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ScienceWeek
SCIENCE-WEEK
A Weekly Email Digest of the News of Science
A journal devoted to the improvement of communication
between the scientific disciplines, and between scientists,
science educators, and science policy-makers.
June 23, 2000 -- Vol. 4 Number 25
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I have steadily endeavored to keep my mind free,
so as to give up any hypothesis, however much
beloved -- and I cannot resist forming one on
every subject -- as soon as facts are shown
to be opposed to it.
-- Charles Darwin (1809-1882)
-----------------------------------------------
Contents of this Issue:
1. Cell Biology:
On Internal Signaling in Biological Cells
-----------------------------------------
Internal information transfer in biological cells is an important
component of intercellular communication -- the signaling network
that holds the body together as a viable entity.
(Includes related background material.)
2. Human Evolution:
Fossil Evidence of Early Hominid Knuckle-Walking
------------------------------------------------
A new study of existing fossils provides strong support for the
idea that knuckle-walking characters were present in the common
ancestor of modern humans and the African apes, and provides
grounds for reconciliation between molecular and anatomical
evidence. (Includes related background material.)
3. Medical Biology:
Plague as a Biological Weapon
-----------------------------
An aerosolized plague weapon is technically feasible, and would
be expected to kill large numbers of people if released over a
city. (Includes related background material.)
4. Theoretical Physics:
Chaos and Nonlinear Dynamics
----------------------------
Recent research indicates that the origin of unpredictable motion
in chaotic thermal convective systems lies in what occurs in
small regions of space and over short time-scales. The results
may be applicable to many chaotic systems in physics, chemistry,
and biology. (Includes related background material.)
5. Atmospheric Chemistry:
Reactions at Atmospheric Interfaces
-----------------------------------
In recent decades, it has become apparent that the atmosphere of
the Earth is a dynamic chemical system with a wide variety of
interactions among its constituents, including heterogeneous
interactions that are important for both climate and the
biosphere. (Includes related background material.)
6. Climatology:
Anthropogenic Atmospheric Aerosols and Global Climate Change
------------------------------------------------------------
Recent studies demonstrate both the importance of anthropogenic
aerosol effects on climate and the complexity of aerosol-cloud
interactions.
7. In Focus:
On Fundamental Particles and Fundamental Forces
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
1. CELL BIOLOGY:
ON INTERNAL SIGNALING IN BIOLOGICAL CELLS
Every multicellular organism is essentially a colony of
biological cells, and in all cases the viability of the colony
(and the viability of the "organism") is critically dependent on
direct or indirect organized communication between the cells
constituting the colony population.
In some cases, certain intercellular communication pathways
in an organism are anatomically explicit (e.g., cell
communication via a nervous system), but in most cases
intercellular communication involves the release and reception of
chemical signals (e.g., hormones) that are transported either by
simple diffusion or by hydraulics (e.g., a circulating blood or
other fluid system).
Although these general principles have been known for more
than a century, it is only during the past few decades that the
molecular details of the mechanisms of communication between
various cells in an organism have become apparent. Research in
this area is not only important for the understanding of the
biology of organisms in general, but it is also of great
consequence for clinical medicine: If any important communication
pathway between the various cells and tissues that constitute the
colony we call the human body fails, or if any mechanism
responsible for the appropriate response of cells to incoming
signals fails, that failure produces disease, often with
reverberations to other communication pathways, and the result
can be severe debilitation or even death of the organism,
followed by rapid disintegration of all the cells comprising the
colony. In essence, intercellular communication (and appropriate
responses by cells to received signals) is what holds the body
together as a viable entity.
... ... J.D. Scott and T. Pawson (2 installations, US CA) present
a review of current research in cell communication, with a focus
on what happens inside cells when cells receive externally-
derived signals. The authors make the following points:
1) Internal signal transmission in cells usually begins
after a chemical messenger responsible for carrying information
between cells (e.g., a hormone) docks temporarily in lock and key
fashion with a specific receptor on the recipient cell surface.
Such receptors are able to relay chemical information into a cell
because they are physically connected to the cell interior
(cytoplasm). The typical receptor is a protein that includes at
least 3 domains: a) an external docking region for a hormone or
other messenger; b) a component that spans the plasma membrane of
the cell; c) a tail that extends a distance into the cytoplasm.
When a messenger binds to the external site, this binding induces
a change in the shape of the cytoplasmic tail, and this change in
shape facilitates the interaction of the tail with one or more
information-relaying molecules in the cytoplasm. These
interactions in turn initiate cascades of further intracellular
signaling.
2) During the last decade, researchers discovered that many
of the proteins involved in internal cell communication consist
of strings of modules, some of which serve primarily to connect
one protein to another. At times, whole proteins in intracellular
signaling pathways apparently contain nothing but such linker
modules. Also, within the past few decades, it has become more
apparent that the cytoplasm of cells is not amorphous, but is
instead densely packed with organelles and proteins, and that
high-fidelity signaling within cells depends profoundly on
configurational interlocking of selected proteins via dedicated
linker modules and adapter proteins. These complexes assure that
enzymes or DNA-binding modules and their targets are brought
together promptly and in the correct sequence as soon as a
receptor on the cell surface is activated.
3) Certain intracellular signaling networks apparently rely
on relatively small adapter proteins, and in some cases such
proteins contain a large number of linker domains. These proteins
are often called "scaffolding molecules", since they permanently
hold groups of signaling proteins together in one location. The
existence of such scaffolds means that certain signaling networks
are hardwired into cells, with such hardwiring enhancing the
speed and accuracy of intracellular information transfer.
-----------
J.D. Scott and T. Pawson: Cell communication: The inside story.
(Scientific American June 2000)
QY: John D. Scott, Oregon Health Sciences Univ. 503-494-2998.
-------------------
Related Background:
CELL BIOLOGY: ON THE SIGNIFICANCE OF INTRACELLULAR CIRCULATION
The prevailing idea in biology is that many physiological and
molecular functions are the sum of individual processes linked in
sequence, although when studied in isolation, many such
individual processes are without apparent function. How such
systems evolve and become regulated continues to be one of the
most important puzzles confronting both biochemists and cell
physiologists.
... ... P.W. Hochachka (University of British Columbia, CA)
presents a review of current theoretical approaches to cell
metabolism and regulation, the author making the following
points:
1) Two views currently dominate research into cell function
and regulation: The first view (Model #1) assumes that cell
behavior is quite similar to that expected in a watery bag of
enzymes and ligands. The second view (Model #2) assumes that
3-dimensional order and structure constrain and determine
metabolite behavior.
2) A major puzzle in the study of cell metabolism is that
essentially all metabolite concentrations are remarkably stable
(are *homeostatic) over large changes in pathway fluxes. The
author calls this the "stability paradox". For muscle cells, for
example, *adenosine triphosphate (ATP) and oxygen are the most
perfectly homeostatic, even though oxygen delivery and metabolic
rate correlate in a 1:1 fashion. In total, more than 60
metabolites are known to be remarkably homeostatic in differing
metabolic states.
