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 Pollution haze over East Coast Dust off West Africa
ATMOSPHERIC GASES ARE “VISIBLE” TOO…IF YOU LOOK IN THE UV OR IR NO2 Columns Observed from the SCIAMACHY Satellite Instrument
WHAT ABOUT CLOUDS? Clouds are made up of water droplets (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.
1.1 Mixing ratio or mole fraction CX[mol mol-1] 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 • Trace gas concentration units: • 1 ppmv = 1x10-6 mol mol-1 • 1 ppbv = 1x10-9 mol mol-1 • 1 pptv = 1x10-12 mol mol-1 Trace gases
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
ATMOSPHERIC CO2 TREND OVER PAST 25 YEARS IPCC  mmol mol-1 is the proper SI unit; ppm, ppmv are customary units
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 75 ppb
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 = Av k k=Boltzmann cnst T = temperature MX= molecular mass of X Also define the mass concentration (g cm-3):
TUESDAY’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
ANNUAL MEAN PARTICULATE MATTER (PM) CONCENTRATIONS AT U.S. SITES, 1995-2000EPA particulate matter assessment document (NARSTO), 2003 PM2.5 (aerosol particles < 2.5 mm diameter) U.S. air quality standard: PM2.5 = 15 mg m-3 (annual mean) Red circles indicate sites in violation of the standard STANDARD IS EXPRESSED AS A MASS CONCENTRATION PER UNIT VOLUME
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 must be treated as a continuous function URBAN Typical U.S. aerosol size distributions by volume RURAL
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
CLAUSIUS-CLAPEYRON EQUATION: PH2O, SAT = f(T) A = 6.11 hPa B = - 5310 K To = 273 K PH2O,SAT (hPa) T (K)
SHORT QUESTIONS 1. Give a rough order of magnitude for the number of molecules present in a typical 1 micrometer aerosol particle. 2. Consider harmful UV radiation for which the ozone layer has an optical depth of 10. The ozone layer has thinned by 6% since 1970, with a corresponding 6% decrease in optical depth. What is the resulting percent increase in this UV radiation reaching the surface of the Earth?3. In an atmosphere with fixed mixing ratio of water vapor, what two processes can cause an increase in relative humidity?
PHASE DIAGRAM FOR WATER http://weather.unisys.com/ Latest Observation for Cambridge, MA (02138) Site: KBOS Time: 12 PM EDT 23 SEP 08Temp: 61 F (16 C)Dewpt: 46 F (7 C)Rel Hum: 58%Winds: E at 10 kntPressure: 1033.2 mb (30.52 in) gas-liquid metastable equilibrium Relative humidity (%) = 100(PH2O/PH2O,SAT) Dew point: Temperature Tdsuch that PH2O = PH2O,SAT(Td)
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
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
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
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
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.
UPTAKE OF WATER BY AEROSOLS: HAZE NaCl/H2O Deliquescence RH; depends on particle composition
SHORT QUESTIONS • What is the fractional increase in mass of water-soluble aerosol particles when relative humidity increases from 90% to 95%? (assume that the aerosols are mainly water). Assuming that visibility degradation is proportional to the cross-sectional area of the particles, what is the effect of this mass increase on visibility?