Next Two Weeks—Stratospheric Chemistry Mid-latitude Ozone Chemistry (and depletion) Polar Ozone Destruction (the Ozone Hole) READING: Chapter 10 of text

Final Projects See web site for more details To allow you to explore in greater detail a topic area that we covered (or did not cover) in class. The topic must deal with atmospheric chemistry or composition. A manuscript which is between double spaced pages This length does not include references or figures. The general idea Typical Formats  Critical Literature Review  NSF-like Proposal  Short Research Project

Ozone and Pop-Culture Day of the Animals (1977) A hole in the ozone layer drives forest animals berserk in Northern California, and a group of vacationing hikers is caught in the fray… See clips on You Tube The Naked Gun II "A love affair is like the ozone layer," says Lieut. Frank Drebin. "You only miss it when it's gone." The “Ozone Awards”, Ozone “air purifiers” Ozone Spa Therapy …

Stratospheric O 3 : Overview Most O 3 (90%) in stratosphere. Remaining 10% in troposphere. “Ozone layer” (15-30 km) is 3000 – 5000 ppb of O 3. Surface O 3 ~ 40 – 100 ppb.

Stratospheric Ozone: Overview sustains terrestrial life by absorbing UV radiation impacts the atmospheric vertical temperature profile important role in Earth’s overall radiative balance ~3,000 ppb <100 ppb

1.What are the natural production and loss mechanisms for stratospheric O 3 ? 2.What is the effect of anthropogenic emissions on stratospheric ozone, both past and future? 3.What are the mechanisms responsible for the drastic ozone decreases in the Antarctic? 4.What role, if any, does the stratospheric aerosol layer play in the gas-phase chemistry of the stratosphere? Stratospheric Ozone: Motivating Questions

1930: Chapman proposed that O 3 continually produced in a cycle initiated by O 2 photolysis. 1840’s Ozone first discovered and measured in air by Schonbein who named it based on its smell ’s: Hartley postulates the existence of a layer above the troposphere, where ozone is responsible for the absorption of solar UV between 200 and 300 nm. 1913: Fabry and Buisson used UV measurements to estimate that if brought down to the surface at STP, O 3 would form a layer ~ 3 mm thick : Dobson first shows that T(z) in stratosphere not constant but increases with z and implicates O 3 absorption. Makes first extensive set of O 3 column measurements with his spectrophotometer Stratospheric Ozone—a brief history

O2O2 O3O3 O hv O2O2 Fast slow small Net O 3 formation Net O 3 loss Odd Oxygen Chemical Familly O x = O 3 + O Mass balance for [O]: Chapman Mechanism

[O x ] = [O] + [O 3 ]  [O 3 ]  [O 3 ] controlled by slow net production and loss via O 2 + hv (R1) and O + O 3 (R4) NOT by fast production and loss of O 3 from O + O 2 (R2) and O 3 + hv (R3) Mass balance for [O x ]: Odd Oxygen Family: O x

Questions 1. The original Chapman mechanism included a fifth reaction: O + O + M  O 2 + M What would be the effect of this reaction on ozone? Is it more important in the lower or in the upper stratosphere? 2. [O]~10 9 molec/cm 3 in the upper stratosphere and ~ 10 6 in the lower stratosphere, if the lifetime of O 3 w.r.t. transport away from equator is ~ month, is it fair to assume O 3 is in a chemical steady state throughout the stratosphere? k O+O3 (US) ~ 4x cm 3 /molec/s; k O+O3 (LS) ~ 3x cm 3 /molec/

Wennberg, et al Science 1994 Stratospheric OH Profile

Hydrogen oxide (HO x ) radical family  Initiation: H 2 O + O(1D)  2OH  Propagation through cycling of HO x radical family (example): OH + O 3  HO 2 + O 2 HO 2 + O 3  OH + 2O 2 Net: 2O 3  3O 2  Termination (example): OH + HO 2  H 2 O + O 2 From troposphere and CH 4 oxidation HO x is a catalyst for O 3 loss but not the only one… HO x = H + OH + HO 2

