Physics of the Atmosphere Physik der Atmosphäre

Transcription

1 Physics of the Atmosphere Physik der Atmosphäre WS 2010/2011 Thomas Leisner Institut f. Umweltphysik

2 Physics of the Atmosphere MVEnv1 Prerequisits: Masters Course experimental Physics: Environmental Physics (MKEP4) or equivalent. Physics of the Atmosphere (this lecture): (Thermo-)dynamics of the atmosphere Radiation Global atmospheric circulation Diffusion and turbulence (advanced topics) Atmospheric chemistry Global cycles of atmospheric constitutents Aerosols Isotopes in atmospheric physics and chemistry Measurement techniques

3 Contents

4 Literature (IUP xxxx: Book - Nr. in the library of the Institut für Umweltphysik, INF 229, 4 th floor, R.410) 1. Physik unserer Umwelt, Die Atmosphäre, Walter Roedel, Springer, Heidelberg, 3. Aufl., Eine ausgezeichnete Übersicht der physikalischen Prozesse in der Atmosphäre (IUP 1511). 2. The Physics of Atmospheres, J. T. Houghton, Cambridge University Press, Cambridge 3rd Edition, Good, concise introduction. 3. Atmosphere Chemistry and Physics, J.H. Seinfeld und S.N. Pandis, John Wiley & Sons, New York, Very comprehensive book on physics and chemistry of troposphere and stratosphere (IUP 1724). 4. Theoretische Meteorologie, Eine Einführung, Dieter Etling, Springer Verlag Heidelberg, 2nd ed., 2002, ausführliche Einführung in Dynamik (UB LN-W ). 5. Fundamentals of Atmospheric Physics, M. L. Salby, Academic Press, 1996: Failry comprehensive introduction to all aspects of atmospheric physics (IUP 1647). 6. Aeronomy of the Middle Atmosphere, 2 nd edition, G. P. Brasseur and S. Solomon, 2005 (IUP 1211). 7. Fundamentals of Physics and Chemistry of the Atmosphere, G. Visconti, Springer-Verlag, 2001; Good and concise text, sometimes surprising mistakes. 8. Fundamentals of Atmospheric Modeling, M. Z. Jacobson, Cambridge University Press, 2005; Covers physics and chemistry of troposphere and stratosphere with the goal to provide the relevant equations for numerical modeling. (IUP 1925) 9. Physics of Climate, J. P. Peixoto, A. H. Oort, American Inst. of Physics, 1992; Dynamics, radiation, thermodynamics and a lot of observational data on climate (IUP 1409).

5 Lecture notes

6 Web Sites of Interest (1) Intergovernmental Panel on Climate Change (IPCC) Presents the comprehensive reports (presently 4th report of 2007) of an international, independent group of scientists on climate and climate change. European Ozone Research Coordinating Unit Results of European research on stratospheric chemistry and ozone loss. NASA Life on Earth Comprehensive site centred on remote sensing of planet earth. German Weather Service (Deutscher Wetterdienst) Information on weather and climate National Oceanic and Atmospheric Administration (NOAA) The Federal Environmental Agency (Umweltbundesamt) Has much information on the state of the environment in Germany. IGBP - International Geosphere-Biosphere Programme Mission: Deliver scientific knowledge to help human societies develop in harmony with Earth s environment. Institute for Environmental Physics - Institut für Umweltphysik Our own web-page Marsilius Kolleg, University of heidelberg

7 Web Sites of Interest (2) - References Journal of Atmospheric Chemistry and Physics, This journal comes in two versions: 1) JACP Discussion 2) JAPC The reviewed journal both are completely in the internet and freely available The Master Chemical Mechanism JPL-Compilation: Chemical Kinetics and Photochemical Data for Use in Stratospheric Modelling NIST-Compilation: IUPAC-Compilation: NASA s Visible Earth Aerosol Inorganic Modelling Home Page:

8 Outline for Today 1. Atmospheric physics a definition 2. Earth System Science 3. Important features of the Earth s atmosphere 4. Spatial and temporal scales in the Earth s atmosphere 5. Some dynamical features 6. Energy 7. Composition and history of the atmosphere 8. Hydrostatic equation

9 The Earth from space First picture of the Earth There are many clouds the Earth is blue/bluish.. The atmosphere is thin

10 Atmospheric Physics a Definition Atmosphere - (greek ατμός, atmós air, pressure, vapour und σφαίρα, sfära sphere ) is the gaseous envelope of a celestial body that is confined due to gravitational attraction. Physics of the atmosphere is the study of all physical phenomena in/of an atmospheric system. Also: Meteorology Atmospheric chemistry Atmospheric science

