Notes

The Development of the Atmosphere
As Earth was forming, the molten planet had no real atmosphere. Instead, Earth contained toxic gases such as hydrogen (H2), helium (He), methane (CH4), and ammonia (NH3). The lighter elements, such as hydrogen and helium, escaped into space, while methane and ammonia sank to Earth’s surface.

Heavy volcanic activity and constant bombardment by meteorites helped to transform the atmosphere. The gases released from these eruptions began to dominate, and the atmosphere became rich in carbon dioxide (CO2) and water vapor (H2O). These heavier compounds sank closer to Earth’s surface, along with the ammonia and methane.

A Changing Atmosphere
Earth’s atmosphere has undergone a variety of changes since the planet first formed. For example, the heavier gases that sank to Earth’s surface experienced a variety of chemical reactions. These reactions transformed ammonia (NH3) into atmospheric nitrogen (N2), which is the dominant gas in the atmosphere today.

The water vapor in the atmosphere also condensed to form the oceans that exist today. Within these oceans, Earth’s first organisms helped to transform carbon dioxide (CO2) into oxygen (O2) through the process of photosynthesis. This process reduced CO2 levels and increased O2 levels in the atmosphere.

Next, you’ll watch a video to learn more about how the organisms altered Earth’s atmosphere.

The Present-Day Atmosphere
The present-day atmosphere is composed of water vapor and dry components. The dry atmosphere contains all the atmospheric components except for water vapor, including nitrogen, oxygen, and trace gases. Nitrogen is the most abundant gas in the atmosphere at 78.1%, followed by oxygen at 20.9%. The remaining 1% includes trace gases like argon, neon, helium, and methane. Even though argon is a trace gas, it is the third most abundant gas in the atmosphere at 0.90%.

Gases such as nitrogen and oxygen circulate in the atmosphere through biogeochemical cycles. In this way, the proportion of nitrogen and oxygen remains constant throughout the atmosphere.

Watch the video on the next screen to learn about separating air into its individual gases.

Air Pressure
Even though we can’t see it, air extends above us for hundreds of miles. Earth’s strong gravitational pull holds the atmosphere in place. Air, like all matter, has mass and weighs down on us constantly. But because it pushes equally from every direction, we do not feel it pressing on us.

Imagine a 1 inch by 1 inch square drawn on the ground. The column of air above that square weighs almost 15 pounds at sea level—equivalent to the weight of a heavy bowling ball. So, at sea level, air pressure is approximately 15 pounds per square inch.

However, as the elevation increases farther from sea level, there is less air in that column. The air pressure is lower. At 18,000 feet above sea level, for example, the air pressure is approximately 7 pounds per square inch.

Layers of the Atmosphere
Earth’s interior is made up of several different layers based on temperature, density, composition, and phase (solid or liquid). Similar to its interior, Earth’s atmosphere also exhibits characteristics that vary based on depth. However, these characteristics are not as obvious as they are within Earth’s interior.

In the following activity, you’ll examine data about Earth’s atmosphere to identify the differences between the layers.

Higher up in the atmosphere, variations in temperature and altitude result in the formation of atmospheric layers, each of which has unique characteristics. The first four layers are the troposphere, stratosphere, mesosphere, and thermosphere. A fifth layer, the exosphere, begins approximately 500 kilometers above Earth’s surface.

You live in the bottommost atmospheric layer, the troposphere. It is denser than the other layers and contains almost 80% of the atmosphere’s mass. Weather and clouds develop in this layer, and airplanes fly here. Earth’s heat warms the troposphere. This layer’s average temperature range, which decreases as altitude increases, is 17°C to -51°C.

Jets and high-altitude balloons fly in the stratosphere. About 90% of the planet’s ozone is located here. Ozone is responsible for filtering out the majority of the Sun’s harmful ultraviolet radiation. As ozone forms, it produces heat, causing the temperature to increase with altitude in this layer. The temperature in the stratosphere ranges from -51°C at the bottom to 5°C at the top.

Scientists know the least about the mesosphere because it is too high for balloons and aircraft but too low for spacecraft. As a result, most knowledge we have about this layer comes from research rockets. The mesosphere is the coldest layer. The temperature decreases with altitude, dropping to -100°C. Gases in the mesosphere create enough friction for meteoroids entering Earth’s atmosphere to burn up. From Earth, they appear in the night sky as meteors, or shooting stars.

The International Space Station (ISS), which is used for space experiments, orbits Earth in the thermosphere. The Sun’s ultraviolet and x-ray radiation are absorbed in the thermosphere, where the temperature increases with altitude. Temperatures vary from -120°C at the bottom of this layer, to 2,000°C near the top. Even with such high temperatures, it would still feel extremely cold to us in the thermosphere because there are few molecules in this layer to transfer heat. Auroras occur in this layer when electrically charged particles from the Sun collide with gases in Earth’s atmosphere.

The exosphere, the outermost layer of the atmosphere, is the boundary between Earth’s atmosphere and outer space. Primarily made up of hydrogen and helium, this layer has molecules that are so widely spaced apart that collisions are highly unlikely. In this region, particles with enough energy are able to escape into space. The temperature in the exosphere varies greatly and can range from 0°C to more than 1,700°C.

The oxygen you need to survive is made of two oxygen atoms bonded together (O2). In the presence of enough solar energy, a third oxygen atom can bond, forming ozone (O3). Ozone can also be produced near Earth’s surface, primarily because of the burning of fossil fuels. Ozone is a hazardous chemical that is linked to a myriad of health and environmental hazards.

