Atmospheric
Aerosols and Global Climate
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
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
- 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 (submitted)
- Mr Yoshiteru Iinuma
- Ms Maja Kuzmanoski
Collaborators:
- Dr David Cohen (ANSTO)
- Dr John Gras (CSIRO
Atmospheric Research)
- Dr. Grainne Moran (UNSW
Chemistry)
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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