Atmospheric Sea Salt Particles Research Articles

Hygroscopic growth of single atmospheric sea salt aerosol particles from mass measurement in an optical trap.

Sea salt aerosol is among the most abundant aerosol species in Earth's atmosphere, and its hygroscopicity is an important parameter to quantify its interaction with solar radiation. Conflicting values for the hygroscopic growth have been reported in the literature, which decreases the accuracy with which their impact on Earth's climate can be modelled. Here we report new values of the hygroscopic growth for a selection of salt compositions representative of atmospheric sea salt. These values are obtained from single optically trapped aqueous droplets with dry radii between 0.3 and 2 μm, using a recently developed method for single particle mass measurement in an optical trap. We compare our results to earlier studies and propose a way to reconcile the apparent discrepancies found in the literature. Within our studies, we also observe the crystallization of CaSO4·2H2O (Gypsum) during the drying of optically trapped sea salt droplets at significantly larger relative humidity of 65-68% than the main efflorescence relative humidity at 50%. This preceding transition occurred in the absence of any contact of the particle with a surface.

A comparison of sea salt emission parameterizations in northwestern Europe using a chemistry transport model setup

Abstract. Atmospheric sea salt particles affect chemical and physical processes in the atmosphere. These particles provide surface area for condensation and reaction of nitrogen, sulfur, and organic species and are a vehicle for the transport of these species. Additionally, HCl is released from sea salt. Hence, sea salt has a relevant impact on air quality, particularly in coastal regions with high anthropogenic emissions, such as the North Sea region. Therefore, the integration of sea salt emissions in modeling studies in these regions is necessary. However, it was found that sea salt concentrations are not represented with the necessary accuracy in some situations.In this study, three sea salt emission parameterizations depending on different combinations of wind speed, salinity, sea surface temperature, and wave data were implemented and compared: GO03 (Gong, 2003), SP13 (Spada et al., 2013), and OV14 (Ovadnevaite et al., 2014). The aim was to identify the parameterization that most accurately predicts the sea salt mass concentrations at different distances to the source regions. For this purpose, modeled particle sodium concentrations, sodium wet deposition, and aerosol optical depth were evaluated against measurements of these parameters. Each 2-month period in winter and summer 2008 were considered for this purpose. The shortness of these periods limits generalizability of the conclusions on other years.While the GO03 emissions yielded overestimations in the PM10 concentrations at coastal stations and underestimations of those at inland stations, OV14 emissions conversely led to underestimations at coastal stations and overestimations at inland stations. Because of the differently shaped particle size distributions of the GO03 and OV14 emission cases, the deposition velocity of the coarse particles differed between both cases which yielded this distinct behavior at inland and coastal stations. The PM10 concentrations produced by the SP13 emissions generally overestimated the measured concentrations. The sodium wet deposition was generally underestimated by the model simulations but the SP13 cases yielded the least underestimations. Because the model tends to underestimate wet deposition, this result needs to be considered critically. Measurements of the aerosol optical depth (AOD) were underestimated by all model cases in the summer and partly in winter. None of the model cases clearly improved the modeled AODs. Overall, GO03 and OV14 produced the most accurate results, but both parameterizations revealed weaknesses in some situations.

(Corrosion Division Morris Cohen Student Award) Impact of Salt Deliquescence on the Humidity-Dependence of Atmospheric Corrosion

In atmospheric environments, corrosion is often approximated as a discontinuous process reliant on the availability of electrolyte to provide ionic conduction between cathodic and anodic sites. A critical question in atmospheric corrosion is when can a surface be considered wet enough for corrosion to occur at a considerable rate? For surfaces contaminated with hygroscopic salts, such as those which comprise atmospheric sea salt particles, the relative humidity (RH) associated with the deliquescence of these salts is often assumed to serve as the wet/dry threshold in this regard. Deliquescence is the bulk phase transformation whereby solid salt spontaneously absorbs water to form an aqueous electrolyte at and above a specific RH, the deliquescence RH (DRH). In this work, we examined the relationship between the hygroscopic phase transitions of NaCl, MgCl2 and ASTM artificial seawater (ASW) and the response of mild steel contaminated with these sea salt simulants at the initial stage of corrosion. The hygroscopic behavior of salt microparticles of each composition, similar in size to natural atmospheric aerosol, was characterized by impedance measurements across an interdigitated electrode sensor. The corrosion behavior of mild steel loaded these microparticles was investigated as a function of RH under isohumidity, room temperature conditions. Both the attack morphology and volume loss rates were quantified using optical profilometry. Ex-situ characterization of the dried salts on the corroded steel and sensor were carried out using SEM/EDS and Raman spectroscopy. Trends in corrosion loss versus RH were not directly reflective of the deliquescence phase transitions observed or predicted for the salts examined. Considerable corrosion was observed below the DRH of the salts and attributed to (1) the presence of non-equilibrium electrolyte due to kinetic limitations during drying, (2) alteration of the deliquescence behavior by the corrosion chemistry and (3) adsorbed electrolyte at the solid salt/metal interface. For NaCl, sustained corrosion was observed down to 33% RH for up to 300 days. At 53% RH and above, attack proceeded at rates comparable to that observed above the DRH (76%) due to the development of hygroscopic corrosion chemistry. There was no significant difference in attack depth and volume loss trends between samples that were originally loaded with droplets and those loaded with NaCl crystals prior to isohumidity exposure. The initiation of corrosion under NaCl deposits well below the DRH can be explained in terms of the presence of adsorbed water associated with the salt crystals in contact with the steel. For MgCl2 and ASW, sustained corrosion was detectable down to 11% and 23% RH, respectively, for up to 30 days. This is well below the DRH of MgCl2 (33%), which is often considered the active component of sea salt mixtures at low humidity levels. Significant admittance was measured across ASW and MgCl2 deposits on the electrode sensors at < 2% RH after 24 hours, likely due to trapping of supersaturated MgCl2 fluid under solid salt crusts. Trends in attack morphology and volume loss above 33% RH of ASW-loaded samples were similar to those loaded with NaCl alone. This may be explained by the similar corrosion chemistry that developed on the NaCl and ASW samples. Together, these results show that DRH of an individual salt contaminant for the systems under study did not serve as the delineation between a wet and a dry surface, nor as the boundary between significant and insignificant corrosion attack. The impact of these findings on humidity-dependent atmospheric corrosion models along with translation to understanding atmospheric corrosion of other alloy systems will be discussed.

