A t m o s f e r a
Atmosphere The Earth is surrounded by a blanket of air, which we call the atmosphere. It reaches over 560 kilometers (348 miles) from the surface of the Earth, so we are only able to see what occurs fairly close to the ground. Early attempts at studying the nature of the atmosphere used clues from the weather, the beautiful multi-colored sunsets and sunrises, and the twinkling of stars. With the use of sensitive instruments from space, we are able to get a better view of the functioning of our atmosphere. Life on Earth is supported by the atmosphere, solar energy, and our planet's magnetic fields. The atmosphere absorbs the energy from the Sun, recycles water and other chemicals, and works with the electrical and magnetic forces to provide a moderate climate. The atmosphere also protects us from high-energy radiation and the frigid vacuum of space. The envelope of gas surrounding the Earth changes from the ground up. Four distinct layers have been identified using thermal characteristics (temperature changes), chemical composition, movement, and density.
Early Earth Timeline About 5.5-6 billion years ago (BYA), the solar nebula begins to collapse About 4.6 BYA, Sun begins fusion About 4.5-4.56 BYA, Proto-Earth formed from planetesimals. 4.44+ BYA, Earth-Moon formed by giant impact. Earth melts, magma ocean. 4.2 BYA, Earth was completely differentiated. 4 BYA, earliest oceans formed, thick atmosphere exists 3.8 BYA, life develops 2.5-3 BYA, photosynthesis leads to O2 in ocean 2 BYA, O2 hits atmosphere
The First Atmosphere The early atmosphere would have been similar to the Sun-- mainly hydrogen and helium, but this atmosphere was lost quickly for two reasons: (1) The gravity of the modest size earth was not strong enough to prevent such light gases from escaping to space. Particularly since the early earth was hot! (2) It appears that around 30 million years after the earth s formation, it was struck by a large object the size of Mars. The result: the origin of the moon and loss of earth s early H, He atmosphere.
Earth s Second Atmosphere A new atmosphere was established by the outgasing of volcanoes the mixture of gases was probably similar to those of today s volcanoes: H 2 0 vapor (roughly 80%) CO 2 (roughly 10%) N 2 (few percent) Small amounts of CO, HCL, HS (Hydrogen Sulfide), SO 2, CH 4 (Methane), Ammonia (NH 3 ), and other trace gases.
Earth s Second Atmosphere Virtually no oxygen in that second atmosphere. Thus, no ozone layer, so ultraviolet radiation flooded the earth s surface. With a huge influx of water vapor and the cooling of the planet, clouds and earth s oceans formed. At that time the sun was about 30% weaker than today why didn t the earth freeze over? The apparent reason: so much CO 2 so there was a very strong greenhouse effect.
The Third Atmosphere While O 2 was increasing, CO 2 decreased due to several reasons: (1) In photosynthesis CO 2 is used to produce organic matter, some of which is lost to the system (e.g., drops to the bottom of the ocean or is buried) (2) chemical weathering, which removes CO 2
Early The Modern Atmosphere No oxygen No ozone layer Lots of UV No land Lots of CO2 and ammonia Intense lightning storms Lots O2 Ozone layer Not much UV More land Less CO2 and ammonia Less lightning storms
Chemical Composition Atmosphere is a mixture of gases and particulate-phase substances Most abundant Nitrogen (78 %) Oxygen (21 %) Trace gases and aerosols make up approximately 1 % (Table 1.1) Some are present in constant concentrations N 2, O 2 and noble gases
Chemical Composition Others vary temporally and spatially: Water vapor (H 2 O) Carbon dioxide (CO 2 ), Carbon monoxide (CO) Ozone (O 3 ) Methane (CH 4 ) Nitrogen oxides (nitrous oxide (N 2 O); nitric oxide (NO); nitrogen dioxide (NO 2 )) Ammonia (NH 3 ) Formaldehyde (HCHO) Sulfur dioxide (SO 2 ) Reduced sulfur compounds (H 2 S, COS, CS 2, (CH 3 )2 S ) Odd hydrogen species (OH,HO 2,H 2 O 2 ) Particulate-phase species Nitrate (NO 3- ), Ammonium (NH 4+ ), Sulfate (SO 4 2+ )
The Rise of Oxygen and the Third Atmosphere In the first two billion years of the planet s evolution, the atmosphere acquired a small amount of oxygen, probably by the splitting of water (H20) molecules by solar radiation. The evidence of this oxygen is suggested by minor rust in some early rocks. The oxygen also led to the establishment of an ozone layer that reduced UV radiation at the surface. With the rise of photosynthetic bacteria (cyanobacteria) and early plants, oxygen levels began to rise rapidly as did indications of rust in rocks Between 2.5 billion years ago to about 500 bya, 0 2 rose to near current levels.
