METO 621
Lesson 22
Summary of Homogeneous Chemistry
Summary of Homogeneous Chemistry
Summary of Homogeneous Chemistry
Mxing ratio profiles for NOy
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 include
N2O5 + H2O → 2HNO3
ClONO2 + H2O → HOCL + HNO3
• The following reaction is critical to the ‘ozone hole’ theory
ClONO2 + HCl → Cl2 + HNO3
• Other surface reactions include
HOCl + 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, nonflammable 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 + hn → 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.