In Friday's case, the bad air quality is a product of high concentration's of ozone near the ground. Ozone high in the atmosphere is a good thing, but it isn't good to breathe. That means ozone near the ground is a bad thing.
Where did it come from? It does form naturally sometimes. Lightning can actually produce it in small amounts. One major source is the exhaust from our vehicles. The combustion exits vehicles and interacts with the intense rays from the sun, and ozone forms. To complicate things, sinking air is trapping those high levels of ozone near the ground.
Showers this weekend will help by stirring up the atmosphere. That allows the ozone to rise from the ground and disperse with the upper level winds. Even if you don't get rain, the atmosphere will be stirred and mixed up enough to help to relieve the high concentrations of ozone near the ground.
That's the simplified version. If you want a more scientific explanation, keep reading....
Car exhaust includes gaseous chemicals that are made up of molecules. Molecules are combinations of atoms that make up virtually everything, including the air we breathe.
The short version: Molecules that make up our car exhaust are broken apart by the sun's intense rays. Oxygen atoms that break off of those molecules can become flammable, so they stablize themselves by attaching to the oxygen molecules we breathe. We breathe O2, which is a molecule made up of two oxygen atoms. The third oxygen atom that attaches itself to the O2 converts the oxygen we breathe to O3, which is Ozone.
The long version: Water Vapor, Carbon Dioxide, Carbon Monoxide, and various hydrocarbons from unburned gas are among the chemicals in vehicle exhaust. Water Vapor is a molecule made up of two hydrogen atoms and an oxygen atom. Carbon dioxide is made up of a carbon atom and two oxygen atoms. Carbon monoxide consists of a carbon atom and an oxgen atom. The oxygen we breathe is actually a molecule that consists of TWO oxygen atoms. A single atom of oxygen is highly reactive and would actually be dangerous to breathe. It can even be flamable.
When car exhaust is spewed into the air, the sun's light rays are strong enough to excite and energize these various molecules. The molecules become so energized, individual atoms break off from the molecule. That means two things: first, new chemicals are produced; second, you have individual oxygen atoms floating around. To keep from becoming flamable, individual oxygen atoms will attach themselves to other atoms and molecules. That helps to stablize the oxygen atoms. Oxygen atoms will often attach themselves to the two oxygen atoms we breathe. That means two oxygen atoms becomes three. A molecule of three oxygen atoms is ozone.
Is that complicated enough? Let's complicate it more...
The weather has played a role here, too. High pressure makes air sink. High pressure has been sitting on top of the Tennessee Valley all week. That sinking air prevents the air from mixing up, which traps the increasing concentrations of ozone near the ground. Showers will return this weekend. Even if you don't get rain, showers will be close enough to stir up the atmostphere and let the air mix. That will allow the ozone to rise and disperse with the upper level winds.
This does not happen frequently in the Tennessee Valley. The Air Quality Alert for Friday was the first in two years. In 2016, Air Quality Alerts were issued on two separate days. In 2012, Air Quality Alerts were issued on three days.
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