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.
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.
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.
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.