Surface Adsorbed Water on NaCl and Its Effect on Nitric Acid Reactivity with NaCl Powders

Adsorbed water has been shown to enhance the ionic mobility on NaCl surfaces at water vapor pressures well below the deliquescence point. This has important implications for the steady-state reactivity of NaCl, in atmospheric sea salt particles, with nitrogen oxides in the atmosphere. In the absence of surface ionic mobility the surface passivates due to the buildup of a product layer. It has also been suggested that the actual reaction probability of the reaction of HNO3 with NaCl surfaces may depend on the water content of the surface. Using X-ray photoelectron spectroscopy and electron microscopy, we show that the specifics (particle size and water exposure) of the NaCl sample preparation can have significant effect on the amount of strongly adsorbed water (SAW) on the surface. Specifically, large single crystals of NaCl(100) do not adsorb water strongly. Crystallites in the size range of 500 μm adsorb small amounts of water strongly. In comparison, small crystallites in the size range of 1−10 μm can adsorb large amounts of water strongly. The strongly adsorbed water remains on the surface when exposed to vacuum at temperatures above 100 °C. The NaCl small crystallites, with the largest amount of strongly adsorbed water, do show a small increase in the initial reaction probability for the reaction with HNO3. This increase in initial reaction probability is at most a factor of 4. By combining the results presented here with our previously reported quantitative measurement of the reaction probability for HNO3 on NaCl(100) single-crystal surfaces we recommend a value of γ = (5.2 ± 3) × 10-3 for the reaction probability of gaseous HNO3 with micrometer-size NaCl particles that contain strongly adsorbed surface water.

Water-Induced Reorganization of Ultrathin Nitrate Films on NaCl: Implications for the Tropospheric Chemistry of Sea Salt Particles

Reactions of sodium chloride, a major constituent of atmospheric sea salt particles which have been observed in the marine boundary layer as well as inland, and nitrogen oxides are known to form gas-phase chlorine species that may ultimately influence tropospheric ozone levels. Therefore, studies were carried out on sodium chloride single crystals which were exposed to nitric acid vapor at ∼298 K to form surface bonded nitrate. Upon exposure to water vapor pressures below the bulk dissolution (deliquescence) of sodium chloride and sodium nitrate, two-dimensional nitrate layers on the surface of sodium chloride crystals reorganized to form separate three-dimensional microcrystallites of sodium nitrate which were observed using transmission electron microscopy. X-ray photoelectron spectroscopy results show that the reorganization of the surface, and the regeneration of a fresh NaCl surface, also takes place under conditions where only 1−2 nitrate monolayers initially existed. These results explain atmospheric observations of highly variable chlorine deficits within individual sea salt particles and suggest that a large portion of the particle chloride, not just the original surface, may become available for reaction.

Estimates of the transport of trace metals from the ocean to the atmosphere

The Bubble Interfacial Microlayer Sampler (BIMS) was used in the North Atlantic to determine the enrichment factors, relative to sodium and the ocean (EFsea), for a series of trace metals on bubble‐derived atmospheric sea salt particles. No enrichment was found for K and Mg within the analytical precision of ±10%. Mean EFsea values determined for Al, Co, Cu, Fe, Mn, Pb, V, and Zn ranged from 10 to 20,000. Positive correlations were found between bubble generation depth and the EFsea values for Fe, Pb, Sc, and Zn. Estimates suggest that the ocean is the primary global source of atmospheric K and Mg, that it is potentially a nontrivial (5–20% of the total) source of atmospheric Cu, V, and possibly Zn, and that it is an insignificant source of Al, Co, Fe, Mn, Pb, and Sc. In the remote North Pacific, during certain periods of the year, the ocean may be the primary source for all these elements present on aerosol particles with diameters greater than ∼3 μm.

