thus the lower density of ice than of liquid H2O. If ice 

 is compressed, the hydrogen bonds shorten and become 

 more like those in the liquid. 



Stable hydrogen bonds also lead to the strong cohesive 

 forces underlying the unusually high surface tension of 

 water. Insects such as water striders take advantage ot the 

 surface tension when they skip across a pond — as if its 

 surface were made of clear, flexible plastic. And the high 

 cohesiveness and surface tension of water explain how 



interaction, the larger the target area onto which more 

 molecules and other nearby droplets can be pulled. 



As the water cluster grows, it remains highly unstable 

 until it reaches a certain critical size — the critical cluster 

 — which is about fifty molecules. Precritical clusters can 

 break up at any time into single water molecules. As it 

 approaches the critical size, though, the cluster is also 

 climbing to the "top" of an energy "hill." If it reaches the 

 top and attains the critical size, it can then "roll down" 



In its vapor form, water is the most important greenhouse gas, 

 so it plays a major role in the climate ot our planet. 



long columns of watery suspensions can be drawn through 

 extensive networks of blood vessels and even into tree 

 canopies several hundred feet above the forest floor. 



Water is the third most abundant chemical com- 

 pound in the Earth's atmosphere, after nitrogen 

 and oxygen. It is present there both as a vapor, or 

 gas. in which the water molecules dart about randomly 

 and independent of one another, and as an aerosol, a 

 mist of tiny liquid droplets or solid ice crystals that are 

 suspended in air because they're too fine to fall to earth 

 as rain or snow. In its vapor form, water is the most im- 

 portant greenhouse gas, so it plays a major role in the 

 climate of our planet. 



But when water takes the form of an aerosol, it is crucial 

 to cloud formation and to the reflection and absorption 

 ot radiation. Water aerosols act as condensation nuclei for 

 clouds — after all, clouds themselves are made up of rela- 

 tively large aqueous aerosols. And water aerosols transtorm 

 radiation in ways that, in turn, feed the factors that shape 

 cloud development. Reaching a better understanding of 

 how water aerosols affect climate has become increasingly 

 important in the past several decades. 



Water aerosols enter the atmosphere when waves break 

 in the ocean or when vapor turns to liquid. The latter 

 process, condensation, is in essence a battle between en- 

 tropy and energy, order and disorder. As water molecules 

 condense into their liquid state, they gain order but lose 

 kinetic energy. The kinetic energy given up by the phase 

 change is dumped as heat into the surrounding air, giving 

 rise to a pocket of thermal instability that will drive yet 

 another change in the weather as it equilibrates. 



The physical process of condensation is "seeded," or 

 nucleated, around tiny molecular impurities or perhaps 

 a dust particle in the air. Once an "embryo" of the new 

 liquid phase forms, more molecules tend to gather around 

 it and glom onto it, attracted by intermolecular forces [see 

 illustration on pages 34—35 ] . The larger the surface area of 

 the growing cluster, or the stronger the intermolecular 



the far side of the energy hill and undergo spontaneous, 

 runaway growth. From fifty molecules, the cluster grows 

 and agglomerates to the size of a condensation nucleus 

 (10 8 molecules), then to a cloud droplet (10 15 molecules), 

 and finally to a raindrop (10 20 molecules). 



In 1998, members of our research team were among the 

 first to measure the chemical identity of the nucleated par- 

 ticles and to show that the chemical interactions among them 

 have a profound influence on the aerosol formation rate. We 

 modeled the rate of evaporation of molecules from clusters, 

 developed a molecular simulation strategy to compute the 

 relevant kinetics, and applied the strategy to water. We found 

 that the molecular interactions between water and the initial 

 nucleating particles — whether dust, sea salt, sulfuric acid, 

 ions, or some other substance — may significantly affect the 

 rate of aerosol formation. That rate affects the distribution, 

 duration, and precipitation processes in clouds, and thus their 

 tendency to reflect, transmit, or absorb the Sun's radiant 

 heat. All those properties in turn influence the reflectivity 

 of the Earth and thus the global climate. 



Atmospheric scientists have yet to determine the exact 

 nature of that influence. One possibility is that it cluster 

 droplets grow more quickly, more clouds may form, 

 helping moderate global warming by providing more 

 cloud cover. On the other hand, faster droplet growth 

 could accelerate the production of rain, causing clouds to 

 dissipate sooner. That would lead to a less cloudy world, 

 and faster warming. 



As we molecular scientists learn more about water, 

 we are continually reminded that we have merely 

 "scratched the surface" of its secrets. The mecha- 

 nisms of its impact on life are still something of a mystery. 

 Coaxing Mother Nature to reveal further secrets about 

 water will require the full interdisciplinary sophistica- 

 tion of today's scientific toolbox. But since water is the 

 wellspring of life, we owe it to ourselves — and everyone 

 else — to explore all we can about its strange and intrigu- 

 ing properties. O 



natural history November 2007 