11 Earth System Sciences IPCC, 2001

12 Masses of Environmental Compartments All Water on Earth R 700km All Air on Earth: Sphere R 1000km 1.41 Mrd. km 3 (standard conditions) Source: Adam Nieman, Atmosphere: g Biosphere: g Hydrosphere: g Earth: g

13 The Role of Physics in Atmospheric Sciences Thermodynamics Phase transitions - condensation and evaporation Adiabatic processes, temperature & pressure gradients Quantum Mechanics Interaction of radiation & matter Chemical processes Hydrodynamics Navier-Stokes equation Classical mechanics Rotating inertial systems, Coriolis and centripetal forces Transport Phenomena Turbulence Diffusion

14 The Atmosphere The atmosphere is a vital part of our environment: provides protective layer for life (stratospheric O 3 ) atmospheric compounds are essential for metabolism (e.g. O 2, CO 2 ) is part (compartment) of the Earth system connects different compartments & is driving force in climate system determines cycling of energy and matter is a complex dynamical system (chaotic motion) is a very thin & extended layer (7-8 km at surface pressure)

15 Kraus, Fig 1.5 Time and Length Scales of Dynamical Processes in the Atmosphere

16 Characteristic transport times Jacob, 1999 Jacobson et al., 2000

17 Composition of the Atmosphere name nitrogen oxygen argon carbon dioxide neon helium water vapour methane krypton hydrogen ozone chemical formula N 2 O 2 Ar CO 2 Ne He H 2 O CH 4 Kr H 2 O 3 relative abundance [%] Nitrogen N 2 : M N2 = 28,015 kg kmol -1 Oxygen O 2 : M O2 = 31,999 kg kmol -1 Argon Ar: M Ar = 39,942 kg kmol -1 Carbon dioxide CO 2 : M CO2 = 44,008 kg kmol -1

18 Mixing Ratios of Atmospheric Trace Gases N 2 O 2 Noble gases H 2 O Xe Kr He Ne Ar CO 2 CH 4 H 2 CO O 3 Hydrocarbons NO X SO 2 HO 2 OH?? 100 s-1000 s s ppt ppb ppm No. of different spezies

19 The Stratospheric Ozone Layer, Where is the Maximum? Mixing Ratio Moles/Mole, ppm Altitude, km Concentration, Molecules/cm 3

20 Which Units? 1) Concentration c = Amount of Volume 2) Mixing ratio trace gas of air Where amount refers to either mass (c m ), number of molecules (c n ), or number of moles (c M ). Examples for units of concentration: micrograms per m 3 or molecules per cm 3 ( number density of a gas). The partial pressure of a species is also a measure of its concentration. x = Amount Amount of of trace gas air + trace gas Amount of Amount trace gas of air Examples: moles of trace gas x M = mole of + ( air trace gas) x unit volume of trace gas = V 6 10 unit volumes of + ( air trace gas) ppm x unit volume of trace gas = V 9 10 unit volumes of + ( air trace gas) ppb x unit volume of trace gas = V unit volumes of + ( air trace gas) ppt

21 Conversion of Units For standard conditions (p 0 = Pa = 1 Atmosphere, T = K) the molar volume is V 0 = cm 3 /mole. For arbitrary temperature and pressure conditions we can use: 1 T p 1 RT 1 RT x = c V = c = c N pt N p M p 0 V n 0 n m A 0 A i Trace gas Molecular Mass g/mole Mixing Ratio x V ppb Number Density c n molecules cm - 3 Concentration c m μg/m 3 Mean molar mass of the (dry) homosphere: Kg/KMole O SO NO NO CH CH 2 O CO

22 Life Times of Atmospheric Trace Gases Micro Scale Urban or Local Scale Regional or Mesoscale Synoptic or Global Scale Noble gases N2 O2, TIME SCALE 1s 100s 1hr 1 day 10 days 1yr 10 yrs 100 yrs Short- Lived Species CH 3 O 2 HO 2 NO 3 Long-Lived Species Moderately Long- CO Lived Species Trop O 3 Aerosols SO 2 H 2 O 2 NO x DMS C 3 H 6 C 5 H 8 CH 4 CH 3 CCl 3 CH 3 Br CFC s N 2 O OH 1m 10m 100m 1km 10km 100km 1000km 10,000km Inter-Hemispheric Mixing Time Intraemispheric Mixing Time Boundary Layer Mixing Time SPATIAL SCALE

