Abstract. A large number of oxygenated organic chemicals (peroxyacyl nitrates, alkyl nitrates, acetone, formaldehyde, methanol, methylhydroperoxide, acetic acid and formic acid) were measured during the 1997 Subsonic Assessment (SASS) Ozone and Nitrogen Oxide Experiment (SONEX) airborne field campaign over the Atlantic. In this paper, we present a first picture of the distribution of these oxygenated organic chemicals (Ox-organic) in the troposphere and the lower stratosphere, and assess their source and sink relationships. In both the troposphere and the lower stratosphere, the total atmospheric abundance of these oxygenated species (ZOx-organic) nearly equals that of total nonmethane hydrocarbons (ZNMHC), which have been traditionally measured.

The process by which liquid cloud droplets homogeneously crystallize into ice is still not well understood. The ice nucleation process based on the standard and classical theory of homogeneous freezing initiates within the interior volume of a cloud droplet. Current experimental data on homogeneous freezing rates of ice in droplets of supercooled water, both in air and emulsion oil samples, show considerable scatter. For example, at ؊33°C, the reported volume-based freezing rates of ice in supercooled water vary by as many as 5 orders of magnitude, which is well outside the range of measurement uncertainties. Here, we show that the process of ice nucleus formation at the air (or oil)-liquid water interface may help to explain why experimental results on ice nucleation rates yield different results in different ambient phases. Our results also suggest that surface crystallization of ice in cloud droplets can explain why low amounts of supercooled water have been observed in the atmosphere near ؊40°C. For almost 200 years, persistent (liquid) fogs have been observed at temperatures well below the frost point, and there has been a continued vigorous interest in understanding why, and how far, water droplets can supercool in the atmosphere. The presence of heterogeneous ice nuclei in supercooled water droplets has been shown to be necessary for glaciating clouds at temperatures above about Ϫ30°C (1). However, ice particle number densities in clouds below Ϫ30°C are often observed to exceed the ice nuclei number densities (2-7). This finding suggests that, in clouds, some supercooled water droplets freeze homogeneously.The conversion of supercooled water droplets and͞or droplets of aqueous salt solutions into ice below Ϫ30°C can occur anywhere in the atmosphere from the surface layer (resulting in ice fogs) (8) to the upper troposphere (in cirrus clouds) (4-6). In addition, ice freezing in the polar stratosphere has been shown to occur via a homogeneous nucleation process (9, 10). Therefore, it is important to elucidate the actual physical process by means of which clouds glaciate in the atmosphere, particularly at cold temperatures and in situations where ice nuclei become less abundant and less effective in promoting the freezing of cloud droplets into ice (1).Radiative properties of ice clouds and their subsequent effect on climate depend strongly on the size of the cloud particles (11), a property closely linked to the rate at which ice particles in the cloud nucleate and grow. In addition, the rates of chemical reactions, which occur in cloud droplets or on cloud surfaces, depend on the phase of the cloud particle (12). Thus it is important to be able to predict under what set of environmental conditions supercooled cloud droplets freeze into ice. Other significant natural processes, which are affected by the physical state of clouds, include lightning (13) and precipitation. The occurrence of lightning can increase the concentration of reactive nitrogen oxides in air, thereby affecting the rate of gas-phase chem...

Nitric acid-containing cloud particles, known as polar stratospheric clouds, play an important role in the springtime ozone destruction over the polar regions. Nitric acid initially condenses in the polar stratosphere to form supercooled solution droplets of mainly nitric acid and water with trace amounts of sulfuric acid. Nitric acid dihydrate (NAD) and nitric acid trihydrate (NAT) later crystallize from this supercooled solution phase to form solid polar stratospheric cloud particles. Until now, experimental data on this crystallization process has been analyzed under the assumption that NAD and NAT nucleation took place in the interior volume of a cloud droplet. However, in this paper, reanalysis of experimental data on the homogeneous freezing rates of concentrated aqueous nitric acid solution droplets provides substantial support for the occurrence of nucleation "pseudoheterogeneously" at the air-aqueous nitric acid solution interface of the droplet. Furthermore, in a following paper, theory that provides compelling evidence for such interfacial nucleation is developed. Together, the reanalysis of laboratory data in this paper and the supporting theoretical arguments in the following paper suggest that the homogeneous nucleation process occurring in atmospheric droplets may be a surface-rather than a volume-related rate process.

