History of Ozone Discovered in 1840 by Christian Friedrich Schönbein. In the 1920’s Gordon Dobson made the first O 3 column measurements. 1 DU = 1x10 -3 m thick layer of pure ozone at standard temperature and pressure. In 1930 Sydney Chapman proposed a mechanism for ozone in the stratosphere.

The Stratosphere from:

Ozone in the Stratosphere Shields the surface of the Earth from harmful UV radiation. From:

Ozone in the Stratosphere From: Jacob

The Chapman Mechanism

From: Jacob

The Chapman Mechanism O2OO3O2OO3 (1) slow (4) slow (2) fast (3)

The O x Family Reaction 1: Production of O x Reaction 4: Loss of O x

The Chapman Mechanism Assuming steady-state for either O or O 3 :

The Chapman Mechanism Everywhere in the stratosphere

O 3 in the Stratosphere Derive an expression for the steady-state of O 3 in the stratosphere: Find steady-state for O

O 3 in the Stratosphere

From: Jacob

Catalytic Ozone Destruction X is a catalyst and may be: X = H, OH, NO, Cl, or Br

The HO x Family in the Stratosphere Source: Water vapor Source: methane, H 2 z T cold trap, T≈190K P H2O ≈2.5x10 -4 torr at 75mbar (3.5ppmv) tropopause stratosphere troposphere

The HO x Family in the Stratosphere

Catalytic Ozone Destruction (HO x )

Cycle I Cycle II Cycle I: important at higher z and only during the day Cycle II: important at lower z and also at night

Catalytic Ozone Destruction (HO x ) Termination occurs when: H 2 O and HNO 3 may mix back to the troposphere and be removed. However:

The NO x Family in the Stratosphere Source: N 2 O Source: Direct emissions of NO to the stratosphere only about 5%

Catalytic Ozone Destruction (NO x ) Active in the daytime -reaction scheme should look familiar?! Active in the day or night -a little different than cycle II To complete the catalytic cycle in the daytime

Null Cycles OxOx OxOx ‘Null cycles’ do not lead to the formation or destruction of O 3. -ties up reactive species (in this case, NO x ) reducing their ability to destroy ozone.

Catalytic Ozone Destruction Cycles may be ‘short-circuited’ through competing reactions: Leads to O 3 destruction Leads to null cycle

Catalytic Ozone Destruction (NO x ) Termination of catalytic cycles occur when: However:

Reservoirs For NO x : –HNO 3, and N 2 O 5 are reservoirs of NO x –NO y = NO x + reservoirs –NO x /NO y gives us an idea of how effective NO x is at destroying ozone. For HO x : –H 2 O and HNO 3 are reservoirs of HO x –HO y = HO x + reservoirs –HO x /HO y gives us an idea of how effective HO x is at destroying ozone.

Mixed Cycles The HO x and NO x cycles are coupled: An increase of stratospheric HO x causes a decrease in the effectiveness of NO x towards destroying O 3.

Transport in the Stratosphere From:

Global Ozone Distribution From:

Global Ozone Distribution From:

Tropospheric NO x and HO x versus Stratospheric NO x and HO x In the troposphere (ozone production): In the stratosphere (ozone destruction): HO x NO x HO x NO x The troposphere has extremely low atomic oxygen concentrations. The stratosphere has high ozone and atomic oxygen concentrations relative to the troposphere.