THE UPPER ATMOSPHERE Learning Objectives: ● Describe the vertical structure of the Earth’s atmosphere as well as its evolution, composition, and chemical profile of each layer. What is an atmosphere? The atmosphere is a protective blanket of air around the Earth. Vertical Structure of the Earth’s Atmosphere A. Five (5) Layers based on Temperature Unlike the changes in pressure and density, which decrease with altitude, the temperature changes are irregular. https://cdn1.byjus.com/wp-content/uploads/2020/10/layers-of-atmosphere.png Troposphere - It is the lowest layer of the atmosphere. - Derived from the Greek word tropos (“a turn or change") and implies an "overturning” of air resulting from the vertical mixing and turbulence characteristic of this layer. - It constitutes 85% of the mass of the atmosphere. - The depth of the troposphere varies considerably, ranging from 8 to 20 km (3.6 to 12 mi), with a mean of about 15 km (9 mi). - The altitude at which the troposphere ends depends largely on its average temperature, being highest where the air is warm and lowest in cold regions. - This is the layer where almost all types of clouds are formed; the site of all weathers. - Its temperature decreases with height. Stratosphere - stratosphere, a name derived from the Latin word strato which means “layer” - the layer above the tropopause; starts just above the troposphere and extends to 50 kilometers (31 miles) high - It contains about 19.9 percent of the total mass of the atmosphere. - It is nearly isothermal. But unlike the troposphere, the temperature in this layer increases as the altitude increases. - The temperature in the lowest part of the stratosphere, remains relatively constant at about -57 °C up to a height of about 20 km. - From there to the top of the stratosphere (called the stratopause), about 50 km (30 mi) above sea level, the temperature increases with altitude. - In the upper stratosphere, heating is almost exclusively the result of ultraviolet radiation being absorbed by ozone. Therefore, as solar energy penetrates downward through the stratosphere, there is less and less ultraviolet radiation available and a resultant decrease in temperature. - In the part of the stratosphere where temperature does not vary with height, heating is the result of both absorption of solar ultraviolet radiation and absorption of thermal radiation from below. Thus, as we move up or down in this region, the reduction in solar heat is offset by the increase in heat given off by Earth. The net result is the straight vertical line of no temperature change. - Contains very little amounts of water vapor. - Only few clouds are found, and little weather occurs in this region. - It is the ideal layer for airplanes to fly. - Within the stratosphere is the ozone layer, a zone of increased ozone concentration at altitudes between 20 and 30 km. Ozone is an extremely important constituent of the stratosphere. It is largely responsible for absorbing the solar energy that warms the stratosphere, and it also protects life on Earth from the lethal effects of ultraviolet radiation. Mesosphere - - The layer where meteors burn into fragments. Extends from 50km to a height of about 80 km (50 mi) above sea level Temperature in the mesosphere decreases with altitude. The top of the mesosphere is the coldest area of the Earth's atmosphere because temperature may locally decrease to as low as 100 K (-173°C). There are few gas molecules in the mesosphere to absorb the Sun’s radiation. The only significant source of heat is absorption of solar radiation near the base of the mesosphere. This is dispersed upward only weakly by vertical air motions, thus temperatures fall rapidly with increasing altitude. Thermosphere - It extends from about 90 km (56 miles) to between 500 and 1,000 km (311 to 621 miles) above our planet. - The air is very thin in this layer. - intensely heated by the sun’s rays - highly bombarded with charged particles - From the word “thermo” which means heat. It is the hottest and thickest layer of the atmosphere; temperature increases vertically to values in excess of 1500 °C - Radiation causes the air particles in this layer to become electrically charged, enabling radio waves to bounce off and be received beyond the horizon. - The few molecules that are present in the thermosphere receive extraordinary amounts of energy from the sun, causing the layer to warm to high temperatures. - The auroras—aurora borealis (or northern lights) in the Northern Hemisphere, and the aurora australis (the southern lights) in the Southern Hemisphere can be seen in this layer. This happens when the solar particles reach Earth’s atmosphere, they collide with atoms of nitrogen and oxygen, stripping away their electrons to leave ions in excited states. These ions emit radiation at various wavelengths, creating the characteristic - colors. Collisions of solar particles with oxygen produce red or green light; collisions with nitrogen produce green and purple light. The space shuttle and the International Space Station both orbit Earth within the thermosphere. Exosphere - “exo” means outside; the very edge of our atmosphere - It starts at an altitude of about 500 km and goes out to about 10,000 km. - Within this region, particles of atmosphere can travel for hundreds of kilometers in a ballistic trajectory before bumping into any other particles of the atmosphere. Particles escape out of the exosphere into deep space. - The uppermost region of Earth's atmosphere as it gradually fades into the vacuum of space. - This layer separates the rest of the atmosphere from outer space. - The air is very thin and mainly composed of extremely low densities of hydrogen, helium and other molecules. - There is a lot of empty space in between gases. There is no air to breathe, and it’s very cold. - The theoretical top boundary of the exosphere is 190,000 km (half way to