3) Several explanations of stability are usually given by
traditional Model #1 studies -- none of which apply to all
enzymes in a pathway, and all of which require diffusion as the
means for changing enzyme-substrate encounter rates. In contrast,
recent developments in our understanding of intracellular
*myosin, kinesin, and dynein motors running on *actin and tubulin
tracks or cables provide a mechanistic basis for regulated
intracellular circulation systems with *cytoplasmic streaming
rates varying over an approximate 80-fold range (from 1 to more
than 80 microns per second).
4) These new studies suggest a Model #2 hypothesis of
intracellular perfusion or convection as a primary means for
bringing enzymes and substrates together under variable metabolic
conditions. In this view, changes in intracellular perfusion
rates causes changes in enzyme-substrate encounter rates and thus
changes in pathway fluxes -- with no requirement for large
simultaneous changes in substrate concentrations. The author
concludes: "The ease with which this hypothesis explains the
stability paradox is one of its most compelling features."
-----------
P.W. Hochachka: The metabolic implications of intracellular
circulation.
(Proc. Natl. Acad. Sci. US 26 Oct 99 96:12233)
QY: P.W. Hochachka [pwh@zoology.ubc.ca]
-----------
Text Notes:
... ... *homeostatic: The term "homeostasis" refers to a
physiological equilibrium necessary in general for the viability
of an organism, and in particular for the operation of many
cellular functions. Homeostatic mechanisms in biological systems
usually involve an element of negative feedback signaling. In
vertebrates, for example, when blood temperature is too high,
temperature receptors provoke a sequence of events involving many
pathways that ultimately results in a lowering of body
temperature. Similar homeostatic mechanisms operate at cellular
levels.
... ... *adenosine triphosphate (ATP): ATP is the most important
chemical energy source in all living cells, intimately involved
in various cell functions and cell metabolism, and an entity in
numerous cyclic chemical pathways involved in the synthesis of
various cell components.
... ... *myosin, kinesin, and dynein motors: It is now recognized
that the interiors of biological cells are structurally complex,
and that this structure is dynamic. Microtubules are part of the
cytoskeleton of biological cells, the quasi-rigid matrix that
among other things determines cell shape. The microtubules are 25
nanometers in diameter, and composed of the protein tubulin. They
occur in regular arrays in various cell organelles, and in the
cytoplasm in general, and they contribute not only to cell shape,
but also to cell motility. Microfilaments are 4 to 6 nanometers
in diameter, highly variable in length, and are found in all
eukaryotic cells. They are composed of a protein called "actin"
and several other accessory proteins, and they are important in
cell locomotion and in the molecular dynamics of muscle cells.
"Motor proteins" are mechanico-chemical enzymes involved in
locomotion or transport, and there are three families of such
proteins: kinesins, dyneins, and myosins. Kinesins and dyneins
are microtubule based motor proteins, while myosin is a
microfilament based motor protein. In general, as mechanico-
chemical enzymes, motor proteins convert energy from hydrolysis
of nucleotides to mechanical force, and since they are involved
in many important cellular events, the molecular details are
currently the focus of intensive research.
... ... *actin and tubulin tracks or cables: See previous note.
... ... *cytoplasmic streaming: Supremely evident to biologists
who study living cells with the light microscope is the fact that
the interiors of cells are often in visible motion, the
cytoplasmic contents circulating in various streams. The
classical name for these movements was "cytoplasmic streaming",
and for nearly a century, such movements remained a profound
puzzle in biology. With the advent of molecular biology,
intracellular streaming motion was recognized as a phenomenon
related to dynamic motor proteins.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 7Jan00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ANALYSIS OF INTRACELLULAR SIGNALING MECHANISMS
In multicellular organisms, chemical messengers of various types
are important entities in the communication between cells
necessary for the function and viability of cells, tissues, and
the organism itself. These messengers usually interact with the
surfaces of cells, particularly with specific receptors on cell
surfaces. Such an interaction is called an "extracellular
signal", and what happens next is a cascade of internal signal
events that effectively transmit the external signal from the
cell membrane to one or more places inside the cell, especially
to the cell nucleus. This sequence in internal signal events
apparently involves specific protein-protein and protein-
phospholipid interactions, with the interactions mediated by
protein domains (regions) of tertiary structure (higher order
configuration) that have evidently been conserved through
evolution. The details of these biochemical interactions are
becoming apparent, at least in some types of somatic cells, so
that molecular biologists are now characterizing the involved
proteins as anchoring (docking) proteins, adaptor proteins,
scaffold proteins, and so on, according to the role played by the
particular protein in the spatial location and translocation and
signal events that eventually produce important reactive or
regulatory responses of the cell. Pawson and Scott (2
installations, CA US), in a review of how extracellular signals
are relayed from the plasma membrane to specific intracellular
sites, discuss the role of scaffold, anchoring, and adaptor
proteins that contribute to signal transduction by recruiting
active enzymes into signaling networks or by placing enzymes
close to their substrates. The authors suggest that the challenge
ahead is to understand both the physiological functions and
regulation of such signaling networks.
QY: Tony Pawson, S. Lunenfeld Res. Inst., Toronto, ON M5G 1X5 CA.
(Science 19 Dec 97) (Science-Week 9 Jan 98)
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
2. HUMAN EVOLUTION:
FOSSIL EVIDENCE OF EARLY HOMINID KNUCKLE-WALKING
There are 185 species of living primates, of which modern
humans, Homo sapiens, are one species. All primates, from bush
babies and tarsiers to gorillas and humans are characterized by
approximately 30 characters that relate to 3 major sets of
adaptations: a) agility in the trees; b) acute brain and visual
functions; c) parental care.
The consensus view in paleoanthropology is that humans are a
very young group that has had a separate evolutionary history for
only 5 to 7 million years, with two sets of characters that
apparently set humans apart from other apes: locomotion on two
legs (bipedalism) and large brain size. Fossil evidence indicates
that by 4.1 million years ago, and perhaps earlier, *hominids
exhibited adaptations to bipedal walking. At present, however,
the fossil record offers little information about the origin of
bipedalism, and despite nearly a century of research on existing
fossils and on comparative anatomy, there is apparently still no
consensus concerning the mode of locomotion that preceded
bipedalism (*Note #1).
... ... B.G. Richmond and D.S. Strait (George Washington
University, US) now present evidence that fossils attributed to
*Australopithecus anamensis and *Australopithecus afarensis
retain specialized wrist morphology associated with knuckle-
walking. This morphology differs from that of later hominids and
non-knuckle-walking anthropoid primates, suggesting that knuckle-
walking is a derived feature of the African ape and human *clade.
The authors suggest this removes key morphological evidence for a
chimpanzee(Pan)-gorilla(Gorilla) clade, and indicates that
bipedal hominids evolved from a knuckle-walking ancestor that was
already partly terrestrial.