Most abundant atmospheric gas Nitrogen (N 2 ) Limited direct role in atmospheric and life processes Precursor for the formation of nitrate used by plants to synthesize proteins Results from atmospheric and symbiotic biological processes Nitrous oxide (N 2 O) Nitric oxide (NO) Nitrogen dioxide (NO 2 ) Dintrogen pentoxide (N 2 O 5 ) Nitrate radical (NO 3 )
Oxygen (O 2 ) Essential for metabolism; Required for the evolution of life Precursor for the production of stratospheric O 3 ; formation of the O 3 layer made life possible O 3 : background surface levels (~ 20 ppbv); peak levels (8-10 ppmv) occur in middle stratosphere Absorbs UV and thermal energy
100% carbon dioxide nitrogen Composition percentage 50% 0% oxygen now 5,000 3,000 0 Time (millions of years)
Atmosphere
The Troposphere The density of the atmosphere decreases rapidly with increasing height. The troposphere has the following characteristics: it is about 12 km (7 mi) thick, the temperature decreases rapidly with altitude, the mean temperatures at the bottom and top are 16 C & -60 C, it is heated from below by conduction and from condensation of water vapor, it is the region where you find precipitation, evaporation, rapid convection, the major wind systems, and clouds, and it is the densest layer of the atmosphere.
The Tropopause/Stratosphere Above the troposphere is a region of relatively constant temperature, -60 C, about 10 km (6 mi) thick called the tropopause. This is where high velocity winds (jet streams) occur. The stratosphere has the following characteristics: it is about 28 km (17 mi) thick, the temperature increases with altitude from about -60 C to 0 C, this is where ozone, an unstable form of oxygen, appears, it is heated as the ozone absorbs incoming ultraviolet radiation.
Mesosphere/Mesopause/Thermosphere Mesosphere temperatures fall with increasisng altitude until they reach the Mesopause at 80Km and -95 o C Above the mesopause is the Thermosphere where temperatures are isothermal for 10Km then rise rapidly with increasing altitude The thermosphere is very sensitive to incoming solar radiation
Thermosphere 80 km and above Temperature increases with altitude as atoms accelerated by solar radiation -95 C at base to 100 C at 120 km Heat content negligible Traces of atmosphere to 1000 km Formerly called Ionosphere
Air Pressure Density of Molecules decreases with height. Although the atmosphere goes up to 184 miles, half of the atmosphere is in the first 18,000 feet or 3.4 miles. Less molecules (same composition) higher up makes it is harder to breath than at sea level. Air Pressure is converted to Sea Level Pressure to observe surface low and high pressures. (otherwise the Rocky Mts. would be always be low pressure and the oceans would be areas of high pressure). Pressure also dependent upon Temperature (We will perform an experiment on this)
Earth-Atmosphere Energy Balance
The Ozone Layer Within the stratosphere, about 19 km to 48 km above your head, lies an atmospheric layer called the ozone layer Ozone is made of oxygen. Although you cannot see the ozone layer, your life depends on it.
The Ozone Layer An ozone molecule is made up of three oxygen atoms bound together. The ozone layer contains a high concentration of ozone and shields you from the Sun's harmful energy. Ozone absorbs most of the ultraviolet radiation that enters the atmosphere. Ultraviolet radiation is one of the many types of energy that come to Earth from the Sun.
Sustaining Ozone
Ozone Depletion Chlorofluorocarbons CFCs were used for years as aerosol propellants and refrigerants. Mostly = CFCl 3, CF 2 Cl 2. They are not water soluble (so they do not get washed out of the atmosphere by rain) and are quite unreactive (so they are not degraded naturally). The C Cl bond is easily broken, though, when the molecule absorbs radiation with a wavelength between 190 and 225 nm. The chlorine atoms formed react with ozone: Cl + O 3 ClO + O 2 In spite of the fact that the use of CFCs in now banned in over 100 countries, ozone depletion will continue for some time because of the tremendously unreactive nature of CFCs.
Depletion of Ozone
Ozone Ozone absorbs much of the radiation between 240 and 310 nm. It forms from reaction of molecular oxygen with the oxygen atoms produced in the upper atmosphere by photodissociation (< 242 nm). O + O 2 O 3
Ozone Depletion In 1974 Rowland and Molina (Nobel Prize, 1995) discovered that chlorine from chlorofluorocarbons (CFCs) may be depleting the supply of ozone in the upper atmosphere.
Chlorofluorocarbons and the ozone layer The ozone layer absorbs the Sun s high-energy ultraviolet (UV) radiation and protects the Earth.
Chlorofluorocarbons and the ozone layer In the stratosphere, the CFCs break down and release chlorine. The chlorine reacts with ozone molecules, which normally block incoming ultraviolet radiation.
Thinning of the Ozone Layer Evidence from satellites of thinning of the Ozone layer led to the Montreal Protocol for reducing CFCs
Darkest blue areas represent regions of maximum ozone depletion Ozone Hole
Increase of atmospheric carbon dioxide Carbon dioxide levels have risen by 30% in the last 200 years. The IPCC projects that, if unchecked, atmospheric carbon dioxide concentrations will range from 650 to 970 ppm by 2100.
Winds Actually Don t move directly towards or away from the pole or equator: In the northern hemisphere they veer to the right (to left in southern hemisphere) the Coriolis Force: An apparent force due to rotating Earth causing different inertial velocities at different latitudes.