Particles of difference Schwartz S. E. Conference on Air Quality II. September 19-21, 2000; McLean, Virginia.

Atmospheric aerosols have received increased attention over the past decade mainly for two reasons: their association with impairment of human health and their possible influences on climate change. The prospect of new regulations on fine particles, PM-2.5, particulate matter of aerodynamic diameter less than 2.5 Ám, makes it mandatory that the processes governing the loading and distribution of these aerosols be sufficiently well understood to permit development of effective and efficient strategies to achieve prospective standards. Likewise, the need to relate observed climate change over the industrial period to the totality of influences makes it mandatory to identify aerosol phenomena that may substantially influence climate change and to quantify these influences. Atmospheric aerosols are also directly associated with reduction in visibility and with long-range transport and deposition of air pollutants. This talk reviews current issues and recent findings involving atmospheric aerosols and points to necessary directions of research.

The key driving force for standards for atmospheric aerosols is their association with impairment of human health. There is a large body of research adducing such an association, ranging from laboratory studies to field epidemiology and hospital admission rates. However the causative agent or agents remain unknown: Are adverse health effects due simply to mass loading, or are they associated with specific aerosol properties?

Early assessments of the climatic influence of anthropogenic aerosols due to light scattering and modification of cloud reflectance established that the cooling influence of these phenomena is comparable to the warming influence of increased concentrations of greenhouse gases and that the aerosol influences may be offsetting a considerable fraction of the warming influence of anthropogenic greenhouse gases. A major distinction is that the aerosols are relatively short-lived in the atmosphere (ca. 1 week) compared to the greenhouse gases (decades to centuries), so that the effect of aerosols is to mask the climate influence of greenhouse gases rather to serve as a solution to greenhouse warming. Aerosols also modify cloud lifetimes by inhibiting precipitation and by causing clouds to evaporate through absorptive heating of the atmosphere. Extinction of solar radiation by aerosol particles reduces the irradiance received at the surface, decreasing crop productivity and inhibiting evaporation, influencing the hydrological cycle.

It is no longer appropriate, if it ever was, to think of atmospheric aerosols as homogeneous spheres of uniform composition and size. Within the United States, and even more globally, not only the mass loading but also the composition, morphology, and size distribution of atmospheric aerosols are highly variable, as a function of location, and at a given location as a function of time. Particles of a given aerodynamic size may differ from one another, and even within individual particles material may be inhomogeneously distributed, as for example, carbon spherules imbedded in much larger sulfate particles. Some of the particulate matter is primary, that is, introduced into the atmosphere directly as particles, such as carbon particles in diesel exhaust. Some is secondary, that is, formed in the atmosphere by gas to particle conversion. Much of the material is inorganic, mainly sulfates and nitrates resulting mainly from energy-related emissions. Some of the material is carbonaceous, in part primary, in part secondary, and of this material some is anthropogenic and some biogenic.

While the heterogeneity of atmospheric aerosols complicates the problem of understanding their loading and distribution, it may well be the key to its solution. By detailed examination of the materials comprising aerosols it is possible to infer the sources of these materials. It may be possible as well to identify specific health impairing agents. The heterogeneity of aerosol particles is thus the key to identifying their sources, to understanding the processes that govern their loading and properties, and to devising control strategies that are both effective and efficient. Future research must therefore take cognizance of differences among aerosol particles and use these differences to advantage.

This page was last updated 2000-09-27.

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