Atmospheric Composition

Primary Post:

Discuss one thing that was confusing to you OR one thing that you found interesting. If it was confusing, discuss what was confusing about it. Are there parts of the concept that you did understand? If so, what are they? Don’t simply say “I don’t understand ____” and leave it at that. What did you do to try and understand the concept? Did you do any outside research online? Did you ask me or a classmate for help?

Secondary Post:

After your post, read your classmates’ posts and see if you can help them figure out a concept that they didn’t quite get. If you did a little outside research online to figure something out, share that with them. If everything was crystal clear to you, then talk about something interesting and go into detail as to why you thought it was interesting. Do not simply define the concept according to how the textbook describes it. I want to know why you think it is interesting – so your own opinion. Does it relate to some other interest you might have? Again – DO NOT just define the concept according to the textbook. Also, simply saying “I never knew that _____” can’t be the only reason why it’s interesting, you will need to elaborate.

You should have a total of two posts. As always, both responses must be substantive and a minimum of 85 words for full credit. See the rubric for this discussion in the Discussion Requirements section of the orientation module.

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3.1 Atmospheric Composition

So the composition of the atmosphere is divided into two categories the heterosphere and the homosphere.

So let’s talk about the heterosphere first. It is the outer shell where all the gases are really separated and stratified like a layer cake except not nearly as dense as a cake – in fact this air has less than .001% of atmosphere’s total mass.

The gases are not evenly mixed, it’s not uniform. They are distributed in layers sorted by gravity according to their weight. The heavier elements (oxygen and nitrogen) are closer to the lower heterosphere, the lighter weight elements (hydrogen and helium) are in the upper heterosphere. The atmosphere is less dense up there – so things can spread out and create layers.

Okay, so now let’s talk about the specific composition of the homosphere. It stretches from the service of the earth up to about 50 mi in altitude (so this includes the troposphere, stratosphere, mesosphere and part of the thermosphere which we will talk about in a little bit). Now it’s called the homosphere because all of the elements mix together creating one mostly uniform space. Hence the name homo, which means ‘same.’ The homosphere is much more dense than the heterosphere, and the air gets denser the closer it is to the surface of the earth.

Its made up of about 78% Nitrogen (sourced mainly from millions of years of volcanic activity), 20% Oxygen (sourced from photosynthesis and therefore varies slightly depending on the availability of vegetation), 1% Argon (sourced from decaying potassium), and then less than 1% of trace elements (including Carbon Dioxide) and water vapor.

Nitrogen and oxygen are essential to life.  We can get oxygen by inhaling it, but that’s not the case with nitrogen – we actually exhale all the nitrogen that we inhale, instead we get it through our food where it is fixed by plants and microorganisms and now fertilizers.

Now lets talk about the trace elements.  They may not be as abundant, but they are super powerful because they represent our natural greenhouse gases. They also have different properties, so we can’t simply write them off because they’re so sparse.  For example the small amount of ozone that is present (known as the ozone layer hanging out up in the Stratosphere) absorbs 98% of UV light, which can be incredibly harmful to people, plants and animals.  It can penetrate our skin and damage DNA molecules, which can lead to cancer.

Collectively these trace gases create the greenhouse effect which keeps the earth a tolerable temperature by letting some radiation in, but not all of it back out. If there were no greenhouse effect, the average temperature would be 0 degrees. So the earth is warmed because of these heat trapping trace, greenhouse gases.  Keep in mind – those more abundant gases of Oxygen and Nitrogen can’t do this, they have no part in the greenhouse effect, so without these trace gases, earth would be a frozen planet unable to support life.

On the other hand, higher concentrations of these gases do the opposite; they enhance the greenhouse effect, causing the planet to warm up too much, which can have a negative impact on ecosystems, and the livelihood of millions of people especially those living in the poorest regions of the world.  So this brings us to the next part of this discussion: How the atmosphere is organized in terms of Temperature  (we just finished talking about how it is organized in terms of chemical composition).

3.2 Atmospheric Temperature

The atmosphere can be divided into 4 zones of different temperatures profiles. They each have pauses in between them which means “to change” so the “pauses” are like transitional areas. We will discuss these starting from the top of the atmosphere and work our way down to the Earth’s surface.

The Thermosphere is the layer furthest from the surface of the Earth.  Its between 80 and 480 km away from the Earth’s surface.  Temperatures here are very high (2200°F) because it’s absorbing so much insolation, but it doesn’t feel hot there.  The reason why is because temperature is a measurement of kinetic (motion) energy (the vibration of molecules).  Heat is created when kinetic energy is transferred b/w molecules – or transferred to us as sensible heat.  So for there to be heat, there needs to be some density of molecules, but the molecules are really sparse in the thermosphere.  So we wouldn’t actually find it hot.