... ... In a commentary in the same journal on the work of
Richmond and Strait, M.Collard and L.G. Aiello (University
College London, UK) state: "Richmond and Strait's study... bears
on the long-standing debate over the evolutionary history -- the
'*phylogenetic' relationships -- of modern humans (Homo),
chimpanzees (Pan), and gorillas (Gorilla). [Molecular] genetic
analyses overwhelmingly indicate that chimpanzees and humans are
more closely related to one another than either is to gorillas.
Until the study by Richmond and Strait, however, the anatomical
evidence largely ran counter to this conclusion... Richmond and
Strait's results offer strong support for the idea that knuckle-
walking characters were present in the common ancestor of modern
humans and the African apes, and provide grounds for
reconciliation between the molecular and anatomical evidence."
-----------
B.G. Richmond and D.S. Strait: Evidence that humans evolved from
a knuckle-walking ancestor.
(Nature 23 Mar 00 404:382)
QY: B.G. Richmond [brich@gwu.edu]
-----------
M. Collard and L.C. Aiello: From Forelimbs to Two Legs.
(Nature 23 Mar 00 404:339)
QY: M. Collard [m.collard@ucl.ac.uk]
-----------
Text Notes:
... ... *hominids: In general, any primate in the human family.
... ... *Note #1: The hominid fossil record is far from
extensive, and a common quip in paleoanthropology is that there
are more working paleoanthropologists than discovered hominid
fossils to be worked on.
... ... *Australopithecus anamensis: Found by Meave Leakey in
4.1- to 3.9-million-year-old sediments near Lake Turkana in
Kenya. The remains include two jaws, a humerus, a tibia, and
isolated teeth. It has a primitive jaw with a shallow palate and
large canine teeth. The tibia indicates that A. anamensis was a
biped.
... ... *Australopithecus afarensis: The famous skeleton of
"Lucy", discovered in the 1970s, is an example of an
Australopithecus afarensis fossil. A. afarensis specimens range
from 3.2 to 2.9 million years in age. The individuals are 1 to
1.2 meters tall, with a brain size of only 415 cubic centimeters
and a generally ape-like face. Primitive characters include long
arms, short legs, and curved finger and toe bones that imply the
use of hands and feet in grasping branches. However, A. afarensis
is fully human in some significant ways, including hindlimbs and
pelvis fully adapted for a type of bipedal locomotion.
... ... *clade: In general, a "clade" is a cluster of taxa
apparently derived from a single common ancestor.
... ... *phylogenetic: The term "phylogeny" refers to the
evolutionary history of a species or group of species in terms of
their derivation and relationships.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 23Jun00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ISOTOPIC ANALYSIS OF THE DIET OF AN EARLY HOMINID
The term "hominid" refers to any primate in the human family
(Hominidae) of which Homo sapiens (modern man) is the only living
specimen. The current consensus, based on molecular evidence, is
that our closest living relatives, gorillas and chimpanzees,
probably separated from our line (or us from them) approximately
6 to 8 million years ago. Humans, gorillas, common chimpanzees
and pygmy chimpanzees, the 4 African species of "great apes",
apparently had common ancestors perhaps 10 to 12 million years
ago. The first record of human footprints, of hominids walking
upright, was discovered at *Laetoli in East Africa, and has been
dated at 3.6 million years ago. This ancestor, Australopithecus
afarensis, probably weighed 25 to 50 kilograms (60 to 120 lbs.)
as an adult (*Note #1). Apparently derived from Australopithecus
afarensis were several species, including Australopithecus
africanus, a species which is believed to have appeared
approximately 3 million years ago. Little is known about the
diets of hominids that predate the Homo genus, because these
hominids did not leave archeological traces such as "*kitchen
middens" and stone tools. Consequently, researchers have made
inferences concerning hominid diet on the basis of craniodental
morphology, gross dental wear, and dental microwear. The current
consensus is that the 3-million-year-old Australopithecus
africanus hominid subsisted on fruits and leaves, similar to the
modern chimpanzee. Early hominid diets are of some theoretical
significance, since one current view is that the emergence of the
more intelligent Homo genus depended on the consumption of high-
quality animal foods that made possible biological changes
resulting in the evolution of a larger brain.
... ... M. Sponheimer and J.A. Lee-Thorp (2 installations, US
ZA) now report a stable carbon isotope analysis of A. africanus
fossils from *Makapansgat Limeworks, South Africa. The authors
sampled 4 of the 14 Australopithecus africanus individuals that
have been unearthed at that location, and also analyzed the
dental enamel of associated 3-million year old animals (65
individual animals from 19 mammalian taxa) in order to place A.
africanus within a broader ecological context. The authors report
their results demonstrate that A. africanus ate not only fruits
and leaves, but they also ate large quantities of carbon-13
enriched foods such as grasses and sedges, or they ate animals
that ate these plants, or both. The authors suggest their results
indicate that early hominids such as A. africanus regularly
exploited relatively open environments such as woodlands or
grasslands for food, and that early hominids may have consumed
high-quality animal foods before the development of stone tools
and the origin of the genus Homo.
-----------
M. Sponheimer and J.A. Lee-Thorp: Isotopic evidence for the diet
of an early hominid, Australopithecus africanus.
(Science 15 Jan 99 283:368)
QY: Matt Sponheimer, Rutgers Univ. New Brunswick 908-932-8789.
-----------
Text Notes:
... ... *Laetoli: The discovery involved a 30-yard-long footprint
trail of three bipedal individuals, the trail made approximately
3.6 million years ago in a newly deposited layer of volcanic ash.
The ash layer also contains the prints of many other animal
forms.
... ... *Note #1: The famous skeleton of "Lucy", discovered in
the 1970s, is an example of an Australopithecus afarensis fossil.
... ... *kitchen middens: In general, a "midden" is a refuse
heap. In this context, a "kitchen midden" is a mound consisting
of shells of edible molluscs and other refuse, the mound marking
the site of a prehistoric human habitation.
... ... *Makapansgat: (Makapansgaat) A cave site northeast of
Johannesburg.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 9Apr99
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ANTHROPOLOGY: DEFINING THE HUMAN GENUS
Taxonomy is the field in biology devoted to the classification of
living organisms, and the term "taxon" is used to indicate the
members of any particular group in the classification scheme:
class, family, genus, etc [*Note #1]. The general problem is how
to incorporate information about evolutionary history and
adaptation into taxonomic classification schemes, with the
problem exemplified by attempts to define our own genus, Homo.
The definition of the genus Homo has always been somewhat
contentious, not least because it is tied, implicitly or
explicitly, to the state of "being human". A series of anatomical
characteristics is found to be apparently unique in Homo -- for
example, an increase in cranial vault height and cranial
thickness, reduced lower facial prognathism (i.e., reduced
projection of the jaw), and in dentition reduction in the size of
the premolars and molars and the length of the molar row -- but
what has always been emphasized by taxonomic definitions is the
size of the Homo brain as revealed by the size of the cranium.
According to the classical scheme, to be Homo is to be a large-
brained *hominine (the apparent human-related fossil group), a
hominine presumably more technologically accomplished than the
ancestor group, the *Australopithecines. At the present time, as
is evident in this report, the taxonomic categorizations related
to the human genus are far from fixed. ... ... B. Wood and M.
