry deposition, also known as adsorption, is a turbulent transfer process. Dry deposition is a downward flux where the surface acts as a “sink.” Two causes of dry deposition are impaction and interception. Impaction is where smaller particles near larger ones cannot follow the normal flow, so they hit a water droplet. Interception is where small particles follow normal flow near an obstruction, and then the particle collides into the obstruction. The rate at which pollution descends towards the surface is determined by the state of turbulence, bacterial activity over soil, or surface tension over water. This is a continuous process because it is not dependent on any precipitation.
Precipitation scavenging is a more efficient process than dry deposition and gravitational settling. This process is the best at removing gases and small particulates. Many of these particulates are condensation nuclei. Condensation nuclei are particles which water vapor condenses upon in the atmosphere. Once many nuclei accumulate and become large enough to fall, it precipitates as rainout or snowout. Washout occurs outside of the cloud. The greater the precipitation, the greater the amount of pollution that can be removed. In comparing the efficiencies of washout, snowout, and rainout, washout tops the list. However, washout depends on the rainfall rate, the size of the rain drops, and the pollutants that it gathers.
In conclusion, air pollutants will eventually find their way out of the atmosphere by gravity, turbulence, or by precipitation.
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© 2018 Weather Forecaster Jennifer Naillon