the Moon). This is the point at which the solar radiation coming from the Sun overcomes the Earth’s gravitational pull on the atmospheric particles. B. Two (2) Major Divisions based on Mixture of Gases Homosphere - the lower part of the atmosphere and up to about 100km or 60 mi - No greater variation in the composition of gases - It is characterized by the homogeneous mixture of gases, except for the ozone layer which envelopes the globe around 30km above sea level. - Regions: troposphere, stratosphere and mesosphere Heterosphere - the upper part of the atmosphere - greater variation in the composition of gases - heterogeneous mixture or no mixing of gases; gases separate in layers Regions: thermosphere and exosphere C. Two (2) layers based on ozone and ions Ozonosphere - found at the bottom 70km of the atmosphere - contains relatively high concentrations of ozone molecules (O3) - The ozone layer is found at 30km above sea level. - Approximately 90 percent of the atmosphere’s ozone occurs in the stratosphere. Ionosphere - a region ionized by UV rays - located from about 48 km (30 mi) to 965 km (600 mi) above sea level - a portion where ions and electrons are trapped by Earth’s magnetic field - They reflect radio waves back to the Earth which play a very important role in the radio transmission. Evolution of the Atmosphere The evolution of the atmosphere could be divided into four separate stages: Origin. Two models are most favored for the origin of the atmosphere: outgassing or accretion. Outgassing is related to the differentiation of the Earth and the release of gases by volcanoes. Assuming that the gases we presently observe were also released by early volcanoes the atmosphere would be made of water vapor (H2O), carbon monoxide (CO), carbon dioxide (CO2), hydrochloric acid (HCl), methane (CH4), ammonia (NH3), nitrogen (N2), & sulfur gases. Chemical/Pre-biological era. There was no oxygen gas on Earth. Oxygen was only in compounds such as water. Complex chemical reactions in the young oceans transformed carboncontaining molecules into simple, living cells that did not need oxygen to live. Instead they made energy out of sulfur and other elements. Microbial era. 2.7 billion years ago, bluish-green microscopic organisms called cyanobacteria flourished in Earth’s oceans. They made gaseous, or free, oxygen from carbon dioxide, water, and sunlight—the process called photosynthesis. As cyanobacteria created more free oxygen, the amount of oxygen in the atmosphere reached one percent of today’s level, which is 21 percent. Life and Earth’s atmosphere evolved together. Over time, tiny photosynthetic organisms produced enough oxygen to react with the methane in the atmosphere, transforming it forever. Until about 430 million years ago, most aerobic organisms lived in the ocean and used oxygen dissolved in seawater. Biological era. Then about 430 million years ago, life on land appeared. Small plants and invertebrates (animals without backbones) evolved the ability to live on land and use oxygen directly from the atmosphere. As plants became firmly established on land, life once again had a major effect on Earth’s atmosphere. Oxygen made up 20 percent of the atmosphere—about today’s level. At any given moment, many chemical compounds form, break apart, and reform in the air we breathe. Most of these atmospheric reactions occur naturally, often triggered by the sun, but human activity also influences them. Life has always affected the atmosphere, but ever since people began to settle down and work the soil, they’ve played increasingly larger parts in atmospheric change. Composition of the Atmosphere Earth’s atmosphere is composed of about 78 percent nitrogen, 21 percent oxygen, 0.9 percent argon, and 0.1 percent other gases. Trace amounts of carbon dioxide, methane, water vapor, and neon are some of the other gases that make up the remaining 0.1 percent. https://www.australianenvironmentaleducation.com.au/wp-content/uploads/2020/08/Composition-ofair-v2-1024x814.jpg PERMANENT GASES OF THE ATMOSPHERE VARIABLE GASES OF THE ATMOSPHERE Constituent Percent by Volume Constituent Percent by Volume Nitrogen 78.08 Water Vapor 0.25 Oxygen 20.95 Carbon dioxide 0.0396 Argon 0.93 Ozone 0.01 Neon 0.002 Methane 0.00018 Helium 0.0005 Krypton 0.0001 Xenon 0.00009 Hydrogen 0.00005 Key Question: Why is nitrogen most abundant gas in the atmosphere? More abundant than oxygen? Nitrogen was already present after the Earth was formed through the process of outgassing. Also, nitrogen is released back to the atmosphere by bacteria where they get their energy by breaking down nitrate and nitrite into nitrogen gas (also called denitrification). The other primary reason is that, unlike oxygen, nitrogen is very stable in the atmosphere and is not involved to a great extent in chemical reactions that occur there. Thus, over geological time, it has built up in the atmosphere to a much greater extent than oxygen. Chemical Profile of the Five (5) Layers of the Atmosphere https://chem.libretexts.org/@api/deki/files/129845/AT01.gif?revision=1 REFERENCES Aguado E. and Burt, J. (2015). Understanding Weather and Climate. Pearson Education. Byju’s (2022). Layers of the Atmosphere. Retrieved from: https://byjus.com/physics/layers-ofatmsophere/ last September 10, 2022. Conrad, J. (2021). Exosphere. NASA Space Place. https://spaceplace.nasa.gov/exosphere/en/ last September 15, 2022. Retrieved from: Lemmons, R. (2022). Thermosphere. Climate Policy Watcher. 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Retrieved from: https://www.aeronomie.be/en/encyclopedia/mesosphere-coldest-layer-earthsatmosphere last September 14, 2022. Rubin, K. (2009). Abundance of Nitrogen in Earth's Atmosphere. Retrieved from:https://www.soest.hawaii.edu/GG/ASK/atmonitrogen.html#:~:text=The%20other%20prim ary%20reason%20is,much%20greater%20extent%20than%20oxygen. last September 19, 2022.