Collard (2 installations, US UK) present an extensive review of
the taxonomic situation concerning the genus Homo, the authors
making the following points:
1) The authors suggest that recent data, fresh
interpretations of the existing evidence, and the limitations of
the paleoanthropological record invalidate existing criteria for
allocating fossil species to Homo.
2) The authors suggest that regardless of any formal
definitions, in current practice fossil hominin species are
assigned to Homo on the basis of one or more out of 4 criteria:
a) absolute brain size at least 600 cubic centimeters; b)
possession of language as inferred from *endocranial casts; c)
possession of a modern human-like precision grip involving a
well-developed and opposable thumb (pollex); d) the ability to
manufacture stone tools. The authors state: "It is now evident,
however, that none of these criteria is satisfactory."
3) The authors present a revised definition for the genus
Homo based on criteria considered verifiable and conclude that
two species, *Homo habilis and *Homo rudolfensis, do not belong
in the genus. The authors suggest the earliest taxon to satisfy
the criteria is *Homo ergaster, or early African *Homo erectus,
which currently appears in the fossil record at about 1.9 million
years ago.
4) The authors suggest that a fossil species should be
included in Homo only if the following can be demonstrated:
... ... a) the species is more closely related to H. sapiens than
it is to the australopiths;
... ... b) the species has an estimated body mass more similar to
that of H. sapiens than to that of the australopiths;
... ... c) the species has reconstructed body proportions that
match those of H. sapiens more closely than those of the
australopiths;
... ... d) the species has a *postcranial skeleton whose
functional morphology is consistent with modern human-like
obligate bipedalism and limited facility for climbing;
... ... e) the species is equipped with teeth and jaws that are
more similar in terms of relative size to those of modern humans
than to those of the australopiths;
... ... f) the species shows evidence for a modern human-like
extended period of childhood growth and development.
5) The authors conclude by suggesting that the adoption of
the above criteria would mean the genus Homo would have both
phylogenetic and adaptive significance. "Researchers can then
explore whether this adaptive shift in hominin evolution
corresponds with changes in climate, analogous evolutionary
changes in other large mammal groups, particular innovations in
the hominin cultural record, substantial expansions in geographic
range, or changes in ecological tolerance as reflected in
reconstructions of hominin habitats."
-----------
B. Wood and M. Collard: The human genus.
(Science 2 Apr 99 284:65)
QY: Bernard Wood [bwood@gwu.edu]
-----------
Text Notes:
... ... *Note #1: The conventional hierarchy of classification in
biology is Kingdom, Phylum, Class, Order, Family, Genus, and
Species. In the literature, organisms are usually referred to by
genus and species in binomial nomenclature, with the genus
capitalized. Human beings are genus Homo, species sapiens,
binomially Homo sapiens. The convention in binomial nomenclature
is to initialize the genus; thus: H. sapiens.
... ... *hominine: The terms hominine, hominin, hominoid,
hominid, are not interchangeable, but their classification
criteria are variously in a state of flux. In general, the
hominoids are a primate superfamily, the hominid family comprises
the great apes within the hominoid superfamily, the hominini are
a "tribe" within the hominids characterized by a number of
features including bipedalism, and the hominini are further
partitioned into the genera Homo and Australopithecus. Concerning
research in human evolution, most paleoanthropologists agree that
what is important is to achieve an understanding of the
evolutionary transitions and transformations, and any
classification scheme must be secondary to this objective. In
other words, in this context, classification must ultimately
reflect phylogeny (the actual evolutionary relationships), and as
knowledge of phylogeny changes, so must the extant classification
schemes.
... ... *Australopithecines: Members of the now extinct genus
Australopithecus, believed to exist between 4.4 and 1 million
years ago, and believed to be precursors of the genus Homo. All
australopithecines are apparently characterized by an ape-like
form, rather than the human-like form of the Homo genus.
... ... *endocranial casts: In general, an "endocast" (steinkern)
is any fossil formed after dissolution of an interior molding
substance. An "endocranial cast" is an endocast involving the
cranium. The interior of the endocast can often reveal details
concerning the absent soft interior substance (in this case, the
brain).
... ... *Homo habilis: In 1964, an early fossil hominin (1.9 to
1.6 million years before the present) was found in Olduvai,
Tanzania, the brain apparently intermediate in size between the
earliest known Homo fossil *Homo erectus and the Australopithecus
group. This new fossil was denoted as a new species by its
discoverers and named Homo habilis. The original set of H.
habilis fossils included a relatively complete hand, its
structure apparently compatible with an ability to make and use
tools. (Homo habilis literally means "handy-man") Considerable
controversy in the paleoanthropology community concerning H.
habilis has continued from 1964 until the present.
... ... *Homo rudolfensis: The original H. habilis species has
more recently been divided into H. habilis and H. rudolfensis,
after a fossil of the latter group was discovered in 1993 and
related to an earlier find in 1967, both dating at approximately
2.4 million years ago. One view is that Homo habilis/rudolfensis
evolved in Africa approximately 2 million years ago and quickly
expanded into Asia to become *Homo erectus/ergaster.
... ... *Homo ergaster: H. ergaster and *H. erectus are the two
immediate precursors of H. sapiens, with H. ergaster believed to
have originated in Africa and to have given rise to H. erectus in
Asia. But as with other hominid fossil groups, precise
evolutionary sequences and geographical loci continue to be
debated.
... ... *Homo erectus: First discovered by Eugene Dubois in 1891
in Indonesia, this fossil group is currently viewed as the
closest precursor to H. sapiens. Formerly called "Anthropithecus
erectus" and "Pithecanthropus erectus". Pithecanthropus erectus
and Sinanthropus erectus ("Peking man", discovered in 1927) were
in 1951 subsumed under the single category Homo erectus, which
was then recognized as a widespread species exhibiting
significant geographical variation.
... ... *postcranial skeleton: In general, this refers to the
skeleton behind the cranium in a quadruped and below the cranium
in a biped.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 9Jul99
For more information: http://scienceweek.com/swfr.htm
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
3. MEDICAL BIOLOGY:
PLAGUE AS A BIOLOGICAL WEAPON
Plague, also called bubonic plague or "Black Death", is a disease
with a notorious history. It is caused by the rod-shaped
bacterium (bacillus) Yersinia pestis, which infects wild rodents.
The bubonic variant of the disease is transmitted to humans from
rodents by the bite of an infected flea. Human to human
transmission occurs by inhalation of respiratory droplets spread
by the cough of patients with plague who have developed pulmonary
lesions, and the result of this is "primary pneumonic plague",
which differs from "bubonic plague" in that bubonic plague
affects the lymph nodes, among other tissues (producing "buboes",
lymph node swellings). The last plague pandemic began in Hong
Kong in 1894 and spread throughout the world. Plague still exists
as an endemic disease in many parts of the world, including the
southwestern U.S.
