QUESTIONS Based on the major source of OH described last class where do you expect OH formation to be high? 2. The Chapman mechanism includes a fifth.

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Presentation transcript:

QUESTIONS Based on the major source of OH described last class where do you expect OH formation to be high? 2. The Chapman mechanism includes a fifth reaction: O + O + M  O2 + M What is the effect of this reaction on ozone? Would it be more important in the lower or in the upper stratosphere? 3. Of all the ozone loss mechanisms we examined, can any operate at night?

SOURCE GAS CONTRIBUTIONS TO STRATOSPHERIC CHLORINE (2004)

CHLORINE PARTITIONING IN STRATOSPHERE

TRENDS IN ATMOSPHERIC CFCs AND HFCs (CFC REPLACEMENTS) Montreal Protocol CFC production is banned [IPCC, 2007]

OZONE TREND AT HALLEY BAY, ANTARCTICA (OCTOBER) Stabilization of chlorine? Farman et al. paper published in Nature 1 Dobson Unit (DU) = 0.01 mm O3 STP = 2.69x1016 molecules cm-2

THE ANTARCTIC OZONE HOLE SEEN FROM SPACE Southern hemisphere ozone column seen from TOMS

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

2006 ANTARCTIC OZONE HOLE: MOST SEVERE OBSERVED Aura OMI – 8 Oct 2006 Ozone Hole Minimum Minimum of 85 DU on 8 Oct 2006 The ozone hole is an austral spring phenomenon – it is not there year-round!

OZONE HOLE IS A SPRING PHENOMENON

VERTICAL STRUCTURE OF THE OZONE HOLE: near-total depletion in lower stratosphere

ASSOCIATION OF ANTARCTIC OZONE HOLE WITH HIGH LEVELS OF ClO Sept. 1987 ER-2 aircraft measurements at 20 km altitude S of Punta Arenas O3 ClO O3 Sep. 16 Edge of Polar vortex Sep. 2, 1987 ClO 20 km altitude Measurements by Jim Anderson’s group (Harvard)

SATELLITE OBSERVATIONS OF ClO IN THE SOUTHERN HEMISPHERE STRATOSPHERE

O3 DEPLETION MECHANISM: ClO SELF-REACTION ClO + ClO + M  ClOOCl + M ClOOCl + hv  ClOO + Cl ClOO + M  Cl + O2 2 x [Cl + O3  ClO + O2] NET: 2O3  3O2  70% of total ozone loss in Antarctic spring An additional catalytic cycle in Antarctic spring involves Br radicals (BrO + ClO)  30% of total ozone loss in Antarctic ozone hole But why are [ClO] so high over Antarctica? Polar stratospheric clouds: conversion of ClOx reservoirs to Cl2 ClNO3 + HCl –PSCCl2 + HNO3 Cl2 + hv  2Cl

WHY THE HIGH ClO IN ANTARCTIC VORTEX 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 Frost point of water

REMINDER: PHASE DIAGRAM FOR WATER gas-liquid metastable equilibrium

HOW DO PSCs START FORMING AT 195K? HNO3-H2O PHASE DIAGRAM Antarctic vortex conditions PSCs are not water but nitric acid trihydrate (NAT) clouds

CHRONOLOGY OF ANTARCTIC OZONE HOLE

HYDROLYSIS OF N2O5 IN AEROSOLS INCREASES THE SENSITIVITY OF OZONE TO CHLORINE N2O5 + H2O –aerosol 2HNO3 N2O5 hydrolysis increases HOx-catalyzed ozone loss. Why?

OBSERVED TREND IN OCTOBER OZONE IN ANTARACTICA recovery? http://ozonewatch.gsfc.nasa.gov/facts/history.html

LONG-TERM COOLING OF THE STRATOSPHERE Sep 21-30, 25 km, 65-75˚S Increasing CO2 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

(lowest Arctic O3 column) 2011 (lowest Arctic O3 column) 2010 (“regular” year)

SKIN CANCER EPIDEMIOLOGY PREDICTIONS "perhaps the single most successful international agreement to date has been the Montreal Protocol” ~ Kofi Annan (UN Secretary General 1997-2006)