Contents
For the past decade, we have been hearing
about how rising levels of greenhouse gases - mainly carbon dioxide,
but also methane, nitrous oxide, and the chlorofluorocarbons - will
lead to global warming. Atmospheric physicists now recognise a new
and far more complex player in the climate game - aerosol particles.
Aerosols are small particles or droplets in the atmosphere, with
sizes on the order of a micrometre.
Atmospheric Aerosols
Aerosol particles are a largely natural,
though highly variable, component of our atmosphere. They may be
created by wind blowing over dusty regions, by evaporation of sea
spray, and by the conversion into tiny particles of some of the
gases emitted by plants. (Volcanoes may inject large amounts of
gases and particles into the stratosphere.) Human agricultural and
industrial activities add more aerosols, for example through mechanical
processes, or by slash-and-burn agricultural practices and increased
desertification. However, it is the emission of sulphur dioxide
from the burning of coal which has aroused most recent concern.
While some of this ends up as acid rain, itself a major environmental
problem, a large fraction is converted into sulphate aerosols.
Aerosols only remain in the atmosphere
for a few days, and so don't have time to travel very far. Hence
the effects of aerosols will largely be concentrated near their
sources, or downwind. The major sources of sulphate aerosols, for
example, are in Europe, eastern North America, and eastern Asia.
The southern hemisphere, by contrast, is quite clean. However, these
patterns are expected to change, as pollution controls act to reduce
emissions of sulphur dioxide in industrialised nations. On the other
hand, developing nations currently place much lower importance on
such issues.
Aerosols affect our environment at
the local, regional, and global levels. At the local level, aerosols
are now becoming recognised as a significant health problem, especially
in regard to respiratory illnesses, including asthma. These conclusions
are still being debated, and better data on both the physical and
chemical properties of aerosols, and illness patterns, are being
actively sought. Aerosols can also have a significant impact on
visibility, with potential economic consequences for tourism, for
example.
Climate Forcing
Atmospheric aerosols influence climate
in two main ways, referred to as direct forcing and indirect forcing.
In the direct forcing mechanism, aerosols reflect sunlight back
to space, thus cooling the planet. (Sooty aerosols from such processes
as biomass burning absorb some of this solar energy, leading to
a local atmospheric heating which may alter stability and convection
patterns.) It has been estimated that pollution of this sort over
the eastern section of the United States has effectively reduced
the annual crop growing season by one week.
The indirect effect involves aerosol
particles acting as (additional) cloud condensation nuclei, spreading
the cloud's liquid water over more, smaller, droplets. This makes
clouds more reflective, and longer lasting. The formation of a cloud
droplet requires two things - an excess of water vapour (supersaturation),
and a seed or "condensation nucleus". Computing the details
of cloud microphysics requires a detailed understanding of the dynamical
processes moving water vapour through the atmosphere, and the physical
mechanisms involved in the formation and growth of cloud particles,
including heating and cooling by solar and infrared radiation.
Calculating the direct forcing effect
is a relatively straight-forward exercise. Given the right data
on the amount and location of sulphur emitted into the atmosphere,
and some reasonable idea of how much is converted into particles
of what size, the rest is fairly simple. It is currently estimated
that in the most heavily polluted regions of the northern hemisphere,
the cooling effects of man-made sulphate aerosols exceed the warming
effects of the past century's increases in greenhouse gases. In
fact, it was the geographic pattern of greenhouse gas warming, plus
sulphate aerosol cooling, which persuaded the Intergovernmental
Panel on Climate Change to conclude that human influences were,
indeed, altering the global climate.
Our Research Group
The research being undertaken by our
group (including projects with other UNSW and Australian scientists,
as well as major international collaborations) covers the full range
of research in this field. This may be conveniently grouped into
3 projects:
We are making a range of measurements
of aerosols in the air column over Sydney, and at ground level,
as well as using data from other sites around Australia. These data
sets are being used both to study seasonal and interannual variations
in aerosols, and also to develop better models to relate aerosol
physical and chemical properties to their optical and radiative
properties.
We are involved two international projects
which are, for the first time, attempting to obtain global aerosol
data using space-borne instruments. In particular, we are developing
sophisticated new algorithms to extract the maximum amount of information
on aerosol properties from such data sets. To complement this activity,
we are also involved in the international field campaign ACE-Asia.
We are developing more sophisticated
techniques to compute the flow of solar radiation through an atmosphere
containing aerosols, clouds and gases. In this area, we have pioneered
a powerful technique known as radiative perturbation theory.
