Denitrification and ozone loss in the Arctic stratosphere

This thesis investigates the mechanism of denitrification o f the Arctic lower stratosphere
and the impact o f denitrification on ozone loss using the SLIMCAT chemical transport
model. The development of a new microphysical model for the simulation of growth
and sedimentation of large nitric acid trihydrate particles is also described.
Model simulations of Arctic denitrification were carried out using thermodynamic equilibrium
schemes based on the sedimentation of either nitric acid trihydrate or ice using
different meteorological analyses. The severity and extent of denitrification in ice-based
model runs was found to be highly sensitive to the meteorological analyses used whereas
nitric acid trihydrate denitrification schemes exhibited considerably less sensitivity.
The response of thermodynamic equilibrium and microphysical NAT-based denitrification
to meteorological conditions has been studied in a series of short idealised simulations.
It was found that microphysical denitrification was considerably more sensitive
to the relative orientation of the polar vortex flow and the region of cold temperatures.
A concentric vortex and cold region are required to promote the long particle growth
times required for strong denitrification in the microphysical model. Reduced rates of
denitrification were evident in the microphysical model at the highest altitudes.
Results from the microphyical denitrification scheme were compared with in-situ and
remote observations of denitrification for two recent cold Arctic winters. There was
remarkable agreement between model and observations of both the magnitude and
location of denitrification despite the simple volume-averaged nucleation rate used in
the model. The limited range of observations did not allow further constraints to be
placed on the microphysical model.
Denitrification was found to enhance Arctic ozone loss by up to 30% during 1999/2000.
Sensitivity studies o f the impact of denitrification on Arctic ozone loss were performed
using thermodynamic nitric acid trihydrate denitrification schemes. Cumulative ozone
depletion was found to increase non-linearly with increasing denitrification. Enhanced
recovery of chlorine radicals to hydrogen chloride in strongly denitrified model runs
offset reduced recovery to chlorine nitrate, limiting the impact of denitrification to the
equivalent of 20 days additional ozone loss