Stratospheric Chemistry EPS March – 04 April 2011 Polar Stratospheric Clouds

ATMOSPHERIC ATTENUATION OF SOLAR RADIATION Solar UV radiation reaching the top of the atmosphere is absorbed by ozone

THE NATURAL OZONE LAYER Based on ozonesonde observations in the 1970s

1 Dobson Unit (DU) is defined to be 0.01 mm thickness at stp; the ozone layer over Labrador is ~300 DU. Mean ratio, column O 3 : air = 5 x 10 -7

Ozone mixing ratio in parts per million

SOLAR SPECTRUM AND ABSORPTION X-SECTIONS O 2 +hv O 3 +hv

CHAPMAN MECHANISM FOR STRATOSPHERIC OZONE (1930) O O 3 O2O2 slow fast Odd oxygen family [O x ] = [O 3 ] + [O] R2 R3 R4 R1

STEADY-STATE ANALYSIS OF CHAPMAN MECHANISM Lifetime of O atoms: …is sufficiently short to assume steady state for O: …so the budget of O 3 is controlled by the budget of O x. Lifetime of O x : Steady state for O x : τ Ox

PHOTOLYSIS RATE CONSTANTS: VERTICAL DEPENDENCE quantum yield absorption X-section photon flux

CHAPMAN MECHANISM vs. OBSERVATION -3 shape determined by k 1 n O2 Chapman mechanism reproduces shape, but is too high by factor 2-3  missing sink!

RADICAL REACTION CHAINS IN THE ATMOSPHERE non-radical radical + radical Initiation: photolysis thermolysis oxidation by O( 1 D) radical + non-radicalnon-radical + radicalPropagation: bimolecular redox reactions non-radical + non-radical Termination: radical redox reaction radical + radical non-radical + M radical + radical + M 3-body recombination

WATER VAPOR IN STRATOSPHERE Source: transport from troposphere, oxidation of methane (CH 4 ) H 2 O mixing ratio

Initiation: Propagation: Termination: OH HO 2 H2OH2O slow fast HO x radical family Ozone loss catalyzed by hydrogen oxide (HO x ≡ H + OH + HO 2 ) radicals

Rate limiting step: Example OH + O 3 -> HO 2+ + O 2 k1 HO 2 + O 3 -> OH + O 2 k2 HO 2 + NO ->->-> OH + NO + O 3 k3 { + O 2 + h  … } d[OH] / dt = -d[HO 2 ] / dt = - k1[OH][O 3 ] + k2[O 3 ][HO 2 ] + k3 * [NO][HO 2 ] ≈ 0 A d[O 3 ] / dt = -k1[OH][O 3 ] – k2[HO 2 ][O 3 ] + k3*[NO][HO 2 ] B To B, add (-1)xA ≈ 0 d[O 3 ] / dt = - 2 k2 [HO 2 ][O 3 ] Rate limiting step for removal of ozone by Reactions 1, 2, 3 OH + O 3 HO 2 + O 3 HO 2 + NO

STRATOSPHERIC OZONE BUDGET FOR MIDLATITUDES CONSTRAINED FROM 1980s SPACE SHUTTLE OBSERVATIONS

NITROUS OXIDE IN THE STRATOSPHERE H 2 O mixing ratio

ATMOSPHERIC CYCLING OF NO x AND NO y

Rate limiting step, NOx: Example NO + O 3 -> NO 2+ + O 2 k1 NO 2 + hν -> NO + O-> O 3 k2 NO 2 + O -> NO + O 2 k3 d[NO] / dt = -d[NO 2 ] / dt = - k1[NO][O 3 ] + k2[NO 2 ] + k3[NO 2 ][O] ≈ 0 A d[O 3 ] / dt = -k1[NO][O 3 ] + k2[NO 2 ] - k3[NO 2 ][O] B To B, add (-1)xA ≈ 0 d[O 3 ] / dt = - 2 k3 [NO 2 ][O] Rate limiting step for removal of ozone by Reactions 1, 2, 3 NO + O 3 NO 2 + O NO 2 + hv

STRATOSPHERIC DISTRIBUTION OF CF 2 Cl 2 (CFC-12)

ATMOSPHERIC CYCLING OF ClO x AND Cl y

SOURCE GAS CONTRIBUTIONS TO STRATOSPHERIC CHLORINE (2004)

CHLORINE PARTITIONING IN STRATOSPHERE

WHAT IS A RATE-LIMITING STEP? From IUPAC: “A rate-controlling (rate-determining or rate-limiting) step in a reaction occurring by a composite reaction sequence is an elementary reaction the rate constant for which exerts a strong effect — stronger than that of any other rate constant — on the overall rate.”

Latitude altitude ftp://hanna.ccmc.gsfc.nasa.gov/pub/modelweb/atmospheric/msis/msise90/

Latitude altitude

Prof. James R. Holton Stratospheric Circulation

OZONE TREND AT HALLEY BAY, ANTARCTICA (OCTOBER) Farman et al. paper published in Nature 1 Dobson Unit (DU) = 0.01 mm O 3 STP = 2.69x10 16 molecules cm -2

SPATIAL EXTENT OF THE OZONE HOLE Isolated concentric region around Antarctic continent is called the polar vortex. Strong westerly winds, little meridional transport Mean October data

THE POLAR VORTEX (Sep-Oct 2006)

THE OZONE HOLE IS A SPRINGTIME PHENOMENON

VERTICAL STRUCTURE OF THE OZONE HOLE: near-total depletion in lower stratosphere Argentine Antarctic station southern tip of S. America

Sep. 2, 1987 Sep km altitude ASSOCIATION OF ANTARCTIC OZONE HOLE WITH HIGH LEVELS OF CLO Sept ER-2 aircraft measurements at 20 km altitude south of Punta Arenas ClO O3O3 O3O3 Edge of Polar vortex Measurements by Jim Anderson’s group (Harvard)

SATELLITE OBSERVATIONS OF ClO IN THE SOUTHERN HEMISPHERE STRATOSPHERE

WHY THE HIGH ClO IN ANTARCTIC VORTEX? Release of chlorine radicals from reactions of reservoir species in polar stratospheric clouds (PSCs)

PSC FORMATION AT COLD TEMPERATURES PSC formation Frost point of water

HOW DO PSCs START FORMING AT 195K? HNO 3 -H 2 O PHASE DIAGRAM Antarctic vortex conditions PSCs are not water but nitric acid trihydrate (NAT) clouds

DENITRIFICATION IN THE POLAR VORTEX: SEDIMENTATION OF PSCs

CHRONOLOGY OF ANTARCTIC OZONE HOLE

TRENDS IN GLOBAL OZONE Mt. Pinatubo

LONG-TERM COOLING OF THE STRATOSPHERE Sep 21-30, 25 km, 65-75˚S Increasing CO 2 is expected to cool the stratosphere

TRENDS IN POLAR OZONE Could greenhouse-induced cooling of stratosphere produce an Arctic ozone hole over the next decade? Race between chlorine decrease and climate change

SKIN CANCER EPIDEMIOLOGY PREDICTIONS