Atmospheric Aerosols and Global Climate

Contents

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.

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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.

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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.

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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.

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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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