Organic carbon in marine atmospheric particulate matter: Concentration and particle size distribution

Organic carbon (OC) was determined in atmospheric particulate matter collected at several remote marine locations in the northern and southern hemispheres (Bermuda, Hawaii, Samoa). The OC concentrations were rather similar at all three locations, generally ranging from 0.2 to 0.4 µg/m³ STP. The major mass of the OC at all three locations was found on the smallest particles (radii less than 0.5 µm). However, the major mass of OC on laboratory generated atmospheric sea salt particles was found on particles with radii from 1‐3 µm, the same size as the major mass of the sea salt in both the laboratory experiments and in ambient marine air. This suggests that most of the organic material in marine atmospheric particulate matter is not associated with sea salt. Gas‐particle interactions involving organic compounds could explain the observed size distribution of the OC‐containing particles and the rather constant concentrations of organic carbon found in marine atmospheric particulate matter at the locations studied.

Alkali and alkaline earth metal chemistry of marine aerosols generated in the laboratory with natural sea waters

Atmospheric sea salt particles, produced in the laboratory by bubbles bursting at the surface of natural seawater solutions, were examined for their alkali and alkaline earth metal content in several particle size categories. No enrichments of K, Ca and Mg relative to the seawater source were found in these atmospheric particles using 1. (1) Sargasso seawater or Narragansett Bay seawater. 2. (2) filtered seawater or unaltered seawater. 3. (3) two different bubble path lengths. Only with the addition of natural surfactants was any enrichment over the seawater ratio found in these aerosols. In this case, K and Mg may have been enriched on the smallest particles. Calculations, indicate that the organic carbon content of aerosols produced from even highly productive natural seawaters cannot account for apparent enrichments of K observed in the environment. These experiments suggest that enrichment of alkali and alkaline earth metals in sea salt particles produced from most natural seawaters is environmentally insignificant.

Factors influencing the organic carbon content of marine aerosols: A laboratory study

Summary and entire article are available on microfiche. Order from American Geophysical Union, 1909 K Street, N. W., Washington, D. C. 20006. Document J76-004; $1.00. Payment must accompany order. A series of laboratory experiments using natural seawater were conducted to examine several factors which influence the organic carbon(OC) content of marine atmospheric sea-salt particles produced by bursting bubbles at the air-sea interface. The organic carbon/Na ratio of these laboratory-generated atmospheric sea-salt particles was found to be dependent on the quantity of organic carbon in the seawater, the nature of the organic compounds in the seawater, the amount of surfactants in the seawater, and the distance the bubbles traveled in seawater before bursting at the air-seawater interface. The dissolved and/or colloidal fractions of the organic material in the seawater used were primarily responsible for the organic carbon content of the atmospheric particles. Filtration of the seawater did not affect the OC/Na ratio of the aerosols produced. The OC/Na ratio of atmospheric sea-salt particles generated from Sargasso seawater was 0.008±0.005, while that of particles generated from Narragansett Bay water was 0.093±0.053.

A laboratory study of iodine enrichment on atmospheric sea-salt particles produced by bubbles

A laboratory model ocean-atmosphere study was undertaken to investigate the mechanisms causing iodine enrichment of up to 500 (compared to the iodine content of sea water) on atmospheric sea-salt particles produced by bubbles in the sea. Experiments were run both by using 131I tracer in sea water and by using iodine analysis by neutron activation in untreated fresh sea water. Particles produced by bubbling in the model ocean were separated into size fractions with a cascade impactor. Gaseous iodine was collected on activated charcoal. The results indicate that organically bound iodine probably accounts for an initial iodine enrichment on the particles and may also explain the characteristic U shape of the iodine enrichment versus particle size curve. It appears that gaseous iodine is also a major factor in determining the iodine enrichment on marine atmospheric particulate matter. The mechanism for production of gaseous iodine is still uncertain.

Difference in chemical composition of atmospheric sea salt particles produced in the surf zone and on the open sea in Hawaii

Difference in chemical composition of atmospheric sea salt particles produced in the surf zone and on the open sea in Hawaii

Difference in chemical composition of atmospheric sea salt particles produced in the surf zone and on the open sea in Hawaii

Cascade impactor samples of atmospheric sea salt particles have been collected at a tower site on the east coast of Hawaii. In an-attempt to differentiate between surf-produced particles and particles produced by bubble bursting on the open sea, samples were collected simultaneously near the tower base and at the top of the tower, 24 meters above sea level. Analyses of the samples by neutron activation for iodine, bromine, and chlorine showed that the total bromine and chlorine concentