TROPOSPHERIC OZONE AND OXIDANT CHEMISTRY Troposphere Stratosphere: 90% of total The many faces of atmospheric ozone: In stratosphere: UV shield In middle/upper troposphere: greenhouse gas In lower/middle troposphere: precursor of OH, main atmospheric oxidant In surface air: toxic to humans and vegetation

THE ATMOSPHERE: OXIDIZING MEDIUM IN GLOBAL BIOGEOCHEMICAL CYCLES EARTH SURFACE Emission Reduced gas Oxidized gas/ aerosol Oxidation Uptake Reduction Atmospheric oxidation is responsible for removal of many pollutants, e.g. methane (major greenhouse gas) CO (toxic pollutant) HCFCs (ClO x sources in stratosphere)

TROPOSPHERE WAS VIEWED AS CHEMICALLY INERT UNTIL 1970 “The chemistry of the troposphere is mainly that of of a large number of atmospheric constituents and of their reactions with molecular oxygen…Methane 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 14 CO place a constraint of ~ 0.1 yr on the tropospheric lifetime of CO [Weinstock, Science, 1969] SECOND BREAKTHROUGH: Tropospheric OH ~1x10 6 cm -3 predicted from O( 1 D)+H 2 O, results in tropospheric lifetimes of ~0.1 yr for CO and ~2 yr for CH 4 [Levy, Science, 1971, J. Geophys. Res. 1973] THIRD BREAKTHROUGH: Methylchlroform observations provide indirect evidence for OH at levels of 2-5x10 5 cm -3 [Singh, Geophys. Res. Lett. 1977]

WHY WAS TROPOSPHERIC OH SO DIFFICULT TO FIGURE OUT? Production of O( 1 D) in troposphere takes place in narrow band [ nm] solar flux I ozone absorption cross-section  O( 1 D) quantum yield   I

~tropopause 10 ppmv 40 ppbv TYPICAL OZONE PROFILE: ~10% OF OZONE COLUMN GLOBALLY IS IN THE TROPOSPHERE

Stratospheric ozone mechanism doesn’t apply to troposphere O 2 +hv O 3 +hv By contrast, in troposphere: no photons < 240 nm  no oxygen photolysis; neglible O atom conc.  no XO + O loss In stratosphere:

Estimate ozone flux F O3 across tropopause (strat-trop exchange) –Total O 3 col = 5x10 13 moles –10% of that is in troposphere –Res. time of air in strat = 1.4 yr Estimate CH 4 source S CH4 : –Mean concentration = 1.7 ppmv –Lifetime = 9 years Estimate CO source S CO : –Mean concentration = 100 ppbv –Lifetime = 2 months UNTIL ~1990, PREVAILING VIEW WAS THAT TROPOSPHERIC OZONE ORIGINATED MAINLY FROM STRATOSPHERE…but that cannot work. F O3 = 3x10 13 moles yr -1 S CH4 = 3x10 13 moles yr -1 S CO = 9.7x10 13 moles yr -1 S CO + S CH4 > 2F O3  OH would be titrated!

OZONE PRODUCTION IN TROPOSPHERE Photochemical oxidation of CO and volatile organic compounds (VOCs) catalyzed by HO x and NO x HO x ≡ H + OH + HO 2 + RO + RO 2 NO x ≡ NO + NO 2 Oxidation of CO: Oxidation of VOC: RO can also decompose or isomerize; range of carbonyl products Carbonyl products can react with OH to produce additional ozone, or photolyze to generate more HO x radicals (branching reaction) OH can also add to double bonds of unsaturated VOCs

GLOBAL BUDGET OF TROPOSPHERIC OZONE (MODEL) O3O3 O2O2 h O3O3 OHHO 2 h, H 2 O Deposition NO H2O2H2O2 CO, VOC NO 2 h STRATOSPHERE TROPOSPHERE 8-18 km Chem prod in troposphere, Tg y Chem loss in troposphere, Tg y Transport from stratosphere, Tg y Deposition, Tg y Burden, Tg Lifetime, days Present-day Preindustrial

OZONE CONCENTRATIONS vs. NO x AND VOC EMISSIONS Box model calculation NO x - saturated regime NO x -limited regimeRidge

SATELLITE OBSERVATIONS OF TROPOSPHERIC NO 2 SCIAMACHY data. May- Oct 2004 (R.V. Martin, Dalhousie U.) detection limit

NO x EMISSIONS (Tg N a -1 ) TO TROPOSPHERE FOSSIL FUEL 23.1 AIRCRAFT 0.5 BIOFUEL 2.2 BIOMASS BURNING 5.2 SOILS 5.1 LIGHTNING 5.8 STRATOSPHERE 0.2

LIGHTNING FLASHES SEEN FROM SPACE (2000) DJF JJA

GLOBAL DISTRIBUTION OF TROPOSPHERIC OZONE Zhang 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 combustion “Killer fog” of December 1952 resulted in 10,000 excess deaths Coal combustion Temperature Altitude inversion sulfate organic carbon black carbon particles < 1km

LOS ANGELES SMOG Respiratory problems, vegetation damage due to high surface ozone troposphere stratosphere 8-18 km temperature inversion ozone altitude Nitrogen oxides (NO x ≡ NO + NO 2 ) Volatile organic compounds (VOCs) UV radiation Ozone (O 3 ) vehicles, industry, vegetation produced by photolysis of oxygen (O 2 )

AIR POLLUTION IN THE US TODAY: Ozone and fine particulate matter (PM 2.5 ) are the two main pollutants 75 ppb (8-h average) 15  g m -3 (1-y av.) Ozone PM 2.5

2008 REVISION TO OZONE STANDARD FROM 84 to 75 PPB CAUSED MORE U.S. AREAS TO BE OUT OF COMPLIANCE

ppb Europe AQS (seasonal) U.S. AQS (8-h avg.) U.S. AQS (1-h avg.) Preindustrial ozone background Present-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 BACKGROUND ?

Currently proposed ppb standard would have extensive non-compliance

OZONE CONCENTRATIONS vs. NO x AND VOC EMISSIONS Air pollution model calculation for a typical urban airshed NO x - saturated NO x -limitedRidge

LARGE SUPPLY OF BIOGENIC VOCs – unrecognized until the 1990s Isoprene (biogenic VOC)Anthropogenic VOCs Jacob et al., J. Geophys. Res. [1993] Switches polluted areas in U.S. from NO x -saturated to NO x -limited regime! recognized in Revised Clean Air Act of 1999

MAPPING OF VOC EMISSIONS FROM SPACE using satellite measurements of formaldehyde confirms dominance of biogenic over anthropogenic VOCs Millet et al. [2008]

TREND OF U.S. EMISSIONS Focus until past decade was on VOC emission controls

DECREASE OF POWER PLANT NO x EMISSIONS OVER THE PAST DECADE Decreasing US NO x emissions from power plants

Growth Trends Population Vehicle Miles Traveled Gross State Product

Emission Trends CO 2 NO x SO x ROG CO

PM10 Trends South Coast State Standard State Standard San Joaquin Valley

Ozone Trends

Historical Ozone Levels Number of Days Stage II > 350 ppb O 3 Stage I > 200 ppb O 3

SOUTH COAST O 3 HISTORY

Improvement Over Past 20 Years -80% -60% -40% -20% 0% Nitrogen Dioxide Sulfur Dioxide Carbon Monoxide OzonePM10Air Toxics (Cancer Risk) Percent Change Approaching Standards Attained Standards

T H A N K S ! !