Abstract. We suggest a chemical model for the composition, structure, and atmospheric processing of organic aerosols. This model is stimulated by recent field measurements showing that organic compounds are a significant component of atmospheric aerosols. The proposed model organic aerosol is an "inverted micelie" consisting of an aqueous core that is encapsulated in an inert, hydrophobic organic monolayer. The organic materials that coat the aerosol particles are surfactants of biological origin. We propose a chemical mechanism by which the organic surface layer will be processed by reactions with atmospheric radicals. The net result of an organic aerosol being exposed to an oxidizing atmosphere is the transformation of an inert hydrophobic film to a reactive, optically active hydrophilic layer. Consequently, processed organic aerosols can grow by water accretion and form cloud condensation nuclei, influencing atmospheric radiative transfer. Radiative transfer may be affected directly by the chromophores left on the surface of the aerosol after chemical transformation. The chemical model yields certain predictions which are testable by observations. Among them is a curve of the percent organic material as a function of particle diameter which predicts that a high fraction of the mass of the upper tropospheric aerosol will be organic. Atmospheric processing of organic aerosols will lead to the release of small organic fragments into the troposphere which will play a subsequent role in homogeneous chemistry. Organic aerosols are likely to act as a transport vehicle of organics and other water insoluble compounds into the atmosphere. We speculate that biomass burning will produce a similar coating of surfactants derived from land sources. Finally, it is pointed out that the radical-induced transformation of the surface layer of aerosol particles from hydrophobic to hydrophilic offers an additional means by which the biosphere, through atmospheric chemistry, can affect the radiative balance.

Halley Bay (75.5øS, 26.8øW) that has occurred annually since the mid-1970s in September and October. This Antarctic ozone "hole" which is characterized by 50% or more decreases in column ozone has now been extensively studied using satellite, aircraft, and ground-based instruments. In addition, further analyses of satellite and ground-based Dobson data have shown northern hemisphere (30øN to 64øN) The Halogen Occultation Experiment (HALOE) was conceived to provide critical data for study of the ozone distribution and those processes which affect ozone levels. The experiment uses the principle of satellite solar occultation to sound the stratosphere, mesosphere, and lower thermosphere. Using this technique, absorption of solar energy in selected spectral bands is used to infer vertical profiles of temperature, pressure, and mixing ratios of key gases involved in the ozone chemistry. The HALOE instrument includes both broadband and gas filter channels Together, these observations form a minimum but adequate set which can be used to derive, under appropriate conditions, the unmeasured concentrations of several other gases needed to test understanding of the chemistry (see Figure 1). In this regard, HALOE 03, H20, and CH4 measurements, for example, can be used to derive OH levels. In turn, these parameters can be used to derive atomic chlorine and, from that, C10 through reactions with 03. Chlorine monoxide can be used with NO2 observations to derive chlorine nitrate (C1ONO2). Since C10

[1] Surface analyses of atmospheric aerosols from different continental sources, such as forest fires and coal and straw burning, show that organic surfactants are found on such aerosols. The predominant organic species detected by time-of-flight secondary ion mass spectrometry on the sulfate aerosols are fatty acids of different carbon chain length up to the C 32 acid. These observations are consistent with literature accounts of functional group analysis of bulk samples, but this is the first direct evidence of fatty acid films on the surface of sulfate aerosols. Surface analysis leads to the conclusion that fatty acid films on continental aerosols may be more common than has been previously suggested.

Abstract. Both satellite water vapor measurements and in situ aircraft measurements indicate that the southern hemisphere lower stratosphere is drier than that of the northern hemisphere in an annual average sense. This is the result of a combination of factors. At latitudes poleward of -50øS, dehydration in the Antarctic polar vortex lowers water vapor mixing ratios relative to those in the north during late winter and spring. Equatorward of-50øS, water vapor in the lower stratosphere is largely controlled by the tropical seasonal cycle in water vapor coupled with the seasonal cycle in extratropical descent. During the tropical moist period (June, July, and August), air ascending in the Indian monsoon region influences the northern hemisphere more than the southern hemisphere, resulting in a moister northern hemisphere lower stratosphere. This tropical influence is confined to levels beneath 60 mbar at low latitudes, and beneath 90 mbar at high latitudes. During the tropical dry period (December, January, and February), dry air spreads initially into both hemispheres. However, the stronger northern hemisphere wintertime descent that exists relative to that of southern hemisphere summer transports the dry air out of the northern hemisphere lower stratosphere more quickly than in the south. This same hemispheric asymmetry in winter descent (greater descent rates during northern hemisphere winter than during southern hemisphere winter) brings down a greater quantity of "older" higher water vapor content air in the north, which also acts to moisten the northern hemisphere lower stratosphere relative to the southern hemisphere. These factors all act together to produce a drier southern hemisphere lower stratosphere as compared to that in the north. The overall picture that comes from this study in regards to transport characteristics is that the stratosphere can be divided into three regions. These are (1) the "overworld" where mass transport is controlled by nonlocal dynamical processes, (2) the "tropically controlled transition region" made up of relatively young air that has passed through (and been dehydrated by) the cold tropical tropopause, and (3) the stratospheric part of the "middleworld" or "lowermost stratosphere", where tropospherestratosphere exchange can occur adiabatically. Satellite water vapor measurements indicate that the base of the "overworld" is near 60 mbar in the tropics, or near the 450 K isentropic surface.

Aerosol particles in the atmosphere have recently been found to contain a large number of chemical elements and a high content of organic material. The latter property is explicable by an inverted micelle model. The aerosol sizes with significant atmospheric lifetimes are the same as those of single-celled organisms, and they are predicted by the interplay of aerodynamic drag, surface tension, and gravity. We propose that large populations of such aerosols could have afforded an environment, by means of their ability to concentrate molecules in a wide variety of physical conditions, for key chemical transformations in the prebiotic world. We also suggest that aerosols could have been precursors to life, since it is generally agreed that the common ancestor of terrestrial life was a single-celled organism. The early steps in some of these initial transformations should be accessible to experimental investigation.A erosol particles in the atmosphere have long been known to scatter and absorb sunlight and hence to have a substantial influence on the temperature of the Earth. Recent real-time observations of the chemical composition of individual aerosol particles have shown an unexpectedly and remarkably high content of organic molecules (1, 2). A conceptual model can account for this observation by an inverted micelle structure in which surfactants form a spherical monolayer enclosing an aqueous interior (3, 4). Analogies in size, form, and composition between these aerosols and single-celled organisms such as bacteria lead us to propose that similar atmospheric particles could have been the precursors of living systems on Earth. Large populations of aerosol particles would have provided an environment for the concentration of prebiotic molecular species and for their chemical transformation through exposure to the fluctuating fields of humidity, temperature, and sunlight available in the atmosphere at different altitudes and latitudes. Coagulation and division of the particles could have resulted in an increased diversity of molecular species and an early mechanism for reproduction and replication of successful molecular populations.The most probable sequence of events in the evolution of living organisms on Earth started with either the terrestrial synthesis or the arrival from elsewhere in the universe of simple carbonaceous compounds (5). It was long ago recognized that hot water or steam would have reacted with metal carbides to form hydrocarbons as the Earth condensed from cooling gases (6). The precursors of biopolymers-amino acids, sugars, purines, and pyrimidines-as well as a range of surfactants such as long-chain carboxylic acids can be rationalized as prebiotic constituents by such means (7). Moreover, molecules of this type, including hydrocarbons with chain lengths in excess of C 10 , have been found in the interior of carbonaceous meteorites (8). These building blocks are thought to have undergone polymerization and other reactions, and through a series of events to have formed structures of greater...