METO 621 CHEM

Lesson 3

Heterogeneous Chemistry

• Heterogeneous reactions play an important part in stratospheric chemistry.

• The interest in these reactions was first sparked by the so called ‘ozone hole’ over the Antarctic, where they play a crucial role through the surfaces of ice crystals.

• However, it is also recognized that reactions on sulfate particles ( the product of volcanic eruptions) can be important away from the poles.

• Heterogeneous processing is particular important in relation to the reservoir species.

Heterogeneous Chemistry

• Such reactions includeN2O5 + H2O → 2HNO3

ClONO2 + H2O → HOCL + HNO3

• The following reaction is critical to the ‘ozone hole’ theoryClONO2 + HCl → Cl2 + HNO3

• Other surface reactions includeHOCl + HCl → H2O + Cl2

HOBr + HCl → H2O + BrCl

N2O5 + HCl → ClNO2 + HNO3

Man’s impact on the Stratosphere

• The concern over a loss of stratospheric ozone is that this will lead to an increase in ultraviolet radiation at the ground, especially those wavelengths near 300 nm.

• The next figure shows the ‘ action spectra’ for biological response. The action spectra also follows closely the absorption spectra for DNA, which leads to the dissociation of the DNA molecule. This could lead to cell mutations – skin cancer.

• We identify a spectral interval of interest, UV-B from 280 to 315 nm.

• Although skin cancer gets most of the publicity, UV radiation also impacts the biota.

DNA absorption and the solar flux at the surface

Human death due to skin melanoma

Supersonic Aircraft• First real concern was the role of supersonic transport (SST)

which were to fly in the stratosphere. These aircraft emit water vapor, CO, and NOX directly into the stratosphere, where they could accumulate over two years.

• Initially it was thought that the injected water vapor was the major problem , after the work of Bates and Nicolet. In the US this led to the formation of the CIAP program (Climatic Impact Assessment Program).

• Then Crutzen (theorist) put forward the NOX catalytic cycle and Johnson (laboratory chemist) showed that the reaction

rates would make the catalytic cycle significant.• In order to show that the original SST fleet was economically

viable, a projected fleet of 500 aircraft emitting 1.2.10+9 kg of NO per year was assumed. Models showed that this led to a reduction in the global ozone column of about 12%.

Rockets and the Space Shuttle

• Solid fuel rockets are used as launch vehicles, and to boost the Space Shuttle in its early stages of flight.

• The oxidizer used in these rockets is ammonium perchlorate. HCl is released into the stratosphere, which it can begin the

chlorine catalytic cycle.• Aluminum is added to the rocket fuel to boost thrust, and the

rocket exhaust also contains aluminum oxide particles, which could contribute to heterogeneous chemistry.

• In general the amount of HCl released is small compared to natural sources of chlorine.

Halocarbons• Fluorinated hydrocarbons were developed in 1930 by the

General Motors Research Laboratories as a non-toxic, non-flammable refrigerant to replace sulfur dioxide and ammonia.

• Dichlorodifluoromethane, CF2Cl2 is a typical example. Du Pont called its product Freon 12, They are also known as CFC’s, choro-fluoro-carbons.

• Because they were chemically inert, they were used as aerosol propellants, as blowing agents for plastic foam production and as solvents, as well as refrigerants.

• This inertness also means that they are not destroyed in the troposphere. Lovelock et al. (1973) detected CFC’s in the troposphere. Molina and Rowland (1974) showed that the CFC’s could be destroyed in the stratosphere, where

CF2Cl2 + h → CF2Cl + Cl

Vertical distribution of CFCl3

Bromine

• Bromine containing compounds can also impact stratospheric ozone. Source gases are mainly CH3Br , methyl bromide, and bromated CFC’s.

• Methyl Bromide has both natural and anthropogenic origins. The natural origin is from the oceans. It is manufactured for use primarily as a soil fumigant.

• The halons are used for several purposes, especially in fire extinguishing agents and fire retardants.

• The particular concern with bromine lies in the very high efficiency with which it can destroy odd oxygen within particular catalytic cycles. It is estimated that it can be about 58 times more efficient than Chlorine.

Equivalent Effective Stratospheric Chlorine

Bromine• The most important factor in determining the high activity of

bromine is that it does not form reservoir species as efficiently as does chlorine, hence most of the bromine stays in the form of BrO or Br.

• There is also a strong bromine-chlorine synergism BrO + ClO → Br + Cl + O2

Br + O3 → BrO + O2

Cl + O3 → ClO + O2

net O3 + O3 → O2 + O2 + O2

• With a mixing ratio of 2.10-11 in the stratosphere, bromine enhances the depletion of ozone by 5 to 20 per cent.

The Ozone Hole

Decline in mean October ozone levels over Halley Bay

Ozone concentrations over Syowa Station

Schematic of the winter polar vortex

Ozone Hole

ClO and O3 in mid-September

Partitioning of Chlorine

Polar Stratospheric Clouds

• There are two main classes of PSC.• Type 1 PSC are small (<1m) HNO3 rich particles. These have

a mass mixing ratio of about 10 ppbm.• Type II PSC are larger (from 10 m to about 1 mm) composed

primarily of H2O-ice with minor amounts of HNO3 as hydrates.

• They can constitute up to 1000 ppbm of the stratosphere.• As noted before, the primary reaction that can be induced on

the surface of the PSC isClONO2 + HCl → Cl2 + HNO3

• The HNO3 is then retained in the PSC.

Polar Stratospheric Clouds

• The overall catalytic cycle is

2(Cl+O3 → ClO + O2)

ClO + ClO + M → (ClO)2 + M

(ClO)2 + h → Cl + ClOO

ClOO + M → Cl + O2 + M

net 2O3 → 3O2

Overall chemistry within the vortex

Ozone loss within the Arctic Vortex

Temperature and chlorine monoxide in the Arctic