... ... T.V. Inglesby et al (19 authors at 10 installations, US)
present a review of medical and public health considerations of
plague as a biological weapon, the authors making the following
points:
1) In 541 AD, the first recorded plague pandemic began in
Egypt and spread across Europe with consequent population losses
of 50 to 60 percent in North Africa, Europe, and central and
southern Asia. The second plague pandemic, also known as the
"Black Death" or "Great Pestilence", began in 1346 and eventually
killed 20 to 30 million people in Europe, approximately one-third
of the European population of that time. Plague spread slowly and
inexorably from village to village by infected rats and humans or
more quickly from country to country by ships. This second
pandemic lasted more than 130 years and had major political,
cultural, and religious ramifications. The third pandemic began
in China in 1855, spread to all inhabited continents, and
ultimately killed more than 12 million people in India and China
alone. Small outbreaks of plague continue to occur throughout the
world.
2) Advances in living conditions, public health, and
antibiotic therapy have made future pandemics of plague
improbable, but plague outbreaks following use of a biological
weapon are a plausible threat. In World War II, a secret branch
of the Japanese army ("Unit 731") is reported to have dropped
plague-infected fleas over populated areas of China, thereby
causing outbreaks of plague. In subsequent years, the biological
weapons programs of the US and the Soviet Union developed
techniques to aerosolize plague directly, eliminating dependence
on the unpredictable flea vector. In 1970, the World Health
Organization (WHO) reported that, in a worst-case scenario, if 50
kilograms of the plague pathogen Y. pestis were released as an
aerosol over a city of 5 million people, pneumonic plague could
occur in as many as 150,000 persons, 36,000 of whom would be
expected to die. The plague bacilli would remain viable as an
aerosol for 1 hour for a distance of up to 10 kilometers.
Significant numbers of inhabitants might attempt to flee, further
spreading the disease.
3) Although US scientists did not succeed in making
quantities of plague organisms sufficient for use as an effective
weapon by the time the US offensive program was terminated in
1970, Soviet scientists were able to manufacture large quantities
of the agent suitable for placing into weapons. More than 10
institutes and thousands of scientists were reported to have
worked with plague in the former Soviet Union.
4) There is little published information indicating actions
of autonomous groups or individuals seeking to develop plague as
a weapon. However, in 1995 in Ohio (US), a microbiologist with
suspect motives was arrested after fraudulently acquiring Y.
pestis by mail. New US antiterrorism legislation was introduced
in reaction to this incident.
5) The authors conclude: "An aerosolized plague weapon could
cause fever, cough, chest pain, and *hemoptysis with signs
consistent with severe pneumonia 1 to 6 days after exposure.
Rapid evolution of disease would occur in 2 to 4 days after
symptom onset and would lead to septic shock with high mortality
without early treatment."
-----------
T.Y. Inglesby et al: Plague as a biological weapon.
(J. Amer. Med. Assoc. 3 May 00 283:2281
QY: Thomas V. Inglesby [tvi@jhsph.edu]
-----------
Text Notes:
... ... *hemoptysis: (bronchostaxis) Spitting of blood derived
from the lungs or bronchial tubes as a result of pulmonary or
bronchial hemorrhage.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 23Jun00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON BIOLOGICAL TERRORISM
Although political events during the past decade have reduced
fears of biological warfare among the major powers, the pathogens
likely to be used in such warfare have not vanished, and there is
now an apparent growing concern about the use of such pathogens
in acts of biological terrorism ("bioterrorism").
... ... Donald A. Henderson (Johns Hopkins University, US)
presents a review of current views, expectations, and contingency
plans, the author making the following points concerning the
scientific fundamentals: 1) The expected scenario following
release of an aerosol cloud of a biological agent is entirely
different from that following an attack of nuclear or chemical
terrorism. A biological agent aerosol release could be silent and
would almost certainly be undetected. The cloud would be
invisible, odorless, and tasteless. It would behave much like a
gas in penetrating interior areas, and the release would not be
suspected for days or weeks later. 2) The implicit assumption has
frequently been that chemical and biological threats and the
responses to them are so generically similar that they can be
readily handled by a single "chembio" expert, usually a chemist.
This is a serious misapprehension. 3) Any of thousands of
biological agents that are capable of causing human infection
could be considered a potential biological weapon, but
realistically only a few pose serious problems. Only a very small
number of species of these pathogens can be cultivated and
dispersed effectively so as to cause cases and deaths in numbers
that would threaten the functioning of a large community. The
current consensus is that there are 11 pathogens "very likely to
be used." *Smallpox, *plague, *anthrax, and *botulism are
considered the top four candidates. The others are *tularemia,
*glanders, *typhus, *Q fever, *Venezuelan equine encephalitis,
*Marburg virus, and *influenza virus. 4) Any group with
sufficient resources could purchase prepared supplies of
aerosolizable organisms and could transport them easily, because
only small quantities are needed to inflict casualties over a
wide area. No mechanisms currently exist for screening to
intercept such materials at state or national borders. 5) Of the
potential biological weapons, smallpox and anthrax pose by far
the greatest threats, but these pathogens have different clinical
and epidemiological properties. Smallpox poses an unusually
serious threat, in part because virtually everyone is now
susceptible, vaccination having stopped worldwide 20 or more
years ago as a result of the eradication of the disease. It is
probable that no more than 20 percent of the world population is
protected; for the unprotected, fatality rates after infection
are 30 percent. Another problem is that there are no longer any
manufacturers of smallpox vaccine, which means large-scale
vaccination immediately after an outbreak is currently not
possible. 6) Concerning an inhalation anthrax epidemic, the
scenario is as dangerous as that for smallpox. After 2 to 3 days
anthrax-infected individuals would appear in emergency rooms and
doctors' offices with a variety of nonspecific symptoms such as
fever, cough, and headache. Within a day or two, patients would
become critically ill and then die within 24 to 72 hours. The
fatality rate for anthrax is 80 percent or greater. 7) The author
concludes: "Once the medical community rallied... in educating
peoples and policymakers everywhere about the dread realities of
a nuclear winter. Perhaps the same should now be done with
respect to the realities of biological weapons, which are now
considered to be a more serious threat than the nuclear ones."
-----------
Donald A. Henderson: The looming threat of bioterrorism.
(Science 26 Feb 99 283:1279)
QY: Donald A. Henderson, Johns Hopkins Center for Civilian
Biodefense Studies, Johns Hopkins Univ., Baltimore, MD 21202 US.
-----------
Text Notes:
... ... *Smallpox: This is an acute eruptive contagious disease
caused by a poxvirus (Orthopoxvirus, a member of the family
Poxviridae). The average incubation period is 8 to 14 days.
Following the incubation period, the onset symptoms are
constitutional: chills, high fever, backache, headache. In from 2
to 5 days, these symptoms subside and the skin eruptions appear.
Considering the temporal course of the disease, a smallpox
epidemic would probably not become evident until 2 to 3 weeks
after release of an aerosol.