However, the ozone layer, which lies about 20 to 30 kilometers above Earth’s surface, is critical to the survival of humans and other species. It protects us from the Sun’s powerful ultraviolet (UV) radiation.

Read the article “Ozone Hole on the Mend.” The term “ozone hole” refers to the seasonal depletion of the ozone layer above Antarctica due to human activity. The article discusses this depletion and the measures taken by scientists to understand and improve the structure and efficiency of the ozone layer.

Every object in the universe is constantly exchanging energy with its surroundings. For example, a bicyclist gains energy as he pedals but loses energy in the form of friction on the road. We can quantify such exchanges with an energy budget.

Similar to how a financial budget tracks inflows and outflows of money, an energy budget quantifies the inflows and outflows of energy for an object.

Earth’s atmosphere works in a similar way. The atmosphere takes in energy from the Sun and from Earth’s interior. This energy is primarily stored in the atmosphere as heat. The atmosphere releases energy by transferring this heat to other places, such as by radiating it out into space or transferring the energy to Earth’s oceans.

Let’s look at how energy exchange works in the atmosphere. If energy inflows to the atmosphere are greater than energy outflows, the excess energy will be stored as heat.

This scenario will cause an increase in the temperature of the atmosphere. But if the atmosphere transfers energy out faster than it takes it in, the atmosphere will begin to lose energy. The result is a drop in the temperature.

So, the rate of energy inflows or outflows influences the temperature of the atmosphere.

We can observe these changes on an hour-to-hour basis. As the Sun rises in a region, energy enters that part of the atmosphere faster than it exits, and the region warms up. As the Sun sets, the balance shifts and energy is lost faster than it is gained. This loss of energy causes the area to cool down.

This effect is not only localized and short term, though. Just as we can quantify the number of calories that we eat and burn, scientists can quantify the amount of energy that is entering and exiting the atmosphere as a whole. This quantification helps us gain insight into the mechanisms that affect the temperature of the atmosphere.

The Ozone Layer
The energy Earth receives from the Sun interacts with a multitude of objects, such as clouds, dust, and landforms on Earth’s surface. Different objects store and use this energy in different quantities.

Consider the ozone layer of the atmosphere—the stratosphere. This layer absorbs harmful UV rays from the Sun. This energy is stored in the ozone layer in the form of heat. As a result, the temperature of the stratosphere increases.

At the same time, if the ozone layer absorbs CFCs from Earth, ozone molecules are destroyed, which depletes the ozone layer. A decrease in the ozone layer has a great impact on Earth because the UV rays are no longer filtered out. The increase in UV rays causes an increase in Earth’s temperature. These types of interactions can drastically affect the energy balance of the atmosphere.

The hydrosphere and atmosphere are both fluid subsystems, meaning that they constantly flow and exchange matter and energy.

In the hydrosphere, energy and matter are exchanged through ocean currents and the water cycle. In the atmosphere, matter and energy exchanges are driven by wind.

Wind Flow and Pressure
Imagine walking into an air-conditioned building on a hot day. As soon as you open the door, a rush of cold air sweeps past you. This sudden rush of air happens because the warm air outside the building is at a lower pressure than the cold air inside.

In the same way, but on a much larger scale, winds in Earth's atmosphere blow due to differences in air pressure.

Why does this happen? Recall that air, like all matter, has mass. And, like all matter, air becomes less dense and rises when it is heated. This rising action creates an area of low air pressure. Cooler air from a high pressure area is then pulled into this low pressure area. Thus, winds on Earth blow from areas of high pressure to areas of low pressure.



Isobars
To better understand weather patterns, meteorologists prepare and study maps of high and low pressure areas.

Air pressure decreases at a higher elevation from sea level. To map air pressure, a line called an isobar joins the areas that have equal air pressure. The word isobar means “same pressure.”

A common unit to measure air pressure is a millibar (mb or mbar).

Air Pressure and Temperature
Areas of high and low pressure are created because the Sun heats Earth unevenly. These differences in air pressure cause the air in Earth’s atmosphere to constantly move from areas of high to low pressure, which allows the air to mix and circulate.

Due to this constant circulation, the atmosphere is essentially uniform in composition across the planet.

Atmospheric circulation also causes various weather phenomena, like cyclones, fog, snowstorms, and polar winds.


Biogeochemical Cycles
Various biogeochemical cycles, such as the carbon cycle, nitrogen cycle, and oxygen cycle, constantly release matter into the atmosphere and pull matter out of it.

The process of photosynthesis, for example, removes carbon dioxide gas from the atmosphere and releases oxygen. This oxygen is then used by other living beings. The result is that a molecule that enters the atmosphere will likely not remain there forever. It will eventually be removed as it moves through one of the biogeochemical cycles.

Scientists have developed a metric called residence time, which calculates the average amount of time that matter remains in any cycle. The following activity will help you understand the concept of residence time.

Residence time tells us how quickly material is cycling into and out of the atmosphere. The atmospheric concentration of compounds with short residence times can be reduced by cutting their rate of emissions. While CO2 has a relatively brief residence time in the atmosphere, the refrigerant carbon tetrafluoride (CF4) has a residence time of more than 50,000 years. This means that its residence in the atmosphere is essentially permanent in regards to human time scales, regardless of any emissions reductions.

So, concentration and residence time both play an important role in a substance’s atmospheric impact.