The Earth s Atmosphere-I. GEOL 1350: Introduction To Meteorology

Transcription

1 The Earth s Atmosphere-I GEOL 1350: Introduction To Meteorology 1

2 Overview What is the composition of Atmosphere? How did the atmosphere arrive at its current state? 2

3 Earth s Atmosphere Earth s atmosphere is a thin, gaseous envelope. Earth radius ~ 6370 km; Atmosphere ~ 500 km Why the atmosphere is so thin? 3

4 Earth s Atmosphere Gravity ---- Reason for Orange skin atmosphere Gravitational attraction has compressed the atmosphere into a shallow layer. 4

5 What is the composition of the Earth s atmosphere? Permanent Gases Name of the gas Molecular weight (g/mol) Percentage Nitrogen N % Oxygen O % Argon Ar % Neon Ne % Helium He % Hydrogen H % Xenon Xe % 5

6 What is the composition of the Earth s atmosphere? Mole: a unit of amount of substance A mole has x atoms or molecules of pure substance being measured Molecular weight for N 2 is gram per mole. 6

7 Variable Gases Name of the gas Molecular weight Percentage Water vapor H 2 O < 4.% Carbon dioxide CO % Methane CH % Nitrous oxide N 2 O % Ozone O % Particles (dust, soot, etc.) % Chloroflourocarbons CFCs % 7

8 Molecular Weight of Air Air is a mixture of gases with different molecular weights What is the molecular weight of the air? Molecular weight of air is the average of the molecular weights of its constituents 8

9 Molecular Weight of Air M air ( M C ) = i i i M i = molecular weight of i-th constituent C i = fractional concentration of i-th constituent M air = g mol 1 9

10 Molecular Weight of Air M air ( M C ) = i M air = M N2 C N2 + M O2 C O2 + M H2O C H2O + i i M air = g mol 1 10

11 Atmospheric Composition Major greenhouse molecules in order of importance (H 2 O, CO 2, O 3 ) Important for chemistry (O 2, O 3, CH 4, N 2 O, CO, CFCs). N 2 is important for background pressure. 11

12 Composition & Structure 12

13 Units for Concentration Parts-per notation is used to describe relative proportions in measured quantities, particularly in low-value. 1 ppmv (parts per million by volume) = ppbv (parts per billion by volume) = pptv (parts per trillion by volume) = Example: Current CO 2 Concentration: 380 ppmv It means that out of every million air molecules, 380 are CO 2 molecules. 13

14 Trace Constituents Charles David Keeling 19N, 155W, 3.3 km Increase in atmospheric CO2 from : Δ(CO 2 ) 377 ppmv ppmv = 62 ppmv Keeling Atmospheric CO 2 Record Longest continuous record Regional CO 2 trend CO 2 seasonal Cycle High in winter (respiration) Low in summer (photosynthesis) 14

15 Glacial ice cores, drilled from great depths, provide a record of atmospheric compositions. Temperature are derived from the geological information. Sharp minima in CO 2 coincide with cold T in ice ages 20 and 140 Kyr ago. CO 2 Temp. 15

16 Surface melt on Greenland 16

17 Snow cover and Arctic sea ice are decreasing Spring snow cover shows 5% stepwise drop during 1980s Arctic sea ice area decreased by 2.7% per decade (Summer: -7.4%/decade) Trenberth 17

18 Sea level is rising: from ocean expansion and melting glaciers Since 1993 Global sea level has risen 41 mm (1.6 inches) 60% from expansion as ocean temperatures rise, 40% from melting glaciers Steve Nerem 18

19 Satellite CO 2 Data Coordinated Observations GLORY 1:34 TES T, P, H 2 O, O 3, CH 4, CO MLS O 3, H 2 O, CO HIRDLS T, O 3, H 2 O, CO 2, CH 4 OMI O 3, aerosol climatology aerosols, polarization CloudSat 3-D cloud climatology CALIPSO 3-D aerosol climatology AIRS T, P, H 2 O, CO 2, CH 4 MODIS cloud, aerosols, albedo 1:26 OCO - - CO 2 O 2 A-band p s, clouds, aerosols CO 2 data can be retrieved from AIRS on Aqua, TES on Aura, and GOSAT 19

20 20 CO 2 Retrieval from Atmospheric Infrared Sounder (AIRS) July 2003

21 Water Vapor 21

22 Water Vapor Tropospheric H 2 O is short lived due to the production and destruction and rapid transport. Lifetime = a few days. H 2 O originates near the equator at the warm ocean surfaces. H 2 O is carried aloft by deep convective cells and horizontally by large-scale eddies that disperse H 2 O across globe. 22

23 Intertropical Convergence Zone (ITCZ) The ITCZ is a belt of convection, which is parallel to the equator, except over the tropical landmasses: South America, Africa, and the maritime continent over Indonesia, where the zone of convection widens. Inside ITCZ, the deep convection is supported by the release of latent heat when moisture condenses. ITCZ moves back and forth across equator following the sun s zenith point. Variation in the ITCZ locations affect rainfall in the tropics. 23

