ATS 621 Fall 2012 Lecture 10

CHAPMAN MECHANISM FOR STRATOSPHERIC OZONE (1930) Odd oxygen family [Ox] = [O3] + [O] slow R2 R1 O2 O O3 fast R3 R4 slow

Apply steady state approximation to Chapman mechanism (daytime): Ozone should decrease with altitude Opposite for [O] See Jacob text for lifetime of total odd oxygen

Ozone lifetimes and transport The chemical lifetime of O3 in the Troposphere ~ 22 days Upper troposphere ~1 mo Lower stratosphere ~several years, sufficiently long to allow transport of O3 (including into the troposphere) Upper stratosphere ~days or LESS (transport not as important in controlling concentrations) We expect a springtime MAXIMUM [Jacob text] Figure 10-1 The natural ozone layer: vertical and latitudinal distribution of the ozone number density (1012 molecules cm-3) at the equinox, based on measurements taken in the 1960s. From Wayne, R.P., Chemistry of Atmospheres, Oxford, 1991.

Brewer-Dobson Circulation (http://www. ccpo. odu In winter, stratospheric winds typically blow from west to east. A jet stream sets up along the zone of greatest temperature change. In the stratosphere, this occurs in winter along the polar night terminator, the line that divides sunlight from the long polar night. (This occurs north of the Arctic Circle and south of the Antarctic Circle.) The region poleward of the northern polar night jet is known as the Arctic polar vortex, while the region poleward of the southern polar night jet is known as the Antarctic polar vortex, which is a region of air isolated from the rest of the stratosphere where the long polar night allows extremely cold temperatures to develop. The degree of isolation, however, is quite different between the Arctic and Antarctic westerly The tropics is a region of the stratosphere that stretches from about 20°N to 20°S. It is here that ozone has its photochemical source region, since it is here that there is enough of the necessary highly energetic ultraviolet radiation from the Sun to create ozone. Ozone is transported out of this region and poleward by a broad circulation pattern. easterly The middle latitudes of the stratosphere is known as the "surf zone.” Because of the equator-to-pole circulation pattern, tropical air contains less ozone than polar air. As a result of weather systems in the middle latitudes, tropical (low ozone) and polar (high ozone) air are mixed together. This gives the surf zone its turbulently mixed appearance.

Brewer-Dobson Circulation (http://www. ccpo. odu “ozone layer” ~27 km “Wave activity" in the extratropical middle and upper stratosphere causes the air to move poleward in the stratosphere, which causes the rising in the tropics, and the sinking in the polar region. The Quasi-Biennial Oscillation, a periodic shifting of stratospheric winds in the tropics from westerly (warm phase) to easterly (cold phase) that occurs every 22 to 34 months.

Dobson units We expect MAXIMUM concentrations in spring; variations are damped closer to equator

CHAPMAN MECHANISM vs. OBSERVATION shape determined by j1nO2 -3 Chapman mechanism reproduces shape, but is too high by factor 2-3 e missing sink!

O3 + X  XO + O2 O + XO  X + O2 Net: O3 + O  2 O2 X is a catalyst Chapman got it almost right… CATALYTIC CYCLES FOR OZONE LOSS: General Idea O3 + X  XO + O2 O + XO  X + O2 Net: O3 + O  2 O2 X is a catalyst The catalyst is neither created nor destroyed…but the rate for the catalytic cycle [odd-O removal in this case] depends on catalyst concentrations

Chapman HOx NOx ClOx

Catalytic cycles bring closer agreement with altitude of maximum:

WATER VAPOR IN STRATOSPHERE                               H2O mixing ratio Source: transport from troposphere, oxidation of methane (CH4)

HOx-CATALYZED OZONE LOSS HOx  H + OH + HO2 hydrogen oxide radical family Initiation: Propagation: Termination: slow H2O OH HO2 fast HOx radical family slow

NITROUS OXIDE IN THE STRATOSPHERE                               H2O mixing ratio

NOx-CATALYZED OZONE LOSS (NOx  NO + NO2) Also emitted Initiation N2O + O(1D) 2NO Propagation NO + O3  NO2 + O2 NO + O3  NO2 + O2 NO2 + h  NO + O NO2 + O  NO + O2 O + O2 + M  O3 + M Null cycle Net O3 + O  2O2 O3 loss rate: Termination Recycling NO2 + OH + M  HNO3 + M HNO3 + h  NO2 + OH NO2 + O3  NO3 + O2 HNO3 + OH NO3 + H2O NO3 + NO2 + M  N2O5 + M NO3 + h  NO2 + O N2O5 + H2O  2HNO3 N2O5 + h NO2 + NO3 Day Night NOy  NOx + reservoirs (HNO3, N2O5, ..)

ATMOSPHERIC CYCLING OF NOx AND NOy

STRATOSPHERIC OZONE BUDGET FOR MIDLATITUDES CONSTRAINED FROM 1980s SPACE SHUTTLE OBSERVATIONS Approximate closure! Source of Ox Gas-phase chemistry only Paul Crutzen shared 1995 Nobel Prize for his work on the NOx catalyzed destruction of ozone

STRATOSPHERIC DISTRIBUTION OF CFC-12

ClOx-CATALYZED OZONE LOSS (ClOx  Cl + ClO) Initiation: Cl radical generation from non-radical precursors (e.g., CFC-12) CF2Cl2 + hn  CF2Cl + Cl Propagation: Cl + O3  ClO + O2 ClO + O  Cl + O2 Net: O3 + O  2O2 O3 loss rate: Termination: Recycling: Cl + CH4  HCl + CH3 HCl + OH  Cl + H2O ClO + NO2 + M  ClNO3 + M ClNO3 + hv  Cl + NO3 Cly  ClOx + reservoirs (HCl, ClNO3) http://www.atmos.washington.edu/2004Q4/211/09_OzoneDep.swf

ATMOSPHERIC CYCLING OF ClOx AND Cly Molina and Rowland shared 1995 Nobel Prize for their work on the ClOx catalyzed destruction of ozone

Monitoring stratospheric ozone: http://ozonewatch.gsfc.nasa.gov/