ATS 621: ATMOSPHERIC CHEMISTRY Fall 2011 Instructor: Colette L. Heald Rm. Class Hours: Tues/Thurs 9:00-9:50am Text: Introduction to Atmospheric Chemistry (D.J. Jacob) Complementary Text: Atmospheric Chemistry and Physics: from Air Pollution to Climate Change (Seinfeld and Pandis)

IMPORTANCE OF ATMOSPHERIC CHEMISTRY Climate Change (…1990s…) Regional Air Pollution (…1950s…) Acid rain (1970s…) Stratospheric Ozone depletion (1985…) Atmospheric CHEMISTRY

URBAN/REGIONAL AIR POLLUTION Global air pollution Tropospheric ozone column: Jun-Aug 1948 – noontime (PA) LA

ACID RAIN Tropospheric ozone column: Jun-Aug Wet sulfate deposition Getting better in the U.S. … but worsening in Asia

STRATOSPHERIC OZONE DEPLETION Tropospheric ozone column: Jun-Aug  Downward trends in stratospheric ozone column on a global scale

Large increases in greenhouse gases and aerosols since pre-industrial times CLIMATE CHANGE

IMPACTS OF ATMOSPHERIC CHEMISTRY GO BACK EVEN FURTHER…. [Zerefos et al., 2007] Paintings by J.M.W. Turner show light scattering from volcanic aerosol Non-volcanic sunset (1828)Volcanic sunset (1833  Babuyan 1831)

AIR QUALITY ALSO DICTATED HOW SOME CITIES EVOLVED… After the great fire of 1666, London’s most fashionable and wealthy districts were established in the West End. The mercantile classes moved East where the air quality was poor … even prior to the Industrial Revolution! Richard Blome’s map, 1673 Docklands (eventually) Merchants/ industry St James Kensington Whitehall

BUT REMAINS A CONTEMPORARY ISSUE…

1. MEASURES OF ATMOSPHERIC COMPOSITION (CHAPTER 1)

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 We will use “species” as a generic term to describe a constituent of the atmosphere Aerosols are the “visible” components of the atmosphere Pollution haze over East Coast Dust off West Africa

BUT ATMOSPHERIC GASES ARE “VISIBLE” TOO… IF YOU LOOK IN THE UV OR IR NO 2 Columns Observed from the SCIAMACHY Satellite Instrument

PREDICTING ATMOSPHERIC CONCENTRATION OF SPECIES X: solve mass balance (or “continuity”) equation for [X](x,t) local change in concentration with time transport (flux divergence; U is wind vector) chemical production and loss (depends on concentrations of other species) [ ] is our general notation for concentration, but “concentration” is an ambiguous word – we will use three different measures: 1. Mixing ratio (or mole fraction) 2. Number density, mass concentration 3. Partial pressure emission deposition

1.1 Mixing ratio or mole fraction C X [mol mol -1 ] remains constant when air density changes  robust measure of atmospheric composition GASMIXING RATIO (dry air) [mol mol -1 ] Nitrogen (N 2 )0.78 Oxygen (O 2 )0.21 Argon (Ar) Carbon dioxide (CO 2 )365x10 -6 Neon (Ne)18x10 -6 Ozone (O 3 )( )x10 -6 Helium (He)5.2x10 -6 Methane (CH 4 )1.7x10 -6 Krypton (Kr)1.1x10 -6 Trace gases Air also contains variable H 2 O vapor ( 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 = 1x mol mol -1

ATMOSPHERIC CO 2 INCREASE OVER PAST 1000 YEARS Intergovernmental Panel on Climate Change (IPCC), 2005 Concentration units: parts per million (ppm) number of CO 2 molecules per 10 6 molecules of air CO 2 CONCENTRATION IS MEASURED AS MIXING RATIO

ATMOSPHERIC CO 2 TREND OVER PAST 25 YEARS IPCC [2005]  mol mol -1 is the proper SI unit; ppm, ppmv are customary units

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

"Perfect Gas" Law PV = N' kT or PV = N RT N' = number of molecules in the air parcel N = number of moles; N' = N x A v k = Boltzmann constant R = Universal Gas Constant; R = k x A v ================================= (In meteorology texts: P =  RT - different "R“ = 287 J/kg/K) k = × m 2 kg s -2 K -1 R = 8.31 J mole -1 K -1

1.2 Number density n X [molecules cm -3 ] Proper measure for reaction rates optical properties of atmosphere Proper measure for absorption of radiation by atmosphere n X and C X are related by the ideal gas law: Also define the mass concentration (g cm -3 ): n a = air density A v = Avogadro’s number P = pressure [Pa] R = Universal gas constant = A v k k=Boltzmann cnst T = temperature [K] M X = molecular weight of X [g/mol]

LAST WEEKEND’S STRATOSPHERIC OZONE LAYER Method: UV solar backscatter Scattering by Earth surface and atmosphere   Ozone layer Ozone absorption spectrum   1 “Dobson Unit (DU)” = 0.01 mm ozone at STP = 2.69x10 16 molecules cm -2 THICKNESS OF OZONE LAYER IS MEASURED AS A COLUMN CONCENTRATION

ANNUAL MEAN PARTICULATE MATTER (PM) CONCENTRATIONS AT U.S. SITES, EPA particulate matter assessment document (NARSTO), 2003 PM 2.5 (aerosol particles < 2.5  m diameter) U.S. air quality standard: PM 2.5 = 15  g 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 Typical U.S. aerosol size distributions by volume URBAN RURAL

1.3 Partial pressure P x [Pa] Dalton’s law:Proper measure for phase change (such as condensation of water vapour) Evaporation of liquid water from a pan: Add a lid: escaping water molecules collide on lid and return to surface; collision rate measures P H2O eventually, flux escaping = flux returning : saturation (P H2O,SAT ) cloud formation in atmosphere requires P H2O > P H2O,SAT T  P H2O,SAT No lid: water molecules escape from pan to atmosphere (evaporation)

CLAUSIUS-CLAPEYRON EQUATION A = 6.11 hPa (= P vap at 0C) B = 5310 K T o = 273 K P H2O,SAT (hPa) T (K) Vapour pressure increases sharply with temperature, due to the large latent heat.

PHASE DIAGRAM FOR WATER Dew point: Temperature T d such that P H2O = P H2O,SAT (T d ) gas-liquid metastable equilibrium Latest Observation for Fort Collins, CO (80523) Site: on CSU campus triple point of water (n=0)

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

WHY CAN YOU SEE YOUR BREATH ON COLD MORNINGS? Draw mixing lines (dashed) to describe dilution of your breath plume w/outside air Your breath 37 o C, ≈ 100%RH warm outside air cold outside air P H2O is plotted on linear scale (vs. log on previous plots) to draw the mixing lines ICE LIQ GAS

RAOULT’S LAW water saturation vapor pressure over pure liquid water surface water saturation vapor pressure over aqueous solution of water mixing ratio x H2O An atmosphere of relative humidity RH can contain at equilibrium aqueous solution particles of water mixing ratio solute molecules in green

AIR POLLUTION HAZE “clean” day “moderately polluted” day Views of Acadia National Park Visibility is limited by high concentrations of aerosol particles that have swollen to large sizes due to high relative humidity

CONVERTING BETWEEN DIFFERENT MEASURES OF CONCENTRATION CxCx PxPx nxnx