J. Rosinski

A study of marine aerosols over the Pacific Ocean

Aerosol samples were collected on a Pacific cruise from 47°N to 55°S. Particle morphology, concentrations, and size distributions were analyzed with an electron microscope; elemental compositions of individual particles were determined with an X-ray energy spectrometer; and chemical compositions of bulk samples were measured with an ion chromatograph. Temporal and spatial variations of aerosol physico-chemical characteristics were studied in relation to ocean currents and atmospheric parameters. The results show that number and mass concentrations of primary particles depend mainly on surface wind speeds. However the ratios between the major ions, e.g., Na+, Cl-, and Mg++, are similar to the ratios in seawater regardless of location or meteorological conditions. The concentrations of secondary aerosols, e.g., non-seasalt sulfate, nitrate, and ammonium particles, show maxima at upwelling regions, such as along the California coast, at the Equator, and near the Chatham Rise where ascending motion brings nutrient-riched deep water into the surface layer. The number concentrations of small sulfate particles and large nitrate-coated particles showed diurnal variations with maxima in the early afternoon and minima at night, indicating that the particles are the products of photo-chemical reactions. Their precursor gases, e.g., CH3SCH3, NO, and NH3 are known to be released from seawater in upwelling regions where biological activities thrive.

Cloud condensation nuclei as a source of ice-forming nuclei in clouds

Abstract For the first time it has been found that sorption ice-forming nuclei (IFN) are produced in a cloud when cloud droplets are evaporating. Ice crystals form by vapor diffusion at the moment when a fraction of the residues of evaporated droplets (newly formed aerosol particles) become IFN. In this study, the concentration of sorption IFN varied from 0 to 6.4 × 103 m−3 at −4.4°C, and it was as high as 7.0 × 103 m−3 at −20°C. The fraction of evaporated cloud droplets producing a transient population of sorption IFN was estimated between 10−5 and 10−4. The concentration of aerosol particles active as IFN operative by condensation-followed-by-freezing was found to be between hundreds and 1.5 × 104 m−3 at temperatures of −6 and −20°C. Evidence now exists that only one measuring technique of concentrations of IFN does not suffice to explain concentrations of ice particles in natural clouds.

Nature of ice-forming nuclei in marine air masses

Abstract Aerosol particles collected over the Pacific Ocean between 7°N and 5°S latitude and 110° to 142°W longitude during the period from 23 May to 19 June 1985 were examined for their ability to nucleate ice by sorption, freezing, and condensation-followed-by-freezing. Ice-forming nuclei (IFN) active by sorption in the temperature range between − 4° and − 17°C were not found in the air masses of pure marine origin. The highest temperatures of freezing of water drops placed on aerosol particles larger than 0.3 μm and below 0.3 μm in diameter were − 12° and − 4°C, respectively. Maximum concentrations of IFN active by condensation-followed-by-freezing at initial ice nucleation temperatures of − 4°, − 6°, and − 7°C were found over some areas to be 40 ± 5, 10, and 401 −1 , respectively; these concentrations were of a patchy character over the Pacific Ocean where upwelling regions were present, and they were found to be independent of temperature for the temperature range from the initial ice nucleation temperatures down to − 17°C. Aerosol particles active as IFN were in the 0.1–0.3 μm diameter size range; they were hydrophobic particles and consisted of a chemical compound or compounds that completely vaporized in 10 −6 mm Hg vacuum, indicating that they were neither bacteria nor proteins. It was found that the IFN always existed in the presence of sulfate-bearing aerosol particles smaller than 0.3 μm in diameter and that the sulfate ion is an integral part of the ice nucleating particle. A relation was also found between SO 4 2− ion concentration, the temperature of ice nucleation (the temperature-concentration curve has a maximum), and the nature of the hydrophobic surface. The result of this relationship is that some of the cloud droplets growing in an updraft can be immune to freezing when the concentration of the SO 4 2− ion in solution is too low. Upon evaporation of droplets, the increasing concentration of SO 4 2− ions reaches the critical concentration at the ice nucleation (freezing) curve and causes evaporating droplets to freeze; this supports the Hobbs-Rangno observations. The freezing of condensing and evaporating droplets is time-dependent and therefore depends on the dynamics of clouds.

Marine aerosols in Pacific upwelling regions

Abstract In a recent research cruise along the American West Coast and the Equator, concentrations of marine aerosols were measured to investigate the temporal and spatial variability in upwelling regions where nutrient-rich, deep-sea water ascends. Aerosol mass concentrations that represent mainly primary production of large sea-salt particles depend primarily on surface wind speed. Aerosol number concentrations, which are dominated by secondary production of sulfate, nitrate, ammonium, and organic particles, are complex functions of solar radiation, sea-surface temperature, wind speed, and wind direction. Both oceanic processes (e.g. production of precursor gases such as CH3SCH3, NOx, NH3), and atmospheric processes (e.g. photochemical reactions and gas-to-particle conversions) are important in the formation of marine secondary aerosols.

