QUESTIONS 1. How does the thinning of the stratospheric ozone layer affect the source of OH in the troposphere? 2. Chemical production of ozone in the troposphere is much faster in summer than winter. Explain. 3. Why don’t reactions of hydrocarbons deplete (or titrate!) all the OH in the troposphere?

CHAIN MECHANISM FOR O3 PRODUCTION: CH4 OXIDATION Initiation: source of HOx (OH production) Propogation: CH4 + OH  CH3 + H2O CH3 + O2 + M  CH3O2 + M CH3O2 + HO2  CH3OOH + O2 CH3O2 + NO  CH3O + NO2 CH3OOH + OH  CH2O + OH + H2O CH3OOH + OH  CH3O2 + H2O CH3OOH + hv  CH3O + OH CH3O + O2  CH2O + HO2 CH2O + OH  CHO + H2O CH2O + hv + O2  CHO + HO2 CH2O + hv  CO + H2 CHO + O2  CO + HO2 (…then CO oxidation…) * * Oxidation from C(-IV) in CH4 through to C(+IV) in CO2 * * * O2 hn O3 hn NO2 NO O3 hn, H2O OH HO2 H2O2 Deposition CO, CH4

METHANE OXIDATION SCHEME NO2 HO2 HO2 HO2 CH4 CH3 CH3O2 CH3OOH CH3O CH2O CHO CO OH O2, M NO HO2 O2 hv hv, O2 In clean troposphere, ~70% of OH reacts with CO, 30% with CH4

NOx EMISSIONS (Tg N yr-1) TO TROPOSPHERE Zeldovich Mechanism: combustion and lightning At high T (~2000K) oxygen thermolyzes: O2  O + O O + N2  NO + N N + O2  NO + O 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

USING SATELLITE OBSERVATIONS OF NO2 TO MONITOR NOx EMISSIONS SCIAMACHY data. May-Oct 2004 (R.V. Martin, Dalhousie U.) detection limit

PAN NO3 N2O5 NO2 NO HNO3 NOX CYCLING hn Combustion lightning ~ 1 day Example of PAN formation from acetaldehyde: CH3CHO + OH  CH3CO + H2O CH3CO + O2 + M  CH3C(O)OO + M CH3C(O)OO+NO2 + M  CH3C(O)OONO2 + M PAN carbonyl oxidation T NO3 O3 O3 M O2 hn N2O5 NO2 NO Combustion lightning H2O OH, M HO2 HNO3 O3 ~ 1 day

PEROXYACETYLNITRATE (PAN) AS RESERVOIR FOR LONG-RANGE TRANSPORT OF NOx

DECOMPOSITION OF PAN LEADING TO OZONE PRODUCTION Chemistry: Changing NOy speciation Origin: Warm Conveyor Belt over Asia ECMWF Evolution: Split by blocking high pressure O3 H CO [Heald et al., 2004]

GLOBAL BUDGET OF TROPOSPHERIC OZONE Chem prod in troposphere, Tg y-1 4300 1600 Chem loss in troposphere, 4000 Transport from stratosphere, 400 Deposition, 700 Burden, Tg 360 230 Lifetime, days 28 42 Present-day Preindustrial O2 hn O3 STRATOSPHERE 8-18 km TROPOSPHERE hn NO2 NO O3 hn, H2O OH HO2 H2O2 Deposition CO, VOC NO+peroxy radicals is rate-limiting, so:

GLOBAL DISTRIBUTION OF TROPOSPHERIC OZONE Climatology of observed ozone at 400 hPa in July from ozonesondes and MOZAIC aircraft (circles) and corresponding GEOS-Chem model results for 1997 (contours). GEOS-Chem tropospheric ozone columns for July 1997. [Li et al., 2001]

1996-2005 NOx EMISSION TREND SEEN FROM SPACE [Van der A et al., 2008]

POWER PLANT EMISSION REDUCTIONS IN THE EASTERN US Effects of NOx controls on large point sources in the Eastern US beginning in the late 1990s Acid Rain Program, NOx SIP Call, NOx Budget Trading Program Focus on coal-burning power plants Improved burner technology, post-burner ammonia scrubbers Northeast Urban Corridor E(NOx) < 20% power plant Ohio River Valley 1997 E(NOx) ~ 50% power plant Ohio River Valley 2005 E(NOx) ~ 20% power plant Courtesy: Greg Frost (NOAA) [Kim et al., 2006]

POWER PLANT POINT SOURCES IN WESTERN US SEEN FROM SPACE North Valmy Intermountain Hunter / Huntington Mohave Navajo Four Corners/ San Juan Cholla/Coronado/ Springerville Bonanza Craig/Hayden Jim Bridger/ Naughton Dave Johnston/ Laramie River Colstrip Reid Gardener Courtesy: Greg Frost (NOAA) [Kim et al., 2009]

Is the third anthropogenic greenhouse gas Tropospheric ozone Is the third anthropogenic greenhouse gas IPCC [2007]

IPCC RADIATIVE FORCING ESTIMATE FOR TROPOSPHERIC OZONE (0 IPCC RADIATIVE FORCING ESTIMATE FOR TROPOSPHERIC OZONE (0.35 W m-2) RELIES ON GLOBAL MODELS …but these underestimate the observed rise in ozone over the 20th century Fitting to observations would imply a radiative forcing of 0.8 W m-2 Preindustrial ozone models } Observations at mountain sites in Europe [Marenco et al., 1994]

RECENT TRENDS IN TROPOSPHERIC OH inferred from methylchloroform observations