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ATMOSPHERIC AEROSOLS: ensembles of condensed-phase particles suspended in air Typical aerosol size distribution number area volume Aerosols are the visible part of the atmosphere: Pollution off U.S. east coast Dust off West AfricaCalifornia fire plumes Slide 2 ORIGIN OF THE ATMOSPHERIC AEROSOL Soil dust Sea salt Size range: 0.001 m (molecular cluster) to 100 m (small raindrop) Slide 3 WHY CARE ABOUT ATMOSPHERIC AEROSOLS? Public health Visibility Ocean fertilization Chemistry Climate forcing Cloud formation Slide 4 OZONE AND PARTICULATE MATTER (PM): THE TOP TWO AIR POLLUTANTS IN THE U.S. # millions of people living in areas exceeding national ambient air quality standards (NAAQS) in 2007 15 g m -3 (day), 65 (annual) 75 ppb (8-h average) 65 g m -3 (day), 15 (annual) Slide 5 ANNUAL MEAN PM 2.5 CONCENTRATIONS (2002) derived from MODIS satellite instrument data Slide 6 SCATTERING OF RADIATION BY AEROSOLS By scattering solar radiation, aerosols decrease visibility and increase the Earths albedo Scattering efficiency is maximum when particle radius = particles in 0.1-1 m size range are efficient scatterers of solar radiation 2 (diffraction limit) green light ( = 0.5 m) Slide 7 VISIBILITY IN U.S. WILDERNESS AREAS Statistics for 20% worst visibility days Deciviews 2001 observationsNatural Background; includes transboundary pollution 300 150 80 40 20 Visual range (km) Park et al. [2006] EPA Regional Haze Rule requires that natural visibility be achieved in all US wilderness areas by 2064 Slide 8 RAOULTS 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 Slide 9 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. Slide 10 UPTAKE OF WATER BY AEROSOLS: HAZE Deliquescence RH; depends on particle composition NaCl/H 2 O Slide 11 FINE AEROSOL COMPOSITION IN NORTH AMERICA Annual mean PM 2.5 concentrations (NARSTO, 2004) Current air quality standard is 15 g m -3 Slide 12 SULFATE-NITRATE-AMMONIUM AEROSOLS IN U.S. (2001) Highest concentrations in industrial Midwest (coal-fired power plants) SulfateNitrate Ammonium Acidity Slide 13 FORMATION OF SULFATE-NITRATE-AMMONIUM AEROSOLS Sulfate always forms an aqueous aerosol Ammonia dissolves in the sulfate aerosol totally or until titration of acidity, whichever happens first Nitrate is taken up by aerosol if (and only if) excess NH 3 is available after sulfate titration HNO 3 and excess NH 3 can also form a solid aerosol if RH is low Thermodynamic rules: Highest concentrations in industrial Midwest (coal-fired power plants) Conditionaerosol pHLow RHHigh RH [S(VI)] > 2[N(-III)]acidH 2 SO 4 nH 2 O, NH 4 HSO 4, (NH 4 ) 2 SO 4 (NH 4 +, H +, SO 4 2- ) solution [S(VI)] 2[N(-III)]neutral(NH 4 ) 2 SO 4, NH 4 NO 3 (NH 4 +,NO 3 - ) solution Slide 14 U.S. SO 2 EMISSIONS Sulfur emissions, Tg a -1 788.3 GLOBAL UNITED STATES Slide 15 OBSERVED TITRATION OF SO 2 BY H 2 O 2 IN CLOUD First aircraft observations by Daum et al. [1984] Slide 16 Slide 17 Ammonia and NO x emissions in the US (2006) Zhang et al. [2011] Slide 18 Distributions of HNO 3 (g) and NO 3 - (aerosol) in surface air HNO 3 (g) NO 3 - (aerosol) EPA network data for 2006 Zhang et al. [2011] Slide 19 NATURAL pH OF RAIN Equilibrium with natural CO 2 (280 ppmv) results in a rain pH of 5.7: This pH can be modified by natural acids (H 2 SO 4, HNO 3, RCOOH) and bases (NH 3, CaCO 3 ) natural rain has a pH in range 5-7 Acid rain refers to rain with pH < 5 damage to ecosystems Slide 20 PRECIPITATION PH OVER THE UNITED STATES Slide 21 CHEMICAL COMPOSITION OF PRECIPITATION Neutralization by NH 3 is illusory because NH 4 + NH 3 + H + in ecosystem Slide 22 Slide 23 LONG-TERM TREND IN US SO 2 EMISSIONS Slide 24 Long-term trend in US NO x emissions Power plant emission trend: NO x SIP Call Slide 25 Sulfate wet deposition and aerosol concentrations, 1980-2010 Leibensperger et al. [2011] Slide 26 Ammonium wet deposition