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2004 Haagen-Smit Prize Winners

The Executive Editors and the Publisher of Atmospheric Environment take great pleasure in announcing the fourth ''Haagen-Smit Award'', designed to recognize outstanding papers published in Atmospheric Environment. From here on the ''Haagen-Smit Award'' has been renamed the ''Haagen-Smit Prize'' and includes cash awards. The Prize is named in honor of Prof. Arie Jan Haagen-Smit, a pioneer in the field of air pollution and one of the first editors of the International Journal of Air Pollution, a predecessor to Atmospheric Environment.

The ''Haagen-Smit Prize/Award'' is given annually to two papers previously published in Atmospheric Environment and covering different science areas. The two 2004 winning papers are described below.

Nomination Letters

Sean TwomeyTwomey, S., Pollution and the planetary albedo, Atmospheric Environment, 8, 1251-1256, 1974.

It is my great pleasure to nominate for the 2004 Haagen-Smit Prize Sean Twomey's 1974 paper, Pollution and the planetary albedo, Atmos. Environ., 8, 1251-1256. This paper marks the first publication to explain how enhanced cloud nuclei from anthropogenic pollution may increase the amount of solar radiation reflected by clouds, thereby increasing the planetary albedo, with potential long term climatic consequences. One might attempt to quantify the importance of this paper by counting citations in the peer-reviewed journals, by its impact on the field of aerosol-cloud-climate research over the intervening decades, or based on its content and clarity. By any of these standards it is an outstanding paper.

The basic premise was simple and a wonderful example of Professor Twomey's incisive thought: if pollution contributes additional nuclei upon which water can condense, then the condensed mass of water will consist of a larger concentration of smaller drops. The reflectance of the cloud will increase because the total surface area of the condensed water in cloud is greater when spread over more droplets. Although pollution alone may directly affect climate by modifying the absorption and scattering properties of the cloud-free atmosphere, the effect of pollution on clouds is potentially even larger; cloud droplets interact with radiation much more strongly than the nuclei upon which they form because they are typically several orders of magnitude greater in size.

This relationship between increased cloud albedo and pollution, today widely known as the Twomey effect or the first aerosol indirect effect, was given little attention when it was first proposed in 1974. However, in the 1980's the climate community recognized that ship tracks, long identified but now more frequently observed in satellite imagery, were visible manifestations of the consequence of injecting additional particles into a cloud layer. Twomey elaborated on his theory in a 1984 paper in Tellus. In a 1991 paper in Atmospheric Environment, Professor Twomey introduced the term cloud susceptibility, a quantification of the sensitivity of enhancing cloud reflectance for a given droplet number concentration and cloud reflectivity. For example, if a cloud was produced in an environment of high concentration of condensation nuclei, introducing additional particles would do little to make the cloud even brighter. Conversely, a cloud forming in a pristine environment with sparse available nuclei would be more prone to albedo change when more nuclei were added. This is why ship tracks are readily observed in some environments, such as the California Pacific coast region, but less frequently in others, for example, west of the Namibian coast in southern Africa. And this is also why quantifying the magnitude of this effect globally is so challenging: it is difficult to identify a background state above which the influence of pollution can be measured.

During the 1990's and up to the present date there has been an explosion of research activity on the Twomey effect. This was due to the gradual recognition by the scientific community of the important implications of Professor Twomey's results, as expressed by the International Panel on Climate Change (IPCC). This scientific body identified the indirect effect of aerosol on clouds as having the greatest uncertainty among all of the anthropogenic agents associated with climate change through radiative forcing. Today the influence of urban-industrial pollution and biomass burning on many other cloud processes, such as cloud formation, cloud maintenance, and precipitation are foci of climate research, and rightly so considering the strong feedback that water in all of its phases contributes to climate forcing. But global quantification of the Twomey effect or the so-called first indirect effect is still a missing piece of the climate puzzle, although it has been verified and quantified locally in some instances. Numerous field campaigns around the globe, including the multi-agency International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) experiment underway right now, originated in part to study the indirect effect of aerosols on clouds.

The work of Professor Twomey on cloud and aerosol interactions, like all of his contributions to atmospheric physics and remote sensing, is so widely recognized that expanding on it here is almost an unnecessary undertaking. That the mechanism he first identified in the 1974 paper now bears his name is testimony to this fact, although Sean certainly would never refer to it as the Twomey effect! Perhaps it is because the climate community is so familiar with his work and his theories have been so broadly accepted that this paper has not been nominated prior to now. Yet Professor Twomey was so clearly ahead of his time that his insights, first published thirty years ago, hardly seem dated. It is my privilege to nominate this paper now and I thank the panel for their consideration.

Nominator: Peter Pilewskie, University of Colorado, Colorado, USA

Prof. Sean Twomey, University of Arizona, 11250 E. Outback Rd., Tucson, AZ 85730, USA

Art StelsonJ SeinfeldStelson, A. W. and J. H. Seinfeld, The relative humidity and temperature dependence of the ammonium nitrate dissociation constant, Atmospheric Environment, 16, 983-992, 1982.

I write to nominate "The relative humidity and temperature dependence of the ammonium nitrate dissociation constant", by A.W. Stelson and J.H. Seinfeld (AE 16, 983-992, 1982) for the 2004 Haagen-Smit Prize. This paper provided the first fundamental data on the ammonium nitrate aerosol equilibrium constant for use in atmospheric applications. It is a very elegant piece of physical chemistry and has proven to be tremendously successful for our understanding of aerosol nitrate formation. The theoretical foundation developed in this paper remains valid to this day and provides the basis for aerosol nitrate representation in all the current-generation models. The current concern over rising aerosol nitrate due to reductions in SO2 emissions illustrates how the Stelson and Seinfeld paper has remained topical over two decades after its publication.

But the Stelson and Seinfeld paper is much more than just a superb treatment of aerosol ammonium nitrate equilibrium. It is a landmark in atmospheric chemistry research because it pioneered the application of thermodynamics to predict atmospheric aerosol composition. It demonstrated that thermodynamic principles and data could describe successfully the observed atmospheric behavior of a simple aerosol system. This has empowered an entire new approach for studying atmospheric aerosol chemistry. The methods first developed by Stelson and Seinfeld are now extensively being used in the research literature to describe the composition of complex inorganic and organic aerosol mixtures.

The Stelson and Seinfeld paper has been cited 241 times according to the Science Citation Index, a clear statement of its importance. It is highly worthy of recognition by the Haagen-Smit Prize.

Nominator: Daniel J. Jacob, Harvard University, Massachusetts, USA

Dr. Arthur W. Stelson, Georgia Department of Natural Resources, 4244 International Parkway, Suite 120, Atlanta, GA 30354, USA

Prof. John H. Seinfeld, Mail Code 210-41, California Institute of Technology, Pasadena, CA 91125, USA

Our congratulations go to all the authors listed on these two outstanding papers.

Hanwant B. Singh and Peter Brimblecombe,
Executive Editors of Atmospheric Environment.

Hetty Verhagen,
Senior Publishing Editor, Elsevier Science.