3.7. Effect of Pollution Sources on Air Temperature Profile

Okay, so, let’s consider two things, first thing to be considered is what is called as the temperature profile as a function of height. So, we are saying that vertical convection happens as a result of thermal forces which means there’s a temperature difference. So, what is the temperature difference that will result in vertical movement of air masses? So, which means that I need to know what is the temperature profile as a function of height.

The temperature profile refers to how temperature changes with height above the Earth's surface. This is important in understanding how air moves and the role temperature differences play in air convection. When the temperature is uneven, air can rise or fall, leading to various atmospheric conditions.

Think of a hot air balloon. The air inside the balloon is heated, making it less dense than the cooler air outside. This temperature difference causes the balloon to rise. Similarly, temperature differences in the atmosphere influence how air masses move.

During daytime, the radiation heats up the soil or the land faster than it heats the air. [...] Therefore, the air closer to the surface is hotter than air above.

In the daytime, sunlight heats the ground quickly, causing the temperature of the soil to rise. This warmth is transferred to the air above, resulting in a temperature gradient where the ground is warmer than the air above. This gradient is essential for understanding air movement.

Imagine standing on a sunny beach. The sand feels much hotter than the air above it. The warm sand heats the air right above it, causing it to rise and create gentle breezes. This process helps explain how temperature differences drive airflow.

But when there is no radiation, say at 7pm, the soil starts cooling, and as a result, the air is still warm but starts cooling more slowly. [...] You can see it very clearly.

At night, the sun sets, and the ground loses heat quickly. The air above, however, retains heat longer. This results in a situation where the ground temperature is cooler than the air directly above it. This temperature difference influences air movement during nighttime.

Consider a campfire at night. The ground around it cools quickly after the fire goes out, while the air might still be warm. If you’ve camped overnight, you might notice how quickly the ground feels cool compared to the warmer air above.

This profile is called as an environmental lapse rate. It is called a lapse rate because it is temperature profile as a function of height. [...] This region is called as the temperature inversion.

The environmental lapse rate describes how temperature decreases with altitude. In some cases, like during inversion, the typical pattern is reversed, where temperature increases with height. This means warmer air is sitting over cooler air, which can trap pollutants close to the surface.

Consider a hot bowl of soup. The steam rises and creates a warmer layer above the cooler air around it. If you put a lid on the bowl, the steam gets trapped, similar to how pollutants can be trapped under a layer of warm air in a temperature inversion.

If I drop a parcel of air coming from a pollutant source, several things can happen to it depending on the environmental lapse rate. [...] This is called atmospheric stability.

When pollutants are released into the atmosphere, their movement depends on stability. In stable conditions, pollutants remain trapped near the ground, leading to higher concentrations. In unstable environments, pollutants can disperse more effectively, reducing concentration in any given area.

Think of spraying water into the air. On a calm day (stable condition), the mist stays close to the ground. But on a windy day (unstable condition), the mist spreads quickly, much like how pollutants disperse depending on atmospheric conditions.

This is the definition of what people call as the mean mixing height which is the intersection of the adiabatic and environmental lapse rates. [...] This concept of stability and mixing height is very important when we are looking at pollutant transport in the atmosphere.

Mean mixing height indicates the altitude where pollutants are mixed into the atmosphere. Understanding this height helps predict how pollutants will spread and concentrate, which is crucial for assessing air quality and environmental health.

Consider a jar of cookie dough. If you stir it just at the top, it stays well mixed—a lot like how pollution spreads evenly within a certain height above the ground. If you only stir at the bottom, ingredients remain concentrated where you stirred, similar to how pollutants can build up under an inversion layer.

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