C. T. Nagamoto

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.

Aerosol particles in the Kuwait oil fire plumes: Their morphology, size distribution, chemical composition, transport, and potential effect on climate

Airborne aerosol samples were collected with an impactor in the Kuwait oil fire plumes in late May 1991. A transmission electron microscope was used to examine the morphology and size distribution of the particles, and an X ray energy spectrometer was used to determine the elemental composition of individual particles. A chemical spot test was used to identify particles containing sulfate. The results show that the dominant particles were (1) agglomerates of spherical soot particles coated with sulfate, (2) cubic crystals containing NaCl and S04=, (3) irregular-shaped dust containing Si, Al, Fe, Ca, K, and/or S, and (4) very small ammonium sulfate spherules. The concentrations of small sulfate particles increased at higher levels or greater distances from the fire, suggesting the transformation of SO2 gas to sulfate particles by photooxidation followed by homogeneous nucleation. The number of soot, salt, and dust particles that were coated with sulfate increased farther from the fire, and the thickness of the coating increased with altitude. This suggested that gas-to-particle conversion had occurred by means of catalytic oxidation combined with heterogeneous nucleation during the plume dispersion. Because the sulfate coating can modify the hydrophobic surfaces of soot and dust particles to make them hydrophilic, most of the particles in the plume apparently were active cloud condensation nuclei that could initiate clouds, fog, and smog, which in turn could affect regional surface temperature, air quality, and visibility. Long-range air trajectories suggested that some aerosols from the fires could have transported to eastern Asia. It seems possible (but is presently unproven) that a severe flood in China in June was influenced by aerosols from the plumes.

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.

The investigation of air quality and acid rain over the Gulf of Mexico

Abstract A research cruise was conducted in the summer of 1986 by a group of scientist from the U.S.A. and Mexico to investigate air chemistry over the Gulf of Mexico. Chemical, physical, meteorological and oceanographic measurements were carried out to survey temporal and spatial variations of diverse parameters throughout the Gulf. Emphases were placed on air-sea-land exchange of gases and aerosols, natural air quality, transport of anthropogenic air pollution, and acid rain deposition to the Gulf. Although the prevailing winds were easterly from the sea during the cruise, the air was highly polluted with continental aerosols, probably caused by local shifting winds and the oscillation between sea breeze and land breeze. Aerosol number concentrations were measured from 10 5 cm −3 at ports to 10 3 cm −3 in the open Gulf. The average aerosol mass concentration was ∼25 μ g M −3 , consisting of 60% insoluble crustal particles that contained Si, Al, Fe; 30% seasalt particles that contained Na + and Cl − ; and 10% anthropogenic sulfate and nitrate particles. Samples of rain water collected near the coast were acidic (pH ∼4). The concentrations of dimethyl sulfide correlated with bio-particle concentrations in surface seawater and could be a significant precursor of atmospheric SO 4 2− particles. The life cycles of the aerosols in the Gulf, including sources, transport, transformation, and wet and dry deposition are discussed.

Characterization of atmospheric aerosols and of suspended particles in seawater in the western Pacific Ocean

Aerosol samples were collected to study the variabilities of marine aerosols at different times and in different ocean areas. The samples were collected during the first three cruises (December 12, 1985, to February 21, 1986; November 11, 1986, to March 1, 1987; September 27 to November 4, 1987) operated by the cooperative program between the United States and the Peoples Republic of China. The concentrations of crustal and pollution elements in aerosols were high over the ocean area close to the China coast and decreased very rapidly with increasing distance from land. For the third cruise, in the ocean area northeast of the Philippines, the concentrations of crustal and pollution elements in aerosols were high, especially Fe, whose values reached 3.15 × 10−5 g m−3. For all three cruises, the mass size distributions of crustal elements and pollution elements in aerosols showed more large particles. The morphology and elemental composition of aerosol particles showed that sea-salt particles may conglomerate with small crustal and pollution particles from land to form large particles. The size distributions of suspended particles (d > 0.3 μm) in surface seawater approximately fitted the Junge size distribution. The concentration of suspended particles in surface seawater decreased with increasing distance from land except at the equator, where particle concentrations were high. In addition, results from rain chemistry show that the relative importance between wet deposition and dry deposition varies with trace elements.

A Study of the Mechanisms of Acid Rain Formation

Abstract Samples of rain, snow, cloud water, aerosols and soil were collected in Colorado to study the mechanisms of acid rain formation. Chemical compositions of various types of samples were analyzed to investigate the stepwise incorporation of impurities into precipitation. Local soil was generally alkaline; atmospheric aerosols, which are mixtures of stirred-up soil particles and anthropogenic pollution, were slightly acidic; cloud condensation nuclei, which initiate clouds at condensation level, had an average pH of ∼6. However, local clouds were very acidic (pH ∼4), indicating that further acidification takes place in clouds by adsorption of acidic gases, e.g., CO2, SO2, and NOx. We found that summer showers formed by coalescence of cloud droplets are likely to be as acidic as cloud water. The chemistry of snow may differ from that of clouds, depending on the mechanisms of snow formation. If snow crystals are initiated by deposition nucleation and grown by diffusion of water vapor from surrounding e...

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).

Freezing Nuclei Derived from Soil Particles

Abstract The majority of hydrosol particles in the submicron range in precipitation are produced by separation from the surfaces of larger hydrosolized aerosol particles. Sail particles may produce freezing nuclei active between –10 and –7C in concentrations up to 100 cm−3 per soil particle, and freezing nuclei active at temperatures warmer than –20C in concentrations greater than 1000 cm−3 per soil particle. It is concluded that neither the role of freezing nuclei in the development of precipitation nor even the distribution of freezing nuclei among cloud particles, raindrops and hailstones can be deduced from the determination of freezing nucleus populations in bulk precipitation samples.