23 What is Actually Important in Atmospheric Chemistry? Ozone - The 'Ozone Problems' Oxidation Capacity Free Radicals Greenhouse Gases Aerosols Clouds - Removal of 'Pollutants' - The 'Driving Force' - Chemistry Radiation Link - 'Liquid - Solid' - Radiation, the white planet

24 Influence of Trace Gases on the State of the Atmosphere Gas Smog Acid Rain Turbidity of the atmosphere Greenhouse Effect Strat. Ozone Degradation CO 2 + +/- Influence on Self Cleaning of the Atmosphere CH 4 + +/- +/- CO + - N 2 O + +/- NO X /- +/- SO VOC's CFC's + + O Gas enhances the effect. - Gas reduces the effect. +/- Depending on conditions the influence can be positive or negative. E.g. NO X (= NO + NO 2 ) can both, enhance or reduce the stratospheric O 3 destruction.

25 Vertical extent of the atmosphere Heterosphere Ionosphere Neutrosphere Homosphere Barry+Chornley, fig 1.15

26 The Floors of the Atmosphere It appears appropriate to divide the atmosphere in individual layers similar to the floors of a building. Customary divisions follow pertinent physical properties of the atmospheric layers: The thermal Structure or stability of individual layers: Troposphere sphere of the weather Stratosphere 'layered' sphere Mesosphere 'intermediate' sphere Thermosphere 'hot' sphere The Degree of Ionisation: Neutrosphere Ionosphere low ion density high ion (electron) density The dominating Mixing Mechanism: Homosphere homogeneous, (turbulent) mixing Heterosphere heterogeneous (diffusive) separation R.G. Fleagle & J.A. Businger (1963), an Introduction to Atmospheric Physics, Academic Press, New York.

27 The Floors of the Atmosphere Ionosphere Thermosphere Heterosphere Mesosphere 1% of Earth s radius: >99.98% of atmosphere The layers of the atmosphere, division according to the categories temperature, mixing mechanism and degree of ionisation. [Brasseur + Solomon 1986]. 1% of Earth s Radius Stratosphere Troposphere Homosphere Tropopause

28 The Vertical Structure of the Atmosphere The vertical structure of the atmosphere: Temperature, pressure and air density, physical processes [Bergman Schäfer 2001]

29 Pressure and Temperature Distribution in the Atmosphere The Atmospheric pressure p 0 at the earth s surface is given by: A 2 4π RE where M A denotes the total mass of the atmosphere, g the acceleration due to gravity of earth, and R E the earth radius. Units of air pressure: bar 1 bar = 10 5 Pa = 10 5 N/m 2 millibar 1 mbar = 10 2 Pa = 1 Hektopascal Torricelli 1 torr = 1 mm Hg = Pa Atmosphere 1 atm = bar = Pa = 760 torr The atmospheric pressure decreases with height, in particular by changing the altitude by dz the force K on an area F will change by the amount: df = - g ρ A dz where ρ denotes the air density. The resulting pressure change dp is given by: dp = df/a = - g ρ dz p 0 = M g (1)

30 The Barometric Height Formula Expressing the air density ρ by molar-mass(m)/molar-volume(v) and substituting V = RT/p (R = gas constant, T = temperature) for one mole leads to: Substituting ρ in the expression for dp: After division by p and integration we obtain: ln dp p = = ln Mg RT p 0 pdz z 0 Mg RT z = p 0 exp And for the pressure p(z) at the altitude z: ( ) RT 0 p z dz ρ = Mg dz M = V For an isothermal atmosphere, i.e. T (and g) being independent of z the above expression can be further simplified (Altitude Pressure Relationship): Mgz z () p z = p0 exp = p0 exp RT zs where the quantity z s is called the atmospheric Scale Height. z s = RT Mg T M Mp RT

31 The Variation of Pressure with Height in the Atmosphere For T = 273 K we obtain z s = 7974 m, i.e. about the known atmospheric scale height of 8 Km. However, in the atmosphere temperatures around 250 K (200 K to 300 K) are more realistic, thus the statement: The atmospheric scale height is 7±1 Km best describes the real conditions in the atmosphere. Alternatively we may define the half pressure heigth : Z 1/2 = (ln2) z s z s 5 km

32 The Altitude Pressure relationship, An Alternative View: The energy E(z) of an air molecule in the gravity field of earth is given by E(z) = M g z. Since the energy is statistically distributed it has to follow an Boltzmann- distribution, i.e. the number of molecules n(z) with the energy E(z) is given by: n ( z ) n( 0) E mgz = exp = exp kt kt Since n(z) p(z), in particular p(0) = p 0 (and with M = N L m and R = N L k, N L = Lohschmidt or Avogadro number) follows the well known pressure altitude relationship (Equation. 2).