and aerosol concentrations, 1980-2010 Leibensperger et al. [2011] Slide 27 Nitrate wet deposition and aerosol concentrations, 1980-2010 Leibensperger et al. [2011] Slide 28 Sulfate-ammonium-nitrate trends in eastern US (E of 100W) Slide 29 TREND IN FREQUENCY OF ACID RAIN (pH < 5) Lehmann et al. [2007] 1994-1996 2002-2004 Slide 30 BUT ECOSYSTEM ACIDIFICATION IS PARTLY A TITRATION PROBLEM FROM ACID INPUT OVER MANY YEARS Acid-neutralizing capacity (ANC) from CaCO 3 and other bases Acid flux F H+ Slide 31 WORLDWIDE MEASUREMENTS OF FINE AEROSOL COMPOSITION Slide 32 CARBONACEOUS AEROSOL SOURCES IN THE U.S. ORGANIC CARBON (OC) 2.7 Tg yr -1 BLACK CARBON (BC) 0.66 Tg yr -1 Annual mean concentrations (2001) BC OC Slide 33 Long-term trends in BC and OC aerosol over the US Annual mean concentrations National trends Observed Model Leibensperger et al. [2011] Slide 34 RADIATIVE FORCING FROM BLACK CARBON (BC) IPCC [2007] Slide 35 BC is emitted by incomplete combustion BC or soot is optically defined and includes both graphitic elemental carbon (EC) and light-absorbing heavy organic matter Diesel engines are large BC sources Freshly emitted BC particle Slide 36 Atmospheric aging and scavenging of BC Emission Hydrophobic BC resistant to scavenging coagulation gas condensation Hydrophilic BC coated with sulfate, nitrate Scavenging Aging time scale ~ 1 d Implications for BC export from source continents: OCEAN aging scavenging Hydrophobic BC aging long-range transport FREE TROPOSPHERE BOUNDARY LAYER Slide 37 BC and OC aerosol during ARCTAS aircraft campaign (spring 2008) Wang et al. [2011] Slide 38 ORGANIC AEROSOL IN STANDARD GEOS-Chem MODEL fuel/industry open fires OH, O 3,NO 3 SOGSOA POA K vegetation fuel/industry open fires 700 isoprene terpenes oxygenates 30 alkenes aromatics oxygenates alkanes alkenes aromatics VOC EMISSIONPRIMARY EMISSION VOC 50 20100 20 Global sources in Tg C y -1 secondary formation Slide 39 TERPENES Terpenes are biogenic hydrocarbons produced in plants by combination of isoprene units (C 5 H 8 ) Monoterpenes: C 10 H 16 -pinene Sesquiterpenes: C 15 H 24 -cadinene Slide 40 SOA MODELING AS GAS-AEROSOL EQUILIBRIUM VOC oxidation generates semi-volatile products: which then partition between the gas and aerosol phase: where the partitioning coefficient is given by and is a strong function of temperature. Values of and p 0 are fitted to smog chamber data Chung and Seinfeld, 2002 M o is the mass concentration of pre-existing organic aerosol Slide 41 POSSIBLE MECHANISMS FOR DICARBONYL SOA FORMATION GASAQUEOUS Oligomers OH Organic acids H* ~ 10 5 M atm -1 Ervens et al. [2004] Crahan et al. [2004] Lim et al. [2005] Carlton et al. [2006, 2007] Warneck et al. [2005] Sorooshian et al. [2006, 2007] Altieri et al. [2006, 2008] Schweitzer et al. [1998] Kalberer et al. [2004] Liggio et al. [2005a,b] Hastings et al. [2005] Zhao et al. [2006] Loeffler et al. [2006] glyoxal H* ~ 10 3 M atm -1 methylglyoxal oxidation oligomerization Slide 42 GLYOXAL/METHYLGLYOXAL FORMATION FROM ISOPRENE GEOS-Chem mechanism based on MCM v3.1 Fu et al. [JGR, 2008] 6%25% molar yields Slide 43 SOA MODELING USING VOLATILITY BASIS SETS Partition semi-volatile VOCs between aerosol and gas: Aerosol fraction for SVOC i Define SVOCs by their stability class: Donahue et al. [2006] Slide 44 SOA VOLATILITY BASIS SET: EFFECT OF DILUTION Donahue et al. [2006] Slide 45 SOA VOLATILITY BASIS SET: CHEMICAL AGING Donahue et al. [2006] As VOCs go through successive oxidation steps, products become more oxygenated and less volatile, but eventually smaller and more volatile Slide 46 IMPLEMENTING OC VOLATILITY CLASSES IN GEOS-Chem Pye and Seinfeld [2010] Combustion Primary OC is actually semi-volatile Mean wintertime OC concentrations: IMPROVE data shown as circles Slide 47 AEROSOL OPTICAL DEPTH IPCC [2007] Global mean AOD is about 0.1, with 25% of that anthropogenic Slide 48 Mt. Pinatubo