Nitrogen oxide (NO x ) radical family Initiation N 2 O + O( 1 D)  2NO Propagation NO + O 3  NO 2 + O 2 NO + O 3  NO 2 + O 2 NO 2 + h  NO + O NO 2 + O  NO + O 2 O + O 2 + M  O 3 + M Null cycle Net O 3 + O  2O 2 Termination Recycling NO 2 + OH + M  HNO 3 + M HNO 3 + h  NO 2 + OH NO 3 + NO 2 + M  N 2 O 5 + M N 2 O 5 + h  NO 2 + NO 3 NO x = NO + NO 2

Cycling of NO x and NO y

What have we learned about NO y ?  Production: N 2 O + O( 1 D) - well understood natural source  Loss: via transport from stratosphere to troposphere. Residence time for air in stratosphere is 1-2 years. Loss rate well constrained  Cycling: O 3 loss related to NO x /NO y ratio. Under most conditions s.s. between NO y species is a good approximation NO x catalytic cycle reconciled Chapman theory with observations…1995 Nobel Prize

Stratospheric O 3 Production and Loss Gas-phase chemistry only Constrained from 1980s Space Shuttle Observations NO x is most important O x sink throughout middle stratosphere Fully accounts for missing O 3 sink in Chapman Mechanism

Year IPCC SYR Figure 2-1 Human Influence on Stratospheric NO x

Questions 1. Of the ozone loss mechanisms we have examined so far, can any operate at night? 2. There are several variations possible for NO x and HO x catalyzed O 3 destruction cycles. Given that NO 3 photolysis can proceed via one of two channels a) NO 3 + hv  NO 2 + O b) NO 3 + hv  NO + O 2 Come up with at least one additional catalytic cycle for NO x. 3. A minor oxidation pathway for NO is HO 2 + NO -> OH + NO 2 What is the net effect of this reaction on ozone?

Lovelock’s First Measurements of CFCs CCl 3 F (pptv) Latitude

Roland and Molina What is the fate of this, non-toxic CFC?

Stratospheric Distribution of CFC-12 CF 2 Cl 2

Chlorine radical family (ClO x =Cl + ClO) Initiation: CF 2 Cl 2 + hv  CF 2 Cl + Cl Propagation: Cl + O 3  ClO + O 2 ClO + O  Cl + O 2 Net: O 3 + O  2O 2 Termination:Recycling: Cl + CH 4  HCl + CH 3 HCl + OH  Cl + H 2 O ClO + NO 2 + M  ClNO 3 + MClNO 3 + hv  ClO + NO 2 O 3 loss rate: Cl not the only halogen…Br chemistry is important too

Cycling of ClO x and Cl y

Questions 1. Can any of the catalytic cycles we’ve studied explain why severe ozone depletion occurs over the South Pole, just after the sun rises there?

Stratospheric O 3 Production and Loss Gas-phase chemistry only Constrained from 1980s Space Shuttle Observations NO x is most important O x sink throughout middle stratosphere Fully accounts for missing O 3 sink in Chapman Mechanism

Early Warning Signs

Atmospheric Trend of CFC-11 Montreal protocol CFC production Is banned

Ozone Column Trend, 60 o S-60 o N Chlorine Or dynamics (transport)? Pinatubo Stabilization of chlorine? [WMO, 1998]

Recent Rise in HCFC’s (CFC Replacements)

What is the effect of increasing stratospheric NO x on the rate of ClO x -catalyzed ozone loss? Give an example of how HO x and NO x are coupled. How might an increase in OH affect ClO x cycles? Coupling Between HO x, NO x, and ClO x Cycles

SPADE Mission

Wennberg, et al Science 1994 Couplings lead to non-linear responses to increases in NO x Observations of HO x, NO x, and ClO x