What are two reasons why a given pollutant may decrease in concentration over a period of several hours?
1 Answer
It typically depends on the pollutant in question however two of the most predictable methods are dilution and volatility.
Explanation:
Many pollutants wind up in water sources and are diluted out. If a pollutant is of a specific, limited, quantity and does not originate from a continuous source such as a mine or a manufacturing plant upstream then the entirety of that pollutant will be washed downstream (assuming that the pollution occurs in a flowing body of water). Some of the pollutant, if it is more dense than water (i.e. organophosphates or heavy metals) may fall to the riverbed as sediment, a large amount however will continue downstream and wind up in a larger body of water (the ocean or a lake) thereby reducing the concentration (diluting) of the pollutant.
Smoke is a major pollutant. To use a relatively obscure albeit interesting example, prior to the harvesting of sugar cane the crops are lit on fire to make it easier to harvest. This produces a lot of smoke (and a spectacular sight to see from an airplane) smoke is volatile, it rises and as it rises it becomes less thick. Winds at higher altitudes then blow the smoke away from the fires reducing the amount of smoke in the air. If you look at a smoke plume it is narrowest and darkest at its base, while it is widest and lightest near the top where the lowest concentration of smoke is.
I hope this helps!