... ... *plague: In this context, this term refers to the acute
infectious disease caused by the bacterium Yersinia pestis, the
disease marked by high fever, toxemia, and prostration. The
pathogen is usually transmitted to man by fleas that have bitten
infected rodents, and there are various forms of the disease. The
incubation period is 2 to 7 days. The fatality rate is near 50
percent, with usually 100 percent fatality for the pneumonic form
of the disease.
... ... *anthrax: This disease is caused by the bacterium
Bacillus anthracis, and is usually transmitted by infected
animals through traumatized human skin. The disease is marked by
hemorrhage and blood effusions in various organs and body
cavities, and by symptoms of extreme prostration. In the context
of this report, the disease entity of concern is "inhalation
anthrax", which is a more serious human disease than anthrax
contracted from an animal through the skin. Inhalation anthrax
produces hemorrhagic pneumonia with shock and is usually fatal
(fatality above 80 percent).
... ... *botulism: This disease is caused by toxins of the
bacterium Clostridium botulinum, an organism common in soil and
sometimes in animal feces. Symptoms appear 18 to 24 hours after
entry of the toxins, and the most severe symptoms are the result
of effects on the neuromuscular system. Death occurs from
respiratory paralysis or cardiac arrest. The fatality rate is
high. Ordinarily, botulism is not an actual human infection,
since the human disease is almost always caused by ingestion of
food contaminated with toxins produced by C. botulinum, which is
anaerobic and grows only under conditions of low or absent oxygen
(e.g., in canned foods). The botulinum toxins are among the most
highly toxic substances known: the lethal dose for a human is
estimated to be in the range 1 to 2 micrograms.
... ... *tularemia: This disease is caused by the bacterium
Francisella tularensis, a pathogen usually transmitted to humans
by biting arthropods (e.g., insects), direct contact with
infected animal tissue, ingestion of contaminated food or water,
and inhalation of aerosols. Apparently, inhalation of only 50
individual F. tularensis bacteria can result in infection.
Symptoms appear within a week. The disease can usually be
controlled with antibiotics.
... ... *glanders: A common disease of horses, mules, and
donkeys, caused by the bacterium Burkholderia mallei. The
inhalation form of the disease may lead to primary pneumonia.
The disease can usually be controlled with antibiotics.
... ... *typhus: A group of acute infectious and contagious
diseases caused by the bacterial group Rickettsaie. These
diseases are characterized by fever, headache, malaise, and
prostration.
... ... *Q fever: Also caused by a Rickettsaie bacterium, but the
symptoms resemble influenza, nonbacterial pneumonia, hepatitis,
or encephalopathy.
... ... *Venezuelan equine encephalitis: This is a viral disease
usually transmitted by mosquitoes from horses to humans. It is
caused by a togavirus, subgroup alphavirus. In humans, the
symptoms are similar to those of influenza.
... ... *Marburg virus: One of the two notorious African
Hemorrhagic Fevers (the other is Ebola virus), highly virulent,
with infections usually ending in death. These viruses have the
highest mortality rate (as much as 90 percent) of all the viral
hemorrhagic fevers. The disease was first recognized in 1967.
... ... *influenza virus: Any of a group of influenza viruses,
all of the family Orthomyxoviridae. The influenza diseases
usually have a sudden onset, are highly contagious, and easily
produce large-scale epidemics. Apparently, if only a few cells of
the respiratory *epithelium are infected by deposited virus
particles, the infection can proceed. The severity of symptoms
and the outcome depends on which strain of the virus is the
pathogen.
... ... *epithelium: In animals and humans, epithelial cells
compose the cell layers that form the interface between a tissue
and the external environment, for example, the cells of the skin,
the lining of the intestinal tract, and the lung airway passages.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 21May99
For more information: http://scienceweek.com/swfr.htm
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
4. THEORETICAL PHYSICS:
CHAOS AND NONLINEAR DYNAMICS
In general, a nonlinear dynamical system is a system
described by time-dependent differential equations such that the
rates of change of one or more dependent variables of the system
depend in a nonlinear fashion on the variables themselves.
Certain nonlinear dynamical systems, some of which are of great
scientific interest, exhibit "chaotic dynamics". In this context,
the term "chaos" refers to unpredictable behavior arising in a
system that obeys deterministic laws but exhibits
unpredictability. The essential idea is that in certain systems
small perturbations may produce a cascade of larger
perturbations, so that eventually the behavior of such systems
cannot be predicted from prior states no matter if the systems
appear simple and obey deterministic laws. Examples of chaotic
nonlinear dynamical systems are the weather and populations of
organisms, and instances of chaotic dynamics have now been
documented in most scientific disciplines.
Because the differential equations for many nonlinear
systems are often intractable (i.e., no explicit quantitative
solutions are possible), a focus of theoretical research on
nonlinear systems has been on analysis of the qualitative
behavior of such systems, in particular on analysis of the "phase
space" and "trajectories" in the phase spaces of such systems.
The idea is essentially as follows: If the state of a system
depends upon N variables, the instantaneous state of the system
can be viewed as a point (phase point) in an N-dimensional space
(phase space; system hyperspace), and as the state of the system
changes, its phase point can be viewed as describing a trajectory
in its phase space. Qualitative analysis of the possible families
of solutions of nonlinear differential equations can provide
information about such phase space trajectories, and there are
certain real systems for which qualitative analysis of the phase
space trajectories of the system has revealed significant
properties of the system otherwise difficult to delineate.
... ... J.P. Gollub and M.C. Cross (2 installations, US) present
a commentary on recent research on chaotic nonlinear dynamics,
the authors making the following points:
1) The techniques of nonlinear dynamics are well-developed,
but the impact of this field has been largely confined to
phenomena in which there are only a few important time-dependent
quantities. Unfortunately, this excludes a vast range of
important problems in which the behavior of one point in space
can be quite different (though statistically similar) to that at
another location. A particular example is convective behavior.
2) The traditional approach to studying nonlinear dynamical
behavior is to plot the dynamical variables of the system as a
multidimensional phase space graph indicating how the behavior
changes over time. For example, a simplified model of the Solar
System consisting of the Sun and 9 planets would require a phase
space with as many as 60 dimensions (3 position and 3 momentum
coordinates for each body). In the case of a convecting fluid, a
complete description of the flow pattern requires knowledge of
the velocity and temperature at a very large number of locations,
so the number of dimensions of the phase plot are enormous (from
thousands to millions, depending on the desired spatial
resolution). As a result, the methods of nonlinear dynamics are
cumbersome and progress has been slow, even though many
interesting examples of spatiotemporal chaos have been explored
both experimentally and numerically.
3) Recent research (D.A. Egolf et al: Nature 404:733 2000)
involving numerical studies of an accepted model of thermal
convection indicates that the origin of unpredictable motion in
chaotic thermal convective systems, at least in one particular
form of spatiotemporal chaos, lies in what occurs in small
regions of space and over short time-scales. These local changes
in the organization of the flow affect the surrounding regions in
such a way that the entire future evolution is affected. The
authors state: "This is something akin to Ed Lorenz's famous
remark [E.N. Lorenz: J. Atmos. Sci. 20:130 1963] that the
localized flapping of a butterfly's wings might change the
weather dramatically over the entire world a few weeks later."
