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1. MEASURES OF ATMOSPHERIC COMPOSITION

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  1. 1. MEASURES OF ATMOSPHERIC COMPOSITION

  2. The atmosphere seen from space WHAT IS THE ATMOSPHERE? • Gaesous envelope surrounding the Earth • Mixture of gases, also contains suspended solid and liquid particles (aerosols) Aerosol = dispersed condensed phase suspended in a gas Aerosols are the “visible” components of the atmosphere California fire plumes Dust off West Africa Pollution off U.S. east coast

  3. WHAT ABOUT CLOUDS? Clouds are made up of water droplets or ice crystals (1-100 mm), much larger than typical aerosols (0.01-10 mm). They are technically aerosols but have unique properties and are in practice considered separately.

  4. ATMOSPHERIC GASES ARE “VISIBLE” TOO…IF YOU LOOK IN THE UV OR IR Nitrogen dioxide (NO2 ) observed by satellite in the UV

  5. remains constant when air density changes e robust measure of atmospheric composition Air also contains variable H2O vapor (10-6-10-2 mol mol-1) and aerosol particles 1.1 Mixing ratio or mole fraction CX[mol mol-1] Trace gases Trace gas concentration units: 1 ppmv = 1 µmol mol-1 = 1x10-6 mol mol-1 1 ppbv = 1 nmol mol-1 = 1x10-9 mol mol-1 1 pptv = 1 pmol mol-1 = 1x10-12 mol mol-1

  6. ATMOSPHERIC CO2 INCREASE OVER PAST 1000 YEARS Intergovernmental Panel on Climate Change (IPCC), 2007 Concentration units: parts per million (ppm) number of CO2 molecules per 106 molecules of air CO2 CONCENTRATION IS MEASURED AS MIXING RATIO

  7. ATMOSPHERIC CO2 TREND OVER PAST 25 YEARS IPCC [2007] mmol mol-1 is the proper SI unit; ppm, ppmv are customary units

  8. EPA SURFACE OZONE AIR QUALITY STANDARD “8-houraverage of 0.08 ppmv not to be exceeded more than 3x/year” EPA air quality standard for surface ozone is 0.075 ppm or 75 ppb

  9. 1.2 Number density nX[molecules cm-3] Proper measure for • reaction rates • optical properties of atmosphere Proper measure for absorption or scattering of radiation by atmosphere nXand CXare relatedby the ideal gas law: na = air density Av = Avogadro’s number P = pressure R = Gas constant T = temperature MX= molecular mass of X Also define the mass concentration (g cm-3):

  10. THIS WEEK’S STRATOSPHERIC OZONE LAYER http://jwocky.gsfc.nasa.gov/ Method: UV solar backscatter l1 l2 Ozone layer Scattering by Earth surface and atmosphere Ozone absorption spectrum 1 “Dobson Unit (DU)” = 0.01 mm ozone at STP = 2.69x1016 molecules cm-2 THICKNESS OF OZONE LAYER IS MEASURED AS A COLUMN CONCENTRATION l1 l2

  11. US AREAS EXCEEDING THE PARTICULATE MATTER (PM2.5) EPA AIR QUALITY STANDARD (2010) PM2.5≡ concentration of aerosol particles < 2.5 mm diameter) U.S. air quality standard: PM2.5 = 15 mg m-3 (annual mean)

  12. SPECIFIC ISSUES FOR AEROSOL CONCENTRATIONS • A given aerosol particle is characterized by its size, shape, phase, and chemical composition – large number of variables! • Measures of aerosol concentrations must be given in some integral form, by summing over all particles present in a given air volume that have a certain property • The aerosol size distribution is a continuous function URBAN Typical U.S. aerosol size distributions by volume RURAL

  13. 1.3 Partial pressure Px[Pa] Dalton’s law: Proper measure for phase change (such as condensation of water vapor) No lid: water molecules escape from pan to atmosphere (evaporation) Evaporation of liquid water from a pan: Add a lid: • escaping water molecules collide on lid and return to surface; collision rate measures PH2O • eventually, flux escaping = flux returning : saturation (PH2O,SAT) • cloud formation in atmoshere requires PH2O > PH2O,SAT • T k e PH2O,SAT k

  14. CLAUSIUS-CLAPEYRON EQUATION: PH2O, SAT = f(T) A = 6.11 hPa B = - 5310 K To = 273 K PH2O,SAT (hPa) T (K)

  15. PHASE DIAGRAM FOR WATER gas-liquid metastable equilibrium Relative humidity (%) = 100(PH2O/PH2O,SAT) Dew point: Temperature Tdsuch that PH2O = PH2O,SAT(Td)

  16. due to accumulation of water vapor from volcanic outgassing early in its history RUNAWAY GREENHOUSE EFFECT ON VENUS …did not happen on Earth because farther from Sun; as water accumulated it reached saturation and precipitated, forming the oceans EARTH VENUS

  17. WHY CAN YOU SEE YOUR BREATH ON COLD MORNINGS? Draw mixing lines (dashed) to describe dilution of your breath plume w/outside air PH2O is plotted on linear scale to draw the mixing lines LIQ Your breath 37oC, ≈ 100%RH ICE cloud! no cloud warm outside air GAS cold outside air

  18. PHASE DIAGRAM FOR WATER gas-liquid metastable equilibrium Relative humidity (%) = 100(PH2O/PH2O,SAT) Dew point: Temperature Tdsuch that PH2O = PH2O,SAT(Td)

  19. AIR POLLUTION HAZE Views of Acadia National Park http://www.hazecam.net/ “moderately polluted” day “clean” day Visibility is limited by high concentrations of aerosol particles that have swollen to large sizes due to high (but <100%) relative humidity

  20. RAOULT’S LAW solute molecules in green water saturation vapor pressure over pure liquid water surface water saturation vapor pressure over aqueous solution of water mixing ratio xH2O An atmosphere of relative humidity RH can contain at equilibrium aqueous solution particles of water mixing ratio

  21. HOWEVER, AEROSOL PARTICLES MUST ALSO SATISFY SOLUBILITY EQUILIBRIA Consider an aqueous sea salt (NaCl) particle: it must satisfy This requires: At lower RH, the particle is dry.

  22. UPTAKE OF WATER BY AEROSOLS: HAZE NaCl/H2O Deliquescence RH; depends on particle composition