This Week—Polar Stratospheric Chemistry READING: Chapter 10 of text SPADE Data Analysis Pt 1: Due Tuesday Nov 13

Today—Ozone Hole Part 1 Characterization of Ozone Hole Finding the “Smoking Gun” With Human Fingerprints

Polar Ozone Loss Severe depletion of stratospheric O 3 occurs every spring (since 1980’s) over the poles (especially South Pole). Example of: far reach of human activities environmental catastrophe and political blame game scientific process and ability of humans to correct

Discovery of Antarctic Ozone Hole

Ozone Watch Web Page Antarctic O 3 Hole A Springtime Event 2006 Ozone Hole Movie From Satellite Observations

Chlorine from CFCs predicted to be most effective ~ 40 km. So why is depletion at 15 – 20 km? Vertical Extent of Ozone Hole

Discovery of Antarctic Ozone Hole The Conundrum: Known catalytic reactions with chlorine not fast enough to explain near complete depletion in couple months. Why only in spring? Why only between 15 – 25 km? Why only in polar region?

Debate Over Causes of Ozone Hole There was also a real scientific debate over the relative roles of chemistry and meteorology. Turns out to be both (of course!)

ClO and O 3 certainly anti-correlated. But how is there so much ClO? What is the mechanism for ClO to destroy ozone so fast? Can’t be ClO + O… Chlorine: The Smoking Gun?

QUESTIONS 1. Where in the stratosphere (altitude) would the ClO dimer mechanism be most important? 2. If CFC concentrations were to double, how much faster would ozone loss by the ClO dimer mechanism occur?

Polar Stratospheric Clouds (PSC’s)

PSC Formation Starts on S.A.L SO 2 H 2 SO 4 OCS oceans volcanoes Troposphere Stratosphere km Extends vertically from 15 to 35 km Aerosol composition primarily H 2 SO 4 -H 2 O Typical particle radius ~ 700 nm Typical particle number density ~ 10 cm -3 Stratospheric Aerosol Layer

Sunrise over Texas From Space Shuttle Tropospheric Clouds Stratospheric Aerosol Layer

Supercooled Ternary Solutions (STS) H 2 SO 4 /HNO 3 /H 2 O Type Ib PSC Crystalline HNO 3 -3H 2 O (NAT) containing particles HNO 3 /H 2 O/ H 2 SO 4 Type Ia PSC Mainly H 2 O- ice particles Type II PSC T 197 K 185 K H 2 SO 4 -H 2 O From stratospheric aerosol layer HNO 3 H2OH2O PSC Formation and Types

PSC Formation Requires Very Low T Arctic T Antarctic T Polar Vortex cuts off warm, ozone/NO x rich mid-latitude air Color shows westerly wind speed

Heterogeneous Chemistry on PSCs Turco, 1987 PSCs convert inactive Cl y to active ClO x

Chlorine Activation in Polar Vortex

Ozone Watch Web Page Antarctic O 3 Hole A Springtime Event 2006 Ozone Hole Movie From Satellite Observations

Antarctic Ozone Hole: Key Points Why only in spring? Why only between 15 – 25 km? Why only in (south) polar region? Wintertime processing on PSCs needed for active chlorine production and denitrification. Sunlight required to generate Cl atoms from Cl 2 and ClOOCl. Where PSCs form (on aerosol layer), and where the ClOOCl mechanism is fastest. PSCs are required and only form under cold conditions achieved during polar winter (and mainly only Antarctic winter).

The Solution: Montreal Protocol The persistent observations of the ozone hole from 1984 – 1987 led to legally binding international agreements. The Montreal Protocol and its amendments eventually called for a near complete ban on the production and use of CFCs. A suitable and easy replacement for CFCs, known as HCFCs, made these acts easier to swallow.

Ozone Recovery Computer model predictions: Antarctic ozone will recover to pre-1980 values by ~2040. Extra-polar ozone should recover by Predictions assume strict adherence to Montreal Protocol.

Questions 1.Large volcanic eruptions can inject sulfur and particles into the stratosphere. How would that affect stratospheric ozone at present? 2.How might eruptions in the distant past affect stratospheric ozone? 3. Increasing greenhouse gases in the troposphere lead to a cooling of the stratosphere. How might global warming affect stratospheric ozone levels?

Effect of N 2 O 5 + Aerosols  2HNO 3 Only gas-phase chemistryIncluding Heterogeneous Chemistry HO x becomes most important catalyst in aerosol layer!