eruption 1991 1992 1993 1994 -0.6 -0.4 -0.2 0 +0.2 Temperature Change ( o C) Observations NASA/GISS general circulation model Temperature decrease following large volcanic eruptions EVIDENCE OF AEROSOL EFFECTS ON CLIMATE: Slide 49 SCATTERING vs. ABSORBING AEROSOLS Scattering sulfate and organic aerosol over Massachusetts Partly absorbing dust aerosol downwind of Sahara Absorbing aerosols (black carbon, dust) warm the climate by absorbing solar radiation Slide 50 AEROSOL INDIRECT EFFECT FROM CLOUD CHANGES Clouds form by condensation on preexisting aerosol particles (cloud condensation nuclei)when RH>100% clean cloud (few particles): large cloud droplets low albedo efficient precipitation polluted cloud (many particles): small cloud droplets high albedo suppressed precipitation Slide 51 Particles emitted by ships increase concentration of cloud condensation nuclei (CCN) Increased CCN increase concentration of cloud droplets and reduce their avg. size Increased concentration and smaller particles reduce production of drizzle Liquid water content increases because loss of drizzle particles is suppressed Clouds are optically thicker and brighter along ship track N~ 100 cm -3 W~ 0.75 g m -3 r e ~ 10.5 m N~ 40 cm -3 W~ 0.30 g m -3 r e ~ 11.2 m from D. Rosenfeld EVIDENCE OF INDIRECT EFFECT: SHIP TRACKS Slide 52 AVHRR, 27. Sept. 1987, 22:45 GMT US-west coast NASA, 2002 Atlantic, France, Spain SATELLITE IMAGES OF SHIP TRACKS Slide 53 Aircraft condensation trails (contrails) over France, photographed from the Space Shuttle (NASA). OTHER EVIDENCE OF CLOUD FORCING: CONTRAILS AND AIRCRAFT CIRRUS Slide 54 Radiative forcing by aerosols is very inhomogeneous in contrast to the long-lived greenhouse gases Present-day annual direct radiative forcing from anthopogenic aerosols Leibensperger et al., 2011 Aerosol radiative forcing over polluted continents can more than offset forcing from greenhouse gases The extent to which this regional radiative forcing translates into regional climate response is not understood global radiative forcing from CO 2 Slide 55 Radiative forcing from US anthropogenic aerosol Leibensperger et al., 2011 Forcing is mostly from sulfate, peaked in 1970-1990 Little leverage to be had from BC control Slide 56 Cooling due to US anthropogenic aerosols in 1970-1990 From difference of GCM simulations with vs. without US aerosol sources, including aerosol direct and indirect radiative effects Surface cooling (up to 1 o C) is concentrated over eastern US Cooling at 500 hPa is more widespread over the northern hemisphere Leibensperger et al., 2011 Slide 57 Observed US surface temperature trend GISTEMP [2010] Contiguous US temperature US has warmed faster than global mean, as expected in general for mid-latitudes land But there has been no warming between 1930 and 1980, followed by sharp warming after 1980 Warming hole over the eastern US; is it due to US aerosol sources? 1955-2000 trend Slide 58 Simulating the 1950-2050 surface temperature trend in eastern US US anthropogenic aerosol sources can explain the warming hole Rapid warming has taken place since 1990s that we attribute to source reduction Most of the warming from aerosol source reduction has already been realized Observed (GISTEMP) GCM (standard) GCM (no anthro US aerosol) GCM (1980 aerosol) 1955-2000 trend Leibensperger et al., 2011 Slide 59 GENERAL SCHEMATIC FOR HETEROGENEOUS CHEMISTRY A(g)A(g) s A(aq) s A(aq) B(aq) BsBs B(g) diffusion surface reaction aqueous reaction GAS AEROSOL Aerosols enable surface and ionic reactions that would not happen in the gas phase; also concentrate low-volatility species in condensed phase interfacial equilibrium Slide 60 FLUX AT THE GAS-PARTICLE INTERFACE A(g)A(g) s A(aq) s diffusion interfacial equilibrium l = mean free path of air (0.18 m at STP) a = particle radius Knudsen number Kn = a/l Kn >>1: continuum (diffusion-limited) regime Kn