TROPOSPHERIC OZONE AND OXIDANT CHEMISTRY

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TROPOSPHERIC OZONE AND OXIDANT CHEMISTRY. The many faces of atmospheric ozone:. In stratosphere: UV shield. Stratosphere: 90% of total. In middle/upper troposphere: greenhouse gas. Troposphere. In lower/middle troposphere: precursor of OH, main atmospheric oxidant. - PowerPoint PPT Presentation

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TROPOSPHERIC OZONE AND OXIDANT CHEMISTRYTroposphereStratosphere:90% of totalThe many faces of atmospheric ozone:In stratosphere: UV shieldIn middle/upper troposphere: greenhouse gasIn lower/middle troposphere: precursor of OH, main atmospheric oxidantIn surface air: toxic to humans and vegetationTHE ATMOSPHERE: OXIDIZING MEDIUM IN GLOBAL BIOGEOCHEMICAL CYCLESEARTHSURFACEEmissionReduced gasOxidized gas/aerosolOxidationUptakeReductionAtmospheric oxidation is responsible for removal of many pollutants, e.g. methane (major greenhouse gas) CO (toxic pollutant) HCFCs (ClOx sources in stratosphere)TROPOSPHERE WAS VIEWED AS CHEMICALLY INERT UNTIL 1970The chemistry of the troposphere is mainly that of of a large number of atmospheric constituents and of their reactions with molecular oxygenMethane and CO are chemically quite inert in the troposphere [Cadle and Allen, Atmospheric Photochemistry, Science, 1970]Lifetime of CO estimated at 2.7 years (removal by soil) leads to concern about global CO pollution from increasing car emissions [Robbins and Robbins, Sources, Abundance, and Fate of Gaseous Atmospheric Pollutants, SRI report, 1967] FIRST BREAKTHROUGH: Measurements of cosmogenic 14CO place a constraint of ~ 0.1 yr on the tropospheric lifetime of CO [Weinstock, Science, 1969]SECOND BREAKTHROUGH: Tropospheric OH ~1x106 cm-3 predicted from O(1D)+H2O, results in tropospheric lifetimes of ~0.1 yr for CO and ~2 yr for CH4 [Levy, Science, 1971, J. Geophys. Res. 1973] THIRD BREAKTHROUGH: Methylchlroform observations provide indirect evidence for OH at levels of 2-5x105 cm-3 [Singh, Geophys. Res. Lett. 1977]WHY WAS TROPOSPHERIC OH SO DIFFICULT TO FIGURE OUT?Production of O(1D) in troposphere takes place in narrow band [290-320 nm]

solar flux Iozone absorptioncross-section sO(1D)quantumyield ffsI

~tropopause10 ppmv40 ppbvTYPICAL OZONE PROFILE: ~10% OF OZONE COLUMN GLOBALLY IS IN THE TROPOSPHEREStratospheric ozone mechanism doesnt apply to troposphere

O2+hvO3+hvBy contrast, in troposphere:

no photons < 240 nmno oxygen photolysis;

neglible O atom conc.gno XO + O lossIn stratosphere:Estimate ozone flux FO3 across tropopause (strat-trop exchange)Total O3 col = 5x1013 moles10% of that is in troposphereRes. time of air in strat = 1.4 yrEstimate CH4 source SCH4:Mean concentration = 1.7 ppmvLifetime = 9 yearsEstimate CO source SCO:Mean concentration = 100 ppbvLifetime = 2 monthsUNTIL ~1990, PREVAILING VIEW WAS THAT TROPOSPHERIC OZONE ORIGINATED MAINLY FROM STRATOSPHEREbut that cannot work.FO3 = 3x1013 moles yr-1SCH4 = 3x1013 moles yr-1SCO = 9.7x1013moles yr-1SCO+ SCH4 > 2FO3 e OH would be titrated!

OZONE PRODUCTION IN TROPOSPHEREPhotochemical oxidation of CO and volatile organic compounds (VOCs)catalyzed by HOx and NOxHOx H + OH + HO2 + RO + RO2NOx NO + NO2Oxidation of CO:

Oxidation of VOC:RO can also decompose or isomerize; range of carbonyl productsCarbonyl products can react with OH to produce additional ozone, or photolyze to generate more HOx radicals (branching reaction)OH can also add to double bonds of unsaturated VOCsGLOBAL BUDGET OF TROPOSPHERIC OZONE (MODEL)

O3O2hnO3OHHO2hn, H2ODepositionNOH2O2CO, VOCNO2hnSTRATOSPHERETROPOSPHERE8-18 kmChem prod in troposphere,Tg y-143001600Chem loss in troposphere,Tg y-140001600Transport from stratosphere,Tg y-1400400Deposition,Tg y-1700400Burden, Tg360230Lifetime, days2842Present-day PreindustrialOZONE CONCENTRATIONS vs. NOx AND VOC EMISSIONSBox model calculation