Next is the Mesosphere from 50 to 80 km above Earth: with the mesopause being the coldest portion of the atmosphere (-90°F). Yes, it’s cold!  It’s not as close to the sun, so the temperatures are not as high as in the Thermosphere.

Okay, so moving on to the Stratosphere (high temps).  11-31 miles above the earth’s surface.  It has high temperatures because it contains the Ozone Layer, which absorbs ultraviolet (UV) radiation.  This absorption energizes the molecules making it much warmer. We’ll discuss the ozone layer in more detail in just a bit.

Finally, there is the Troposphere, this is where 90% of the total mass of the atmosphere lies. And the bulk of all water vapor, clouds and air pollution are.  It acts like a lid – the warmer air of the stratosphere traps the cooler air in the troposphere preventing the air to mix into the stratosphere.  Temperature decreases with increased altitude from the Earth’s surface at an average rate of 3.5°/1000 ft. this is called the normal lapse rate – that’s just an average.  The actual lapse rate which depends on local weather conditions is called the environmental lapse rate which we’ll talk about much more in upcoming chapters.

3.3 Atmospheric Function

This is the last way to think about the atmosphere.  Recall, we’ve talked about composition and temperature already. Function refers to the action or the work that the atmosphere does, and is categorized into 2 zones.  The ionosphere and the ozonosphere – together, they remove most of the harmful wavelengths from the sun and charged particles.

The Ionosphere is located within the thermosphere and into the mesosphere. It absorbs gamma rays, x-rays, and some shortwave UV rays – it changes atoms to positively charged ions.

The other layer is the Ozonosphere, or the Ozone Layer which is part of the stratosphere.  Ozone is a highly reactive oxygen molecule (O3).  It absorbs the shortwave, UV radiation especially UVB rays.  However, UVA is not absorbed – it has longer wavelengths and is less intense, but it penetrates the skin more deeply, causing skin cancer (it can be decades before you see the effects though). There are daily and seasonal fluctuations of ozone in the stratosphere, but recently, ozone is being depleted through anthropogenic (human induced) pollutants, namely CFCs which are Chlorofluorocarbons.  CFCs break down ozone through a chemical reaction so it can’t protect us from UV rays.

Watch the short video below to learn more about this issue.

3.4 Pollutants in the Atmosphere

Pollutants are gases, particles, and other substances in amounts that are harmful to humans.  They can come from both human activity and nature. They’re not new.  Romans complained about foul air in the cities 2,000 years ago.  Natural sources of pollutants include: plants and trees, decaying plants soil, oceans, and fire.  Pollen can cause high amounts of particle pollution triggering asthma or other side affects negative to humans.  We call these particulates, aerosols.

Increasingly humans are contributing to the atmospheric pollution, which is becoming known as the anthropogenic atmosphere – in which we are creating enough gases and aerosols to define a new atmospheric zone.  Air pollution is worse in cites.  41% of people in the US live with unhealthful levels of pollution.  Most of the pollutants or the ones that concern us the most are a result of combustion of fossil fuels in transportation.

Photochemical smog comes from the interaction of sunlight and combustion of fossil fuels.  High temperatures in auto engines produce NO2 – Nitrogen Dioxide which then does 2 things:

1. Interacts with water vapor to form nitric acid(acid Rain)

2. Interacts with Oxygen and VOC (volatile organic compounds – hydrocarbons) to create Ozone – the principle component to photochemical smog.  Ozone in the troposphere is bad (only in the stratosphere is it a good thing) and can cause all kinds of respiratory problems.  A photochemical reaction occurs in which UV radiation liberates one oxygen from NO2 (nitrogen dioxide). Then the free oxygen atom combines with an oxygen molecule (O2) to make O3 Ozone.  It damages biological tissues.  Children are impacted worse.

Industrial smog and sulfur oxide are bad for your health as well and are produced by Industry.  Industrial pollution produces carbon dioxide and sulfur oxides, which react with Oxygen.  These are dangerous to our health because of their deteriorative and corrosive characteristics.

Particulates and Aerosols are another kind of pollutant that occurs both anthropogeniclly and naturally.  ‘PM’ refers to particulate matter which are fine particles that impact human health, like haze, smoke, dust, and soot – we breath those in and they get trapped in our lungs.

Natural factors of pollutants include: wind and dust which can come from dry lake beds/terrain and overgrazed lands.

Finally, there are temperature inversions which are differences in atmospheric density in the troposphere can concentrate pollution in the troposphere.  You’ve seen these when there seems to be a perfect layer of fog or smog hanging over the city. Inversions happen when normal temperature patterns reverse so that warmer air is on top of cooler air in the troposphere.  This traps pollutants in – it’s a natural process but when too many pollutants are trapped under warmer air, it can be very bad for human health, plus it just looks gross!