Ice-forming nuclei of maritime origin

Abstract Aerosol particles collected over the Pacific Ocean between 14 February and 7 May 1984 were examined for their ability to nucleate ice by freezing, sorption, and condensation-followed-by-freezing. Aerosol particles nucleating ice by freezing in the temperature range from −7.3° to −11.1°C along the western coast of North America were in the size range from 6 to 8 μm in diameter. A very low concentration of particles smaller than 0.5 μm nucleating ice at −12°C was present over the South Pacific. No ice-forming nuclei, active by sorption in the temperature range between −5 and −17°C, were found. Concentrations of IFN active by condensation-followed-by-freezing were 100 m −3 at −3.3°C and 3 × 10 4 m −3 (301. −1 ) at and below −4.0°C. These concentrations were found to be independent of temperature for the temperature range from −4° to −14°C. The fraction of the aerosol population nucleating ice below −4°C was ∼ 10 −3 . Aerosol particles supplying IFN were below 0.5 μm in diameter and seemed to consist mostly of organic matter. They were present only over the South Equatorial Current and were associated with biological activity in that current. The formation of frozen droplets by condensation-followed-by-freezing in clouds is time-dependent and consequently depends on the evolution of clouds.

Cloud condensation nuclei as a real source of ice forming nuclei in continental and marine air masses

Cloud condensation nuclei (CCN) constitute a source of ice-forming nuclei (IFN) in continental and marine air masses. The mode of ice nucleation studied was condensation-followed-by-freezing. Concentrations of IFN formed by evaporation of cloud droplets and subsequent condensation of water vapor was found to be for marine aerosol particles 40 to 110 m−3 at Sw = 0.2% and 80 to 700 M−3 at Sw = 5%; for continental air masses it was 40 to 300 m−3 at Sw=0.2% and 5%. Concentrations of IFN of marine origin increased with increasing Sw, at a constant temperature; they were constant over a wide temperature range at a constant Sw. IFN concentrations in continental air masses were found to be independent of Sw at a constant temperature; they were constant for the Sw range of 0.2 to 5% (70% of data) and 0.5 to 5% (30% of data). The IFN produced through the evaporation-condensation cycle of marine aerosol particles have properties that are identical to IFN present in a marine atmosphere—their concentrations are independent of temperatures at Sw≥0.25%.

Ice-forming nuclei in air masses over the Gulf of Mexico

Abstract Aerosol particles collected over the Gulf of Mexico during the period from 20 July to 30 August 1986 were examined for their ability to nucleate ice by condensation-followed-by-freezing. Ice-forming nuclei (IFN) in the 0.1–0.4 μm-diameter size range nucleated ice at a temperature of −4°C; their concentrations were between 2 and 10 m −3 . Fractions of aerosol particles in that size range nucleating ice at the initial (the highest) temperatures were between 10 −8 and 10 −7 . Peaks in the concentration of dimethyl sulfide (DMS) (0800 h) preceded peaks in ice-nucleating temperatures (1300 h) by 5 h; this is sufficient time for DMS molecules to be oxidized to sulfates and to produce mixed aerosol particles through coagulation of different-sized aerosol particles and absorption of sulfur-bearing gas molecules. Fractions of aerosol particles larger than 0.2 μm in diameter containing SO 2− 4 ions were larger than 0.90; most of the time they were 0.99–1.00. All IFN displayed characteristic features of mixed IFN, that is of marine origin (part of IFN concentration independent of temperature) and of continental origin (part of IFN concentration dependent on temperature).

Ice-forming nuclei over the East China Sea

Abstract Ice-forming nucleus (IFN) population was determined for aerosol particles collected on filters over the East China Sea. The mode of ice nucleation studied was condensation-followed-by-freezing; experiments were performed in a dynamic chamber at a constant temperature and with continuous cooling at S w =0.3%. Ratios of IFN concentrations determined at a constant temperature to those determined during continuous cooling simulating an updraft were close to one (35%) and larger than one (65%). Ratios close to one indicate the absence of any deactivation of IFN during the growth of droplets; for ratios larger than one the dilution takes place during the entire cooling process, thus producing more dilute solutions of water-soluble salts in droplets and apparently deactivating some ice nucleating sites. IFN concentration maxima were found at 0600 and 1800 local time. Concentrations of IFN were found to be inversely proportional to the amount of air pollutants, but at a very high SO 4 2− ion concentration IFN concentrations increased.

Latent ice-forming nuclei in the Pacific Northwest

Abstract Some cloud condensation nuclei (CCN) constitute a reservoir of latent ice-forming nuclei (IFN) active by condensation-followed-by-freezing and by sorption. Evaporated droplets occasionally left aerosol particles that acted as sorption IFN at temperatures as high as −5°C and water vapor supersaturation over ice of 0.2%. The newly formed aerosol particles (residues of evaporated droplets) are all mixed particles. The discovery of IFN produced from CCN promotes new insights into the process of ice formation in clouds; in an evaporating parcel of a cloud the rate of formation of ice particles will be enhanced by continuous production of IFN. Aerosol particles left behind after evaporation of a cloud may provide a source of IFN for formation of some of the cirrus clouds.

Ice-forming nuclei in transvaal, Republic of South Africa

Abstract Concentrations of ice-forming nuclei (IFN) active by sorption and by condensation-followed-by-freezing were assessed during February and March, 1986 in Transvaal, Republic of South Africa. Concentrations were found to be very low; at Sw just below 0% and at 2%, IFN concentrations below 10 m−3 at a temperature of −15°C were present on 44 and 28% of the days, respectively. Number concentrations and the rates of activation of IFN active through condensation-followed-by-freezing were found to be larger than those active by sorption. IFN active by condensation-followed-by-freezing usually nucleated ice at higher temperatures than the IFN active by sorption. The number concentration of IFN active through condensation-followed-by-freezing was found to be independent of water vapor supersaturation over liquid water in the range between 0 and 6% in the temperature range from −12° to −22°C. Evaporation and condensation of water vapor on IFN active through condensation-followed-by-freezing did not deactivate them.