NOx-saturatedregimeNOx-limited regimeRidge

SATELLITE OBSERVATIONS OF TROPOSPHERIC NO2SCIAMACHY data. May-Oct 2004(R.V. Martin, Dalhousie U.)detectionlimitNOx EMISSIONS (Tg N a-1) TO TROPOSPHERE

FOSSIL FUEL 23.1

AIRCRAFT 0.5BIOFUEL 2.2BIOMASSBURNING 5.2SOILS 5.1LIGHTNING 5.8STRATOSPHERE 0.2LIGHTNING FLASHES SEEN FROM SPACE (2000)

DJFJJAGLOBAL DISTRIBUTION OF TROPOSPHERIC OZONEZhang et al. [2010]TES thermal IR satellite observations for 2006, seasonal means at 500 hPa

Maximum values at northern mid-latitudes in spring-summer due to anthropogenic pollution;

High values in tropical regions affected by seasonal biomass burning;

Minimum values over tropical oceans due to chemical loss

LONDON FOG

Aerosols a.k.a.particulate matter (PM) from domestic+industrial coal combustionKiller fog of December 1952 resulted in 10,000 excess deathsCoal combustionTemperatureAltitudeinversionsulfateorganic carbonblack carbonparticles< 1km

LOS ANGELES SMOG

Respiratory problems, vegetation damage due to high surface ozonetropospherestratosphere8-18 kmtemperatureinversionozonealtitude Nitrogen oxides (NOx NO + NO2) Volatile organic compounds (VOCs)UV radiationOzone (O3)vehicles, industry, vegetationproduced by photolysisof oxygen (O2)

AIR POLLUTION IN THE US TODAY:Ozone and fine particulate matter (PM2.5) are the two main pollutants75 ppb (8-h average)15 mg m-3 (1-y av.)http://epa.gov/airtrends/2010/

OzonePM2.5

2008 REVISION TO OZONE STANDARDFROM 84 to 75 PPB CAUSED MORE U.S. AREAS TO BE OUT OF COMPLIANCE0 20 40 60 80 100 120 ppb Europe AQS(seasonal)U.S. AQS(8-h avg.)U.S. AQS(1-h avg.)PreindustrialozonebackgroundPresent-day ozone background at northern mid-latitudes Europe AQS (8-h avg.) Canadian AQS (8-h avg.)Mexican AQS(1-h avg.)AND INCREASED THE IMPORTANCE OF THE OZONE BACKGROUND20082014?

Currently proposed 60-70 ppb standard would have extensive non-complianceOZONE CONCENTRATIONS vs. NOx AND VOC EMISSIONSAir pollution model calculation for a typical urban airshed

NOx-saturatedNOx-limitedRidgeLARGE SUPPLY OF BIOGENIC VOCs unrecognized until the 1990s

Isoprene (biogenic VOC)Anthropogenic VOCsJacob et al., J. Geophys. Res. [1993]Switches polluted areas in U.S. from NOx-saturated to NOx-limited regime!recognized in Revised Clean Air Act of 1999MAPPING OF VOC EMISSIONS FROM SPACEusing satellite measurements of formaldehydeconfirms dominance of biogenic over anthropogenic VOCs

Millet et al. [2008]1970-2003 TREND OF U.S. EMISSIONS

Focus until past decade was on VOC emission controlsDECREASE OF POWER PLANT NOx EMISSIONSOVER THE PAST DECADE

Decreasing US NOx emissions from power plantsGrowth Trends

PopulationVehicle Miles TraveledGross State Product26Emission Trends

CO2NOxSOxROGCO27PM10 Trends

South Coast State StandardSan Joaquin Valley2890.00.10.20.30.40.51980198219841986198819901992199419961998Max 1-hr Ozone (ppm) State StandardSouth CoastSan Joaquin ValleyOzone Trends29Historical Ozone Levels

Number of DaysStage II > 350 ppb O3Stage I > 200 ppb O330In 1999 SoCAB 111 days above the state std of 90 ppb and 39 days above the federal std of 120 ppb

South Coast is the only EXTREME ozone non-attainment area in the countrySOUTH COAST O3 HISTORY0.00.10.20.30.40.50.619501955196019651970197519801985199019952000YearsMax O3 (ppm)1952 Prof. Haagen-Smit discovers smog formation1963 Clean Air Act (CAA)1970 EPA created, CAA Ammend.1976 Volvo introduces first car to use 3-way catalyst1984 Smog Check Program1990 CAAA1994 Smog Check II1971 CARB adopts 1st auto NOx standardsEPA promulgates NAAQSNAAQSImprovement Over Past 20 Years-80%-60%-40%-20%0%NitrogenDioxideSulfurDioxideCarbonMonoxide Ozone PM10 Air Toxics(CancerRisk)Percent Change Approaching StandardsAttained Standards32

33

T H A N K S ! !34

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