33 The Atmospheric Scale Height for some Gases Gas Molecular Weight Scale Height z s p(20km)/p(0) (hypothetical!) g/mol Km N Air O CO CFCl

34 Separation of the Atmospheric Constituents? Statement: Heavy molecules (molecules with high molecular weight) are enriched close to the surface. In a hypothetical purely diffusive mixed atmosphere each species would, indeed, assume a vertical distribution according to its individual scale height (Equation. 2, pressure altitude relationship). Where the scale height z s would be inversely proportional to its molecular weight. Table 3 gives some examples. In reality the atmosphere is well mixed by turbulence up to the 'Turbopause' at about 100 Km altitude (see Fig. 5), this layer is called the homosphere. The diffusive separation of constituents only occurs above about 100 Km in the "Heterosphere".

35 Turbulent Mixing of the Atmosphere Molecular diffusion constants, D Turbulent diffusion constants, K K, D in cm 2 /s Turbulent Diffusion constant K (according to estimates of several authors) and molecular diffusion constants (D(N 2 -O 2 ) and D(N 2 -H)) as function of altitude. The D is proportional to the mean free path of the gas and thus inversely proportional to pressure, therefore D increases exponentially with altitude. Below 100 Km altitude (in the Homosphere) turbulent mixing dominates, thus there is a uniform scale height for all atmospheric constituents (based on the mean molecular weight of air). Above ca. 100 Km (the "Turbopause") in the Heterosphere eventually diffusive mixing dominates, therefore the atmospheric constituents separate in the sense that each species attains an individual scale height.

36 Atmospheric Temperature and Mean Molecular Mass Heterosphere Homosphere Wikimedia Commons

37 The Atmosphere and Poisson's Equations of State TV κ 1 = pv κ = const. const. p T κ 1 κ T = = γ p const. f κ= f + 2 f κ 1 κ = γ = f /3 = /5 = /5 = 1.4 2/ /7 = /

38 The (Dry) Adiabatic Laps Rate Scales are so large, that changes of state (e.g. lifting of an airmass) are adiabatic processes in good approximation: γ γ p p γ 2 ( ) 0 p p p ( ) p z T = T = T T z = T With the barometric hight formula: z p T z T e T e 0 0 p 0 γ z 0 zs zs ( ) = = Temperature gradient: ( ) dt z 0 dz z z S S γ z γ T e zs γ = = T z Using the scale hight z S =Mg/RT: ( ) γ zs ( ) = p z p e ( ) dt z γmg γmg = T( z) = = const. dz RT ( z) R 0 z γ ( ) dt z dz K m

39 The Concept of Potential Temperature Air : Poisson 'sequation of state : κ κ T0 κ 1 κ 1 p0 T p = = const. With the surface pressure p 0 = 1013 mbar. we obtain the potential Temperature Θ: κ 1 κ p 0 Θ= T p with ( κ-1)/ κ for air z z 3 z 2 Displacement of a fluid parcel upwards (from z 2 to z 3 ): Θ P =const. Θ P < Θ(z 3 ) Density of the fluid parcel larger than density of the surrounding fluid restoring force F dθ > 0 Stable Conditions dz T(z) F F Θ(z) Δz Θ P Θ

40 History of the Atmosphere Oxygen-Catastrophe: O 2 = 1% of atmosphere drastic shift in biosphere bacteria: CO 2 + H 2 O HCHO + O 2 i.e. O 2 was waste product O 2 - below 15%: nothing burns - above 25%: burning is instant

41 The energy supply of the atmosphere What is the source of energy for the atmosphere? The Sun! cold formation of one large circulation pattern dominated by convection heat gradient gravitation? hot? cold

42 Barry+Chorley (1998) Some dynamical features

43 Summary Atmosphere crucial for live Atmosphere is a complex and non-linear system, interacting with the other geophysical compartments (ocean, land, ice sheet ) The primary source of energy is solar radiation Strongest variability (T, p, ) is in the vertical Large range of spatial and temporal scales Hydrostatic equation: zs ( ) = ( ) p z p 0 e z