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? 4. Methylperoxy radicals produced from methane oxidation can self-react to form methanol: CH 3 O 2 + CH 3 O 2  CH 3 OH + CH 2 O + O 2 What is the effect of this reaction on OH levels?

CHAIN MECHANISM FOR O 3 PRODUCTION: CH 4 OXIDATION Initiation: source of HOx (OH production) Propogation: CH 4 + OH  CH 3 + H 2 O CH 3 + O 2 + M  CH 3 O 2 + M CH 3 O 2 + HO 2  CH 3 OOH + O 2 CH 3 O 2 + NO  CH 3 O + NO 2 CH 3 OOH + OH  CH 2 O + OH + H 2 O CH 3 OOH + OH  CH 3 O 2 + H 2 O CH 3 OOH + hv  CH 3 O + OH CH 3 O + O 2  CH 2 O + HO 2 CH 2 O + OH  CHO + H 2 O CH 2 O + hv + O 2  CHO + HO 2 + O 2 CH 2 O + hv  CO + H 2 CHO + O 2  CO + HO 2 (…then CO oxidation…) Oxidation from C(-IV) in CH 4 through to C(+IV) in CO 2 * * * * * O3O3 O2O2 h O3O3 OHHO 2 h, H 2 O Deposition NO H2O2H2O2 CO, CH 4 NO 2 h

METHANE OXIDATION SCHEME CH 4 CH 3 CH 3 O 2 CH 3 OOH CH 3 O CH 2 OCHO CO OHO 2, M NO HO 2 OH O2O2 hv hv, O 2 OH O2O2 hv HO 2 NO 2 In clean troposphere, ~70% of OH reacts with CO, 30% with CH 4

NO x EMISSIONS (Tg N yr -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 Zeldovich Mechanism: combustion and lightning At high T (~2000K) oxygen thermolyzes: O 2  O + O O + N 2  NO + N N + O 2  NO + O

LIGHTNING FLASHES SEEN FROM SPACE (2000) DJF JJA

USING SATELLITE OBSERVATIONS OF NO 2 TO MONITOR NO x EMISSIONS SCIAMACHY data. May-Oct 2004 (R.V. Martin, Dalhousie U.) detection limit

NITROGEN DIOXIDE FROM THE OMI SATELLITE (MARCH 2006) March 2006

NO X CYCLING HO 2 NO NO 2 h O3O3 O3O3 O2O2 Combustion lightning HNO 3 OH, M O3O3 NO 3 N2O5N2O5 M H2OH2O PAN carbonyl oxidation T  ~ 1 day Example of PAN formation from acetaldehyde: CH 3 CHO + OH  CH 3 CO + H 2 O CH 3 CO + O 2 + M  CH 3 C(O)OO + M CH 3 C(O)OO+NO 2 + M  CH 3 C(O)OONO 2 + M

PEROXYACETYLNITRATE (PAN) AS RESERVOIR FOR LONG-RANGE TRANSPORT OF NO x

DECOMPOSITION OF PAN LEADING TO OZONE PRODUCTION H Origin: Warm Conveyor Belt over Asia Evolution: Split by blocking high pressure ECMWF Chemistry: Changing NO y speciation CO O3O3 Heald et al., 2004

NO x EMISSION TREND SEEN FROM SPACE Van der A et al., 2008

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 GLOBAL BUDGET OF TROPOSPHERIC OZONE NO+peroxy radicals is rate-limiting, so:

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 GLOBAL DISTRIBUTION OF TROPOSPHERIC OZONE Li et al. [2001]

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

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

RECENT TRENDS IN TROPOSPHERIC OH inferred from methylchloroform observations