24 O 3 is concentrated in the stratosphere. It increases sharply above tropopause. 24

25 The zonal mean O 3 mixing ratio is largest in the tropics, where the flux of solar UV and photodissociation of O 2 are large. 25

26 Column Ozone (Dobson Unit) Dobson Unit (DU) is the most common unit for measuring O3 concentration. 26

27 Column Ozone (Dobson Unit) Dobson Unit (DU) is the most common unit for measuring O3 concentration. One DU is the number of molecules of O3 that would be required to create a layer of pure O mm thick at 0ºC and 1 atm pressure. One DU would contain about 2.69x10 16 O3 molecules for every square centimeter of area at the base of the column. Over the earth surface. The ozone layers average thickness is about 300 DU or a layer that is 3 mm thick. 27

28 28

29 Ozone Hole Reduction of O3 by 30% in the southern hemispheric spring over Antarctica and first reported in Through the 1990s, column O3 in Sep and Oct have continued 29 to be 40-50% lower than the preozone hole value.

30 Overall cause of O3 depletion is the presence of chlorine-containing source gases (primarily CFCs and related Halocarbons). Column O3 in Sep 21-30, 2006 NASA The Cl-catalyzed O3 depletion can take place in the gas phase, but it is dramatically enhanced in the presence of polar stratospheric clouds (PSCs). The PSCs form during extreme cold winter. Low T at pole form cloud particles and are composed by nitric acid (Type I PSC) or Ice (Type II PSC). Both types provide surfaces for chemical reactions that lead to O3 destruction. 30

31 Methane (CH4) CH4 is produced primarily by bacterial and surface processes that occur naturally. Anthropogenic sources such as mining and industrial activities may constitute ~20% of CH4 production. CH4 is long-lived and therefore well mixed in the troposphere, where it is ~1.7 ppmv. CH4 decreases with altitude as a result of oxidation. This process ultimately leads to the formation of stratospheric water vapor. 31

32 Nitrogen Compounds N2O is produced primarily by natural means relating to bacterial processes in soils. Anthropogenic sources of N2O include nitrogen fertilizers and combustion of fossil fuels, which accounts for ~25% of the total production. N2O is long lived and well mixed in the troposphere. N2O mixing ratio is ~ 300 ppbv. N2O decreases with altitude in the stratosphere due to dissociation of N2O into NO. NO can destroy ozone catalytically. It is produced as a by-product of inefficient combustion. 32

33 How did the atmosphere arrived at its currents state? The earth s first atmosphere (4.6 billion years ago) consisted mostly of He, H 2; Most scientists feel that this early atmosphere escaped into space from the earth s hot surface. Outgassing (volcanic eruption) releases CO 2, N 2, H 2 O, forming the earth s second atmosphere 33

34 Eruption of Mt. St. Helen in 1980, adding to our atmosphere s composition: water vapor, carbon dioxide and other gases. Volcanic eruption: water vapor 85% carbon dioxide 10% nitrogen 1-5 % sulfur 1-5% 34

35 Origin of the atmosphere Water vapor condensed to form ocean Carbon dioxide dissolved into the ocean and locked up in carbonate sedimentary rocks Nitrogen, chemically stable, accumulated Where did the oxygen in the atmosphere come from? 35

36 Where did the oxygen in the atmosphere come from? Photodissociation: 2H 2 O 2H 2 + O 2 O 2 began an extremely slow increase in concentration as energetic rays from the sun split H 2 O into H 2 and O 2. H 2, being lighter, rose and escaped into space, while O 2 remained in the atmosphere. 36

37 Photo-dissociation uv step 1 Oxygen formation uv H O H H O H Escape to space 2 H 2 O 2 2 H 2 O + ultra violet radiation 2 H 2 + O 2 37

38 Where did the oxygen in the atmosphere come from? This slow increase in oxygen may provide enough of O 2 for primitive plants to evolve, perhaps 2 to 3 billion years ago. After plants evolved, the atmospheric O 2 increased more rapidly as a result of photosynthesis (combine CO 2 and H 2 O to produce O 2 ). Photosynthesis: O billion years ago 38

39 Where did the oxygen in the atmosphere come from? O 2 reach the present composition about several hundred million years ago. 39

40 Summary 1. Earth s atmosphere is rich in nitrogen and oxygen as well as smaller amounts of other gases, such as water vapor, carbon dioxide, and other greenhouse gases. 2. Nitrogen occupies about 78% and oxygen is about 21%. Greenhouse gas, CO 2, increases in its concentration by more than 25% since early 1800s. 3. The earth s first atmosphere (4.6 billion years ago) consisted mostly of He, H 2. Outgassing (volcanic eruption) releases CO 2, N 2, H 2 O, forming the earth s second atmosphere. After plants evolved, the atmospheric O 2 increased more rapidly as a result of photosynthesis. 40