Academic staff:
Research Associate:
Satellite Observation
of Aerosol Properties
It is very important that we are able
to build up a complete picture of aerosols across the globe, so
that we may understand how they vary in both time and space. It
is even more important, however, that such observations be on-going,
so we may monitor any build-ups which may be occurring. After all,
since our present climate is a reflection of our current atmosphere
- including its greenhouse gases and aerosols - it is the changes
in these components which will lead to climate change. The only
way to achieve an on-going global coverage is by satellite observation.
Up until now, however, this has been
somewhat beyond our capability. While satellites are used routinely
to monitor many of the gases in our atmosphere, aerosols are a different
matter. Unlike the molecules of a particular gas, such as carbon
dioxide, which are all the same, aerosol particles are quite variable
in size, shape and composition. Thus, the amount of sunlight which
they reflect, and which may then be detected by a satellite, depends
on such factors as the aerosol properties and sun-satellite geometry.
A single observation rarely provides enough information. (It is
also necessary to be able to separate out the light reflected by
the ground from light scattered by aerosol particles.)
One way to improve on this situation
is to view the same segment of atmosphere from a number of different
directions, either using two (or more) satellites which happen to
make nearly simultaneous observations, or by using a satellite instrument
specifically designed to take several looks as it passes above.
The French POLDER instrument, on board the Japanese ADEOS satellite,
is just such an instrument, as is MISR, on NASA’s recently
launched Terra. We are developing a set of unique algorithms to
extract the maximum information from such multi-directional data
sets. (Dr. Michael Box is a member of the International POLDER Science
Working Team, and is collaborating with Terra scientists as well.)
In 1997, the World Climate Research
Program, in conjunction with NASA, called for a complete re-analysis
of the past two decades of atmospheric/environmental satellite data,
as well as a full assessment of the potential of all current and
planned environmental space missions, with the intention of finding
new ways to extract more information on global aerosol distributions,
and their radiative forcing of our climate. We were asked to join
this project, and Dr. Michael Box is now a member of the International
Aerosol Radiative Forcing Science Team. Our contribution to this
effort will primarily be the development of new algorithms, as well
as the use of the final data sets to study some of the impacts of
aerosols.
Academic staff:
Research Associate:
Graduate students:
- Dr. Claudia Sendra (graduated)
- Dr. Yalong Tian (graduated)
- Mr. Qin Yi (graduated)
Collaborators:
- Dr. Ross Mitchell (CSIRO)
- Dr. Denis O'Brien (CSIRO)
Retrieval of the Albedo and Phase Function
from Exiting Radiances Using Radiative Perturbation Theory, M. A.
Box and C. Sendra, Applied Optics, 38, 1636-1643, 1999.
Radiative Transfer
The flow of both solar and terrestrial
radiation through the Earth's atmosphere involves the physical processes
of emission, absorption, and scattering, and is governed by the
equation of radiative transfer. In its full complexity, this equation
is not trivial to solve, yet in the case of a numerical weather
prediction, or global climate model, we might be required to 'solve'
this equation millions of times. Clearly efficiency - the trade-off
between speed and accuracy - is crucial. We are working with a number
of codes, several of which are regarded as international standards.
In addition, we have pioneered the use of a perturbation technique
which has enormous potential in many applications, including those
situations where the atmosphere is not horizontally homogeneous.
To date, we have used this technique
in a number of applications, of which two deserve to be mentioned.
The computation of UV indices, as presented on TV weather reports,
requires the integration over wavelength of the product of the flux
of solar radiation and the biological susceptibility for sunburn.
This normally requires solving the radiative transfer equation many
times. Using perturbation theory, we were able to achieve essentially
identical results from a single solving of the radiative transfer
equation.
The second application has been at
the heart of the algorithms we are developing to extract aerosol
information from multiangular satellite data. In this approach,
we make a first guess as to the aerosol properties, and use this
to compute the expected satellite observations. We then invert the
difference between the actual and predicted measurements to obtain
the difference between the actual and first guess aerosol properties.
We have used solution techniques of
varying speed and accuracy in a number of applications to do with
the radiative effects of aerosols. At one extreme, we have collaborated
in the inclusion of aerosols and their effects in an Australian
numerical weather prediction model. At the other, we have carefully
analysed the radiative impacts of a set of standard aerosol optical
models, under a range of physically realistic scenarios.
