Role of Wind and Inversions on Air Pollution

chemicals remain in the atmosphere for approximately one hundred years, although it depends on the type of CFC.

Once it was determined that CFCs were a problem, the Montréal protocol was developed, which helped to reduce the numbers of CFCs we release into the atmosphere. Because of this protocol, we no longer have aerosol spray cans but instead use aerosol pumps, which do not have CFCs in them. One problem, however, is that we may not be able to completely recover from the ozone losses we have already achieved.

Now, getting back to air pollution and ozone, you need to talk about ground-level ozone or what we call bad ozone. Ozone near the earth at its surface can be a strong irritant to the lungs. Children, elderly persons, and those with lung diseases are particularly susceptible to the effects of ozone near the ground. Ozone is the main chemical you will see in smog.

How to get all of this ozone? As mentioned, it is primarily caused by chemical reactions we see between two types of molecules. These are nitrogen oxides and VOCs or volatile organic compounds. In other words, when you mix emissions from cars, large boilers, power plants, chemical plants, and refineries, you will get ozone but only if it reacts with sunlight. Once made, ozone can travel for very long distances.

The effects of ozone are largely related to the lungs. It is a chemical irritant that can cause irritation of the respiratory passages, inflammation of the same, coughing, chest pain, and worsening of pre-existing lung diseases. High levels of ozone will also affect ecosystems of all types, particularly those with a lot of vegetation and wildlife.

ROLE OF WIND AND INVERSIONS ON AIR POLLUTION

We have talked briefly about thermal or temperature inversions. Where they become more important is when talking about air pollution in urban areas. Let's first talk about these thermal inversions and why we get them. We need to understand this because air quality and our weather are tightly connected in many ways.

A temperature inversion involves the situation where cool air is trapped beneath warm air. You know by now that this is not it normally works. When this happens, the cool air

has no place to go and it sits stagnant over an area on the earth's surface. Fog is actually a situation of a thermal inversion when it sits above the grass. It is humid air that is trapped along the earth's surface. Most thermal inversions will eventually disperse along with the wind or whenever the sun warms up the cool air above the inversion layer. The most common type of thermal inversion is called radiation inversion.

There are many reasons you can get a thermal inversion. Let's look at these:

• Ground Topography - if there are low-lying areas, such as valleys or ponds, cool air can settle into them below a layer of warm air. This will intensify any inversion you get.

• Time factors – thermal inversions often occur in the evening hours. The surface begins to cool so it doesn't make it as much heat. Cool air begins to develop near the surface, largely because it cools faster than the air above it.

• Seasonal factors – you will see inversions in the wintertime because the nighttime hours last longer. The land does not absorb as much heat so that the surface is cooler to begin with.

• Wind factors – you need fairly strong winds to mix cool and warm air together. If the winds are fairly light, however, you have a tendency toward getting more thermal inversions.

• Precipitation - rain also mixes layers of the atmosphere, which blocks the formation of a thermal inversion. Snow will also keep the land cool, worsening the inversion.

Thermal inversions tend to trap air pollution near the earth's surface. Under normal conditions, air pollution will travel away from an area because of the wind, rainy conditions, or natural dispersion into the higher atmosphere. If you see pollution gathering in a certain area that cannot escape to these mechanisms, it will build up further.

The three things that will determine how bad the pollution event is likely to be are these: the strength of the immersion, its duration, and the height of the area trapped beneath the inverted area. Certainly, the amount of pollution produced also plays a role. A strong

inversion basically means there is a high thermal difference between the inversion layers and the mixing layers beneath it. If the inversion area is low to the ground, air pollution will be worse because there is a smaller volume in which to have the pollution collect.

When it comes to air pollution, there are four layers of the atmosphere to consider. Closest to the ground is the smog area or the mixing layer. Above this is air that is cooler. Above this is what's called the inversion layer, which is the same thing as the warm temperature layer trapping the cooler area below it. Finally, there is the top layer we call the cold air layer. This is the normal cold air you would see in the higher atmosphere.

There are two types of smog that you can see in thermal inversion mixing layers. The first is a mixture of smoke and fog, which comes wherever factories burn coal. We call this industrial smog or London smog. We don't see much of this in developed countries anymore. The second type is called photochemical smog, seen mostly in Los Angeles, which gives it its name. This comes from automobile emissions and from power plants that burn fossil fuels. The smog is much different and contains other chemicals altogether. We will talk about the composition of smog in a minute.

London smog contains high amounts of sulfur dioxide. It was quite common in London during the beginning of the industrial age and cause a great many health problems throughout primarily the nineteenth century. The high amounts of sulfur dioxide come from the fact this is a major combustion byproduct of coal. When it mixes with air on foggy days, it creates a substance that is both acidic and corrosive and has added particulate matter within it.

Beginning in the early 1940s, people in Los Angeles started noticing a different type of smog that appeared like a yellow haze in the sky. It increased the risk of cancers and cause a great deal of respiratory irritation. We call this photochemical smog because it needs sunlight in order to develop. The components of this smog are the OCs, nitrogen oxides, ozone, and organic acids called PANs, which stands for peroxyacyl nitrates.

Is it possible to have a permanent inversion? Yes, there are relatively permanent inversions in the stratosphere between seven and thirty-one miles above the earth. The stratosphere essentially traps the troposphere layer and while it doesn't cause short-