Aerosols and Global Climate
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.
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.
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.
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.
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.
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
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.
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:
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.
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.
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.
Observation of Aerosol Properties
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.
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.)
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 NASAs 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.)
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.
Claudia Sendra (graduated)
Yalong Tian (graduated)
Qin Yi (graduated)
Ross Mitchell (CSIRO)
Denis O'Brien (CSIRO)
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.
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.
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.
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.
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.
Peter Loughlin (graduated)
Christian Werner (graduated)
Merlinde Kay (graduated)
Thomas Trautmann (Mainz, Germany)
Lance Leslie (UNSW Mathematics)
Anthony Davis (Los Alamos National Laboratory)
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.
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,
Chemical, Physical and Optical Properties
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.
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
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.
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
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.
Ghassan Taha (graduated)
Yoshiteru Iinuma (graduated)
David Cohen (ANSTO)
John Gras (CSIRO Atmospheric Research)
Content and Wavelength Selection for Multispectral Radiometers.
G.P. Box, M.A. Box and J. Krucker. J. Geophys. Res. 101, 19211
- 19214. 1996
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.
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.
Chemistry in the Sydney Region
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.
Remote Sensing of Aerosols
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.
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 technique has shown its value in a variety
of applications, and many more are waiting to be explored. Peter
Loughlins 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.
of Satellite Data Sets
generation space platforms such as NASAs 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.
of our Recent Graduates
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.
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 Earths
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
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
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.
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.
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.
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 Ghassans appointment completes the circle.
He is working on SAGE satellite data.