Academic staff:
Research Associate:
Graduate students:
- Dr. Peter Loughlin (graduated)
- Dr. Christian Werner (graduated)
- Ms. Merlinde Kay (graduated)
- Mr. Paul Douriaguine
Collaborators:
- Dr. Thomas Trautmann (Mainz, Germany)
- Prof. Lance Leslie (UNSW Mathematics)
- Dr. Anthony Davis (Los Alamos National
Laboratory)
Applications of Radiative Perturbation
Theory to Changes in Absorbing Gas, M. A. Box, P. E. Loughlin, M.
Samaras and T. Trautmann, J. Geophys. Res. 102, 4333-4342, 1997.
Radiative effects of absorbing aerosols
and the impact of water vapour, M. J. Kay and M. A. Box, J. Geophys.
Res. 105, 12,221-12,234, 2000.
Aerosol Chemical,
Physical and Optical Properties
Aerosols vary widely in space and time
and thus continuous monitoring of their physical and optical properties
is important. Ground-based measurements are an important aspect
of this. (In the near future, we hope to have access to airborne
data as well.) Our group's efforts are focussed in three areas.
A multispectral rotating shadowband
radiometer has been used to make measurements of aerosol optical
thickness at the UNSW site in Sydney since December 1995. Measurements
are made at 6 different wavelengths, 4 of which are primarily sensitive
to aerosols, one to ozone and the other to water vapour. These data
are being used in a number of ways to study aerosols in Sydney,
and their seasonal variations.
The optical thicknesses are inverted
to give aerosol size distribution. The radiometer measurements are
also being used to derive total column ozone and water vapour, which
are then compared with satellite derived measurements of these quantities.
Another aspect of the work being done is the correlation between
the radiometer data, which is a column measurement, and measurements
made at the surface, such as the nephelometer measurements, and
other measurements, routinely made by NSW EPA.
The inversion of optical thicknesses
to determine size distribution is not a trivial task and the solution
is not unique and may be unstable. We have a continuing interest
in the development and refinement of inversion algorithms, and are
developing techniques for extracting the maximum information possible
from the data available. As well as applying such algorithms to
our own data, we will be applying them to data taken in international
field campaigns such as ACE-Asia, in which we are collaborating
with NASA.
It is the aerosol optical properties
which are most important in visibility and radiative forcing. These
depend on a number of factors including size distribution, chemical
composition, and relative humidity. We have recently begun a project
to study the relationship between the physical and chemical properties
of aerosols and their optical properties. This involves the use
of chemical thermodynamic models and uses data collected in a number
of Australian cities. In the future, we will also be obtaining aerosol
chemistry data here in Sydney.
Academic staff:
Graduate students:
- Mr Ghassan Taha (graduated)
- Mr Yoshiteru Iinuma (graduated)
- Ms Maja Kuzmanoski
Collaborators:
- Dr David Cohen (ANSTO)
- Dr John Gras (CSIRO Atmospheric
Research)
Information Content and Wavelength
Selection for Multispectral Radiometers. G.P. Box, M.A. Box and
J. Krucker. J. Geophys. Res. 101, 19211 - 19214. 1996
Multispectral radiometer monitoring
of aerosols in Sydney. G. Taha and G.P. Box. in Proceedings of 14th
International Clean Air Conference, Melbourne, Oct. 18-22, 1998.
Possible Research
Projects
Here are a number of possible postgraduate
(or even honours) research project areas in which we are ready to
provide supervision. They should be regarded as indicative only,
and actual projects will be arranged with individual students based
on their background and interests, as well as the current questions
which are important in our science.
Aerosol Chemistry in the Sydney
Region
Aerosol particle levels are closely
correlated with poor health, but the details remain obscure. We
intend to collect aerosol samples in two size ranges from various
parts of greater Sydney, and perform chemical analyses on them (in
conjunction with both the School of Chemistry and ANSTO). In addition,
we expect to be in a position to place instruments on the UNSW Aviation
Department flying training aircraft to obtain above-ground data.
We will then attempt to correlate these results with the relevant
health statistics using multivariate analysis. Students with a background
in Physics and/or Chemistry and/or Statistical Analysis could make
a valuable contribution to such research. Alternatively, students
with an interest in air pollution meteorology may care to run appropriate
models to track the flow of these pollutants across the Sydney basin.
Ground-based Remote Sensing of Aerosols
While the PhD research of Ghassan Taha
has laid the foundation for the remote sensing of aerosol size distribution
and other parameters, this is an ongoing challenge. Because aerosols
are such a variable atmospheric component, continual monitoring
is essential. As well as maintaining our remote sensing program
here at UNSW (including the airborne data mentioned above), a student
might be able to contribute to international field campaigns such
as ACE-Asia, or to the validation of satellite-derived aerosol data.
