1.1. Air-Water Exchange

We look at evaporation of a pollutant from a lake okay. So, there is a lake and there is air. There are some volume and we are talking about this thing.

Air-water exchange refers to the process where pollutants evaporate from water bodies (like lakes) into the air. This is significant for understanding how pollutants disperse in the environment. In this example, we consider a scenario where a pollutant is present in a lake, and we want to analyze how it transfers from the water to the air.

Imagine you have a pot of water on the stove. If you drop some food coloring into the water, that color represents a pollutant. As the water heats up and begins to evaporate, the food coloring will eventually leave the water and enter the air. This illustrates the concept of air-water exchange with pollutants.

So, for this system, rate of accumulation of A in the system equals rate in minus rate out.

In any system such as a contaminated lake, we can apply the mass balance principle. We denote 'A' as the pollutant. The rate at which the pollutant accumulates in the system (the lake) equals the rate at which it enters the lake minus the rate at which it leaves. In our scenario, we examine a case where pollutants can enter the lake through, for example, a pipeline and leave through evaporation.

Think of a bathtub with a faucet and a drain. If you turn on the faucet (adding water, analogous to adding pollutants) and simultaneously leave the drain open (allowing water to leave, analogous to evaporation), the bathtub will fill up until the water level stabilizes. The same principles apply to pollutant concentration in a lake.

This means that there is an initial mass that is sitting there already and that we have estimated it somehow.

When pollutants are introduced into a body of water, they create an initial concentration based on how much was released. Over time, as pollutants evaporate, the concentration will change. We need a method to estimate how the concentration evolves over time, factoring in the rate of evaporation and any residual pollution.

Consider a sponge soaked in ink. Initially, the ink represents a high concentration of the pollutant. If you place the sponge in an open area, the ink will begin to evaporate. Over time, the color will fade, just like pollutant concentration decreases in a lake as it evaporates.

Evaporation flux has units of MA by L square into T, area is L square, MA by T.

Evaporation can be quantified as a flux, which refers to the amount of substance that passes through a unit area per unit time. Understanding this helps model how quickly pollutants might evaporate from a lake into the atmosphere. The calculation involves both the area of the water surface and the characteristics of the pollutant.

Visualize a flat pond covered in a layer of oil. The surface area of the pond (the area available for evaporation) plays a crucial role. A larger surface area will allow more oil to evaporate faster compared to a smaller pond, showcasing how this concept applies to different sized water bodies.

If no information is given, you can do problems in multiple ways.

When modeling the evaporation of pollutants, assumptions about the environment (like whether the water is well-mixed or stagnant) can significantly change the outcome. If the lake is well-mixed, it behaves like a CSTR (Continuous Stirred Tank Reactor), where the concentration is uniform throughout. If stagnant, the concentration may vary from one region to another.

Think about stirring a glass of lemonade versus leaving it still. When stirred, the flavors blend uniformly, similar to a well-mixed lake, while the still glass has concentrated spots of flavor. Both scenarios demonstrate how conditions affect the rate and effectiveness of evaporation of pollutants.

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