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Contents
NAMBLEX 2002 is a major field experiment, funded by the Natural Environment Research Council
(NERC), investigating the chemistry of atmospheric constituents in
the marine environment. The project, NAMBLEX, brings together again
the UK consortium comprising the Universities of Leeds, East Anglia,
Leicester, UMIST and Bristol, which have a successful history of
working closely together in the ACSOE, SOAPEX and
other field campaigns. The scientific rationale and aims of the
experiment are summarised below. This website provides forecasts for
the back trajectories of air masses arriving at Mace Head (picture
above), on the west coast of Eire, where the experiment is based. Each
back trajectory shows the path of an air mass over a period of five
days before its arrival above Mace Head. The date labelling each
trajectory corresponds to its arrival time. The trajectories are not
exact and so a cluster of 7 trajectories is calculated for each
arrival time. The spread of the cluster is very small (compared to
trajectory length) at the arrival time, but as the cluster is followed
backwards in time its spread typically increases because the wind
varies in space and time so that each trajectory is blown by a
slightly different wind. The dispersal of the cluster is described as
`chaotic'. Changes in chemical composition at Mace Head can change
rapidly when the origin of `air masses' changes, as seen by a dramatic
displacement in the endpoints of the back trajectories. Times when the
cluster spread is large indicate periods when a change in air mass is
likely.
Click on the blue `forecast' buttons to see the predicted path of air
masses to Mace Head for different arrival times using the latest wind
forecasts (the arrival time is T+.. hours after the start of the wind
forecast). The trajectories are calculated using
computer models in two steps. First, the ECMWF numerical weather
prediction (NWP) model produces forecasts of winds and temperatures on
60 altitude levels ranging from the ground to the upper
atmosphere. Their model represents the atmosphere at each level using
a grid which covers the globe (like a series of concentric egg
shells). Every afternoon the latest surface, radiosonde (weather
balloon) and satellite observations are collected from around the
world and assimilated with the model to give their best guess of the
atmospheric state at the previous midday (12GMT). The NWP model is
then run forwards from 12GMT. When finished at night, these
meteorological forecasts used to calculate the trajectories.
The Mace Head site has been used previously by the consortium and
experiences a wide range of clean and semi-polluted air masses:
trans-Atlantic, arctic/sub-arctic, tropical maritime, European outflow
and locally influenced. The site allows investigation of oxidative and
particle physico-chemistry in a wide range of air mass types and
histories. Anti-cyclonic flow off Europe and air transported across
the ocean from the USA enable the investigation of the long range
effect of anthropogenic aerosol on chemical composition and cloud
droplet formation in the marine environment.
The trajectory model was written by John Methven for the U.K. Universities Global Atmospheric
Modelling Programme (UGAMP), a national programme conducting
fundamental research into atmospheric science, funded by the Natural Environment Research Council
(NERC).
NAMBLEX 2002: Scientific Background
The hydroxyl radical (OH), formed from the photolysis of ozone
at wavelengths below 340 nm in the presence of water vapour, initiates
the oxidation of almost all trace gases released into the boundary
layer. Peroxy radicals produced by this oxidation chemistry are
responsible for ozone production or ozone destruction, depending upon
the level of nitrogen oxides (NOx) co-present. The oxidizing capacity
of the troposphere is essentially a combination of the above processes
and possibly others involving halogen and nitrate radicals.
Increasing evidence suggests that the oxidizing capacity has been
perturbed in recent years due to human activity through emission of
methane, carbon monoxide, non-methane hydrocarbons (NMHCs) and
NOx. These perturbations may be causing changes in the natural
atmospheric composition, for instance increasing tropospheric
concentrations of the greenhouse gas ozone, which has important
implications for climate and human health. Any change in the oxidizing
capacity of the atmosphere has many consequences for the long-term
stability of the Earth's climate.
A major aim of the study, to be achieved through the comparison of
measured and modelled species, is to test our understanding of basic
photo-oxidation chemistry across a range of clean and polluted
environments, which are experienced at Mace Head. Validated mechanisms
can be reduced and steady-state mechanisms derived which are
appropriate to dynamical climate models (full mechanisms are too
large), giving more confidence in the accuracy of predicted changes in
climate, or legislation designed to reduce pollution, in a future when
anthropogenic perturbation of the troposphere is expected to increase
significantly.
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