Radiative Transfer Theory
Different calculational problems in
radiative transfer are often best handled by different computational
techniques. We currently have codes which solve the radiative transfer
equation using the following techniques: Gauss-Seidel; Discrete
Ordinates; Delta-four-stream; Two-stream; and Spherical Harmonics.
In addition, we have a variety of data sets which characterise the
various components of the atmosphere, especially aerosols. Among
possible projects are the development of additional codes; the optimisation
of all codes so that each may run with any relevant data sets; and
a variety of applications such as the radiative impacts of aerosols.
Radiative Perturbation Theory
Our radiative perturbation technique
has shown its value in a variety of applications, and many more
are waiting to be explored. Peter Loughlin’s thesis work needs
to be extended and packaged as a community code for researchers
and weather forecasters to easily compute ultraviolet indices. Recently
we have modified the international benchmark code DISORT to provide
accurate perturbation calculations over a wide range of atmospheric
conditions. Extension of this work to higher order terms in the
perturbation series is an interesting challenge. An even greater
challenge is to apply this technique to problems involving horizontally
inhomogeneous media such as clouds, a subject of current worldwide
interest.
Analysis of Satellite Data Sets
Current generation space platforms
such as NASA’s Terra with its 5 superb instruments are providing
scientists with unprecedented amounts of high quality data, most
of which is available to the international science community for
analysis. In addition, we have many key contacts at NASA and other
agencies with whom we regularly collaborate. The number of projects
using such data sets is limited only by time and imagination. Currently
we are working on the analysis of MISR data to obtain aerosol information
over oceans, but there are many other exciting opportunities here.
Highlights of our
Recent Graduates
1. Peter Loughlin completed
his PhD in May of 1995, having worked under Dr. Michael Box, and
also with Dr. Ian Plumb of CSIRO. Two weeks later he was in Mainz,
Germany, as a Post Doctoral Research Assistant working with Dr.
Andreas Bott. As well as taking language classes, he started working
on a different radiative transfer model, coupled to a cloud microphysics
model. This model was two dimensional, with highly detailed particle
size distribution, which allowed marine clouds to be studied with
high internal detail.
After two and a half years, Peter decided
that the warm sunny beaches of home were more to his liking. Upon
returning at the end of his contract, he decided on a slight career
change, and took up a position as Orbital Dynamics Analyst with
Optus. This position is responsible for their geostationary telecommunications
satellites, controlling their position with respect to the Earth
station, and hence the customers using the payload. His work is
concerned with understanding the orbit of the spacecraft, and how
that orbit is influenced by perturbations from the Sun, Moon, and
the Earth’s triaxial gravitational field. The Orbital Analyst
is responsible for planning manoeuvres to ensure the spacecraft
stay within their 0.05 degree station boxes, and to do so in such
a way as to minimise fuel usage.
2. Claudia Sendra completed
her PhD in 1997, and since then has been teaching Physics in a number
of Sydney high schools, and also pre-university Physics courses,
both in Australia and in Spain.
3. Christian Werner completed
his PhD in mid 1999. He firstly made use of his expertise in the
numerical modelling of weather and climate by working in the field
of environmental modelling for CANCES (the Centre for Advanced Numerical
Computation in Engineering and Science), and later in the High Performance
Computing Support Unit of UNSW. Both of these jobs provided many
opportunities to further his skills in this area. In particular,
he was able to continue working on a high resolution numerical environmental
prediction model, which can be applied to a wide range of important
problems, such as air pollution, weather, and salination.
Recently he has joined Enron, a Houston
based company which is the pioneer in weather derivatives risk management:
that is, they try to manage the risks associated with adverse weather.
Enron is a leading market maker in the electricity, gas, bandwidth
and weather derivatives areas, and a highly innovative company.
He is currently using his expertise in weather modelling in the
analysis, trading and structuring side of weather derivatives in
the Asia-Pacific region. It is a fast paced, dynamic place to work.
4. Yalong Tian completed his
PhD in 2000, and is now working as a Professional Officer with the
Ionospheric Prediction Service. His current main task is to enhance
the radio propagation prediction software by adding new options
and upgrades. In addition, he carries out research on Solar Terrestrial
Physics.
5. Ghassan Taha completed his
PhD in the middle of 2000, and went straight to a position as a
Research Scientist at NASA Langley Research Center, in connection
with the University of Arizona. These are both institutions where
Michael and Gail worked (20 years ago), and so Ghassan’s appointment
completes the circle. He is working on SAGE satellite data.
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