Although such sensitivity to localized fluctuations has never
been confirmed as the source of the unpredictability of the
weather, it is apparently the origin of chaotic dynamics in
thermal convection.
4) The authors conclude: "The methods used by Egolf et al
should apply to many other forms of chaos in spatially extended
systems (physical, chemical, and biological) for which reliable
model equations are available, so that the key processes leading
to the complex dynamics can be identified. Applications to areas
as diverse as cardiology and atmospheric dynamics might be
expected eventually. Moreover, it is not unreasonable to imagine
that insight into the processes leading to unpredictability will
also lead to progress in modifying or controlling the dynamics of
these systems."
-----------
J.P. Gollub and M.C. Cross: Chaos in space and time.
(Nature 13 Apr 00 404:710)
QY: J.P. Gollub [jgollub@haverford.edu]
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 23Jun00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
EXPERIMENTAL EVIDENCE FOR MICROSCOPIC CHAOS
In the study of physical systems, the term "chaotic behavior" has
a specific meaning: the behavior of a system is said to be
"chaotic" if its final state is so sensitive to the system's
precise initial conditions that the behavior of the system is in
effect unpredictable and cannot be distinguished from a random
process, even though the behavior of the system is strictly
determinate in a mathematical sense. In other words, a
deterministic system characterized by extremely sensitive
instabilities, despite the system being determinate, can exhibit
behavior that is unpredictable, and the system is then called
"chaotic". During the past several decades, the analysis of such
chaotic systems has intrigued both physicists and mathematicians.
In general, in the study of physical systems, the term "phase
space" refers to a multidimensional space, each point of which
(phase point) completely represents the state of the system. For
example, in the study of dynamical systems, each phase point in
the phase space completely represents the values of all the
generalized coordinates and corresponding momenta. As the phase
point of a system moves in the phase space (e.g., changes with
time), the phase point follows a trajectory in the phase space,
and this trajectory is called the "phase point trajectory". In
the mathematical analysis of a particular phase space and its
phase point trajectories, "*Lyapunov exponents" are coefficients
that describe the rates at which nearby phase point trajectories
converge or diverge, and the Lyapunov exponents can be shown to
provide estimates of how long the behavior of a dynamical system
is predictable before chaotic behavior sets in. Chaotic behavior
of a system is characterized by the existence of positive
Lyapunov exponents. ... ... Gaspard et al present the results of
an experimental study of "microscopic chaos". The authors point
out that many macroscopic dynamical phenomena, for example in
hydrodynamics and oscillatory chemical reactions, have been
observed to display erratic or random time evolution, despite the
deterministic character of their dynamics -- a phenomenon known
as "macroscopic chaos". On the other hand, it has been long
supposed that the existence of chaotic behavior in the
microscopic motions of atoms and molecules in fluids or solids is
responsible for their equilibrium and non-equilibrium
properties. But, the authors state, this hypothesis of
microscopic chaos has never been verified experimentally. The
authors now report direct experimental evidence for microscopic
chaos in fluid systems, the study involving the *observation of
brownian motion of a colloidal particle suspended in water. The
authors report finding a positive lower bound on the sum of
positive Lyapunov exponents of the system composed of the
brownian particle and the surrounding fluid. They suggest their
results and quantitative analysis provide strong experimental
evidence for microscopic chaos. They conclude: "On the assumption
that the system is deterministic, and given our knowledge of the
molecular structure of the fluid, this evidence supports, in
particular, the hypothesis that large systems -- which may be
treated by statistical mechanics -- are typically chaotic. The
result also supports the role of dynamical instability in non-
equilibrium fluids."
-----------
P. Gaspard et al (7 authors at 3 installations, BE US):
Experimental evidence for microscopic chaos.
(Nature 27 Aug 98 394:865)
QY: P. Gaspard
-----------
Text Notes:
... ... *Lyapunov: A.M. Lyapunov (1857-1918) developed a general
theory of dynamic stability applicable to both linear and
nonlinear systems. His work was largely buried and forgotten
until it was exhumed nearly 30 years after his death.
... ... *observation of brownian motion: The experiment here
involved a colloidal particle of 2.5 microns diameter moving in
suspension in deionized water at 22 degrees Celsius, with
recorded observations of 145,612 positions over a total time
interval of approximately 2430 seconds, the observations
involving a microscope and video camera, the smallest resolution
stated as 25 nanometers. Particles of this size undergo
sedimentation, which may confound the results with non-Brownian
effects, but the authors report studies of non-sedimenting
smaller particles substantiate their observations, the larger
particle simply allowing tracking observations for a longer time.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 18Sep98
For more information: http://scienceweek.com/swfr.htm
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
5. ATMOSPHERIC CHEMISTRY:
REACTIONS AT ATMOSPHERIC INTERFACES
In recent decades, it has become apparent that the
atmosphere of the Earth is a dynamic chemical system with a wide
variety of interactions among its constituents, interactions that
are important for both climate and the biosphere.
In chemistry, the term "heterogeneous" refers to a mixture
of phases (e.g., solid-gas or liquid-gas), and a "heterogeneous
chemical reaction" is any reaction involving entities from two
different phases.
The term "troposphere" refers to the lowest 10 to 20
kilometers of the atmosphere (with the lower boundary the surface
of the Earth), and the term "stratosphere" refers to the layer
above the troposphere up to approximately 50 kilometers of the
atmosphere.
The term "aerosol" refers to a dispersion in which a finely
divided solid is suspended in air and the particles are of
colloidal dimensions. The term "colloidal dimensions" refers to
the range approximately 1 nanometer to 100 nanometers in
diameter.
... ... John H. Seinfeld (California Institute of Technology, US)
presents a commentary on current research in atmospheric
chemistry, the author making the following points:
1) Reactions within or on the surfaces of liquid or solid
particles are known to play an important role in the
stratosphere, but much less is known about such heterogeneous
chemistry in the troposphere. What little is known has focused on
the ubiquitous aqueous droplets that have generally been assumed
to behave as well-mixed microscopic chemical reactors. But this
view is probably too simplistic, since reactions at the droplet
surface may also make a significant contribution to tropospheric
heterogeneous chemistry.
2) There exists a large variety of tropospheric surfaces,
ranging from sea salt, mineral dust, and combustion-generated
aerosols, to the surface of the Earth itself. Together with the
extensive array of gas-phase species in the troposphere, this
leads to a wide range of potential heterogeneous interactions.
Although knowledge of tropospheric heterogeneous chemistry is
incomplete, some key processes have been identified. These
include conversion of NO(subx) [NO + NO(sub2)] to HNO(sub3) by
the reaction of N(sub2)O(sub5) with moist aerosols and
heterogeneous hydrolysis of N(sub2)O(sub3) to form nitrous acid
(HONO). Photochemical production of formaldehyde at the air-snow
interface constitutes an important local HCHO source in Arctic
surface air. There is also recent speculation that mineral dust,
long believed to be a nonreactive substrate, can interact with
N(sub2)O(sub5), O(sub3), and HO(sub2) radicals to influence gas-
phase chemistry.