Homogeneous freezing of nitric acid hydrate particles can produce a polar freezing belt in either hemisphere that can cause denitrification. Computed denitrification profiles for one Antarctic and two Arctic cold winters are presented. The vertical range over which denitrification occurs is normally quite deep in the Antarctic but limited in the Arctic. A 4 kelvin decrease in the temperature of the Arctic stratosphere due to anthropogenic and/or natural effects can trigger the occurrence of widespread severe denitrification. Ozone loss is amplified in a denitrified stratosphere, so the effects of falling temperatures in promoting denitrification must be considered in assessment studies of ozone recovery trends.

[1] Airborne measurements of oxygenated volatile organic chemicals (OVOC), OH free radicals, and tracers of pollution were performed over the Pacific during Winter/ Spring of 2001. We interpret atmospheric observations of acetaldehyde, propanal, methanol, and acetone with the help of a global 3-D model and an air-sea exchange model to assess their oceanic budgets. We infer that surface waters of the Pacific are greatly supersaturated with acetaldehyde and propanal. Bulk surface seawater concentration of 7 nM (10 À9 mol L À1) and 2 nM and net fluxes of 1.1 Â 10 À12 g cm À2 s À1 and 0.4 Â 10 À12 g cm À2 s À1 are calculated for acetaldehyde and propanal, respectively. Large surface seawater concentrations are also estimated for methanol (100 nM) and acetone (10 nM) corresponding to an undersaturation of 6% and 14%, and a deposition velocity of 0.08 cm s À1 and 0.10 cm s À1 , respectively. These data imply a large oceanic source for acetaldehyde and propanal, and a modest sink for methanol and acetone. Assuming a 50-100 meter mixed layer, an extremely large oceanic reservoir of OVOC, exceeding the atmospheric reservoir by an order of magnitude, can be inferred to be present. Available seawater data are both preliminary and extremely limited but indicate rather low bulk OVOC concentrations and provide no support for the existence of a large oceanic reservoir. We speculate on the causes and implications of these findings.

We attempt to explain the experimental and molecular dynamics simulation evidence that suggests that the freezing of atmospheric aerosols occurs beginning at the droplet surface. By using the capillarity approximation, we derive the reversible work of formation of a crystal nucleus in the cases where it forms homogeneously within a (supercooled) bulk liquid and where it forms “pseudoheterogeneously” at the surface. Comparing the works of formation in these two cases, one obtains a condition that must hold in order for pseudoheterogeneous (surface) crystallization to be thermodynamically more favorable than homogeneous (bulk) crystallization. This condition is satisfied when at least one crystal facet is only partially wettable by its own melt.

Mechanisms for the formation of Type I (nitric acid‐based) polar stratospheric clouds (PSCs) are discussed. If the pre‐existing sulfate aerosols are liquid prior to PSC formation, then nitric acid particles (Type Ib) form by HNO3 dissolution in aqueous H2SO4 solution droplets. This process does not require a nucleation step for the formation of HNO3 aerosols, so most pre‐existing aerosols grow to become relatively small HNO3‐containing particles. At significantly lower temperatures, the resulting supercooled solutions (Type Ib) may freeze to form HNO3 ice particles (Type Ia). If the preexisting sulfate aerosols are initially solid before PSC formation, then HNO3 vapor can be deposited directly on the frozen sulfate particles. However, because an energy barrier to the condensation exists a nucleation mechanism is involved. Here, we suggest a unique nucleation mechanism that involves formation of HNO3/H2O solutions on the sulfate ice particles. These nucleation processes may be highly selective, resulting in the formation of relatively small number of large particles.

Upper Atmosphere Research Satellite observations indicate that extensive denitrification without significant dehydration currently occurs only in the Antarctic during mid to late June. The fact that denitrification occurs in a relatively warm month in the Antarctic raises concern about the likelihood of its occurrence and associated effects on ozone recovery in a colder and possibly more humid future Arctic lower stratosphere. Polar stratospheric cloud lifetimes required for Arctic denitrification to occur in the future are presented and contrasted against the current Antarctic cloud lifetimes. Model calculations show that widespread severe denitrification could enhance future Arctic ozone loss by up to 30%.