3) Despite its benign source, sea salt aerosol has proved to
be a surprisingly complex and interactive tropospheric chemical
constituent. It has been recognized for some time that HCl
volatilizes from sea salt aerosols as a result of displacement by
strong acids such as HNO(sub3) or H(sub2)SO(sub4) gases. More
recently, atmospheric measurements, laboratory experiments, and
kinetic simulations have shown that reactive chlorine-containing
gases may be released from sea salt aerosols by heterogeneous
processes.
-----------
John H. Seinfeld: Meeting at the interface.
(Science 14 Apr 00 288:285)
QY: John H. Seinfeld [seinfeld@caltech.edu]
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 23Jun00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
DIRECT OBSERVATION OF HETEROGENEOUS ATMOSPHERE CHEMISTRY
... "Aerosol time-of-flight mass spectrometry" is a technique for
determining the size and chemical composition of single
particles, and by sampling many particles consecutively, the
properties of a particle mixture can be defined as it changes
over time. The essentials of the technique involve 1)
accelerating the particle to a terminal velocity determined by
its mass; 2) measuring the velocity (from which the mass is
calculated); 3) laser desorption and ionization of the particle;
4) analysis of the ions to determine the chemical composition of
the desorbed species. ... ... Gard et al (12 authors at 2
installations, US) report a study of the heterogeneous
replacement of chloride by nitrate in individual sea-salt
particles in the troposphere, the procedure involving continuous
monitoring over time with the use of aerosol time-of-flight mass
spectrometry. Modeling calculations show that the observed
chloride displacement process is consistent with a heterogeneous
chemical reaction between sea-salt particles and gas-phase nitric
acid, leading to sodium nitrate production in the particle phase,
accompanied by liberation of gaseous HCl from the particles. The
authors suggest such single-particle measurements, combined with
a single- particle model, make it possible to monitor and explain
heterogeneous gas/particle chemistry as it occurs in the
atmosphere, and provide a new approach to the study of
atmospheric chemistry.
-----------
QY: Kimberly A. Prather [prather@citrus.ucr.edu]
(Science 20 Feb 98) (Science-Week 5 Mar 98)
For more information: http://scienceweek.com/swfr.htm
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
6. CLIMATOLOGY:
ANTHROPOGENIC ATMOSPHERIC AEROSOLS AND GLOBAL CLIMATE CHANGE
As was pointed out in the previous report in this issue (SW 23
Jun 00 report #5), aerosols are an important component of
atmospheric chemical dynamics, with the contributions of aerosols
to global and regional climate far from simple.
... ... S.E. Schwartz and P.R. Buseck (2 installations, US)
present a commentary on recent research on anthropogenic
atmospheric aerosols, the authors making the following points:
1) Most considerations of global climate change caused by
human activities have focused on the warming influence of
greenhouse gases. However, aerosols are another important
atmospheric constituent that influences climate and that has been
affected by human activities. In general, aerosol particles
increase scattering and absorption of shortwave (solar)
radiation, increase cloud reflectance, enhance cloud lifetimes,
and suppress precipitation. These phenomena are all thought to
exert a cooling influence on climate. Recent data indicate that
anthropogenic aerosols reduce cloud droplet size and suppress
precipitation downward of major urban areas and industrial
facilities, which is consistent with earlier hypotheses.
2) The influences of aerosols on climate are more complex
than those of greenhouse gases. Bulk aerosol composition is
highly variable spatially and temporally because of different
sources and production mechanisms and short atmospheric residence
times (from less than a day to more than a month). Particles
sizes range from nanometers to microns, and within the same size
class, particles can exhibit widely different compositions and
morphologies, with different constituents present within the same
particle (e.g., 10 nanometer carbon spherules can be found
embedded within much larger sulfate particles). The
inhomogeneities in properties and geographical distribution of
aerosols make it difficult to characterize their influences on
climate and to represent these influences in models.
3) Recent analysis of the consequence of absorption of
shortwave radiation by aerosols indicates that the heating of the
atmosphere can evaporate clouds. Clouds exert both cooling and
warming influences on climate: cooling in the shortwave (because
of their reflectance), and warming in the longwave (because of
absorption and re-emission of thermal infrared radiation). The
shortwave component dominates, so a reduction in cloud coverage
would result in a net warming influence.
5) The authors conclude: "Recent studies demonstrate both
the importance of aerosol effects on climate and the complexity
of aerosol-cloud interactions. Unfortunately for those would like
a quick and accurate assessment of anthropogenic climate forcing
over the industrial period, the studies also demonstrate that
there is much to be learned before such an assessment can
confidently be given."
-----------
S.E. Schwartz and P.R. Buseck: Absorbing phenomena
(Science 12 May 00 288:989)
QY: S.E. Schwartz [ses@bnl.gov]
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 23Jun00
For more information: http://scienceweek.com/swfr.htm
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7. IN FOCUS:
ON FUNDAMENTAL PARTICLES AND FUNDAMENTAL FORCES
"Although many different kinds of elementary particles have been
discovered, almost all the matter in the Universe is made of four
kinds: protons, neutrons, electrons, and neutrinos. These
interact via four basic forces: gravity, electromagnetism, and
the strong and weak nuclear forces. Each of these forces is
characterized by a few numbers. Each has a range, which tells us
the distance over which the force can be felt. Then, for each
kind of particle and each force, there is a number which tells us
the strength by which that particle participates in interactions
governed by that force. These are called the coupling constants.
One of these is the electrical charge, which tells how strongly a
particle may interact, or be attracted by, other charged
particles. The parameters of the standard model consist primarily
of the particles and these numbers that characterize the four
forces... The strength by which any particle is affected by
gravity is proportional to its mass. The actual force between two
bodies is given by multiplying the two masses together, and then
multiplying the result times a universal constant. This constant
is called Newton's gravitational constant; it is one of the
parameters of the standard model. The most important thing to
know about it is that it is a fantastically small number. Its
actual value depends on the units we use, as is the case with
many physical constants. For elementary particle physics, it is
natural to take units in which mass is measured by the proton
mass. In these units you or I have a mass of about 10^(28), for
that is how many protons or neutrons it takes to make a human
body. By contrast, in these units the gravitational constant is
about 10^(-38). This tiny number measures the strength of the
gravitational force between two protons. The incredible smallness
of the gravitational constant is one of the mysteries associated
with the parameters of particle physics. Suppose we had a theory
that explained the basic forces in the Universe. That theory
would have to produce, out of some calculation, this ridiculous
number, 10^(-38). How is it that nature is so constructed that
one of the key quantities that govern how it works at the
fundamental level is so close to zero, but still not zero? This
question is one of the most important unsolved mysteries in all
of physics."
-----------
Lee Smolin: _The Life of the Cosmos_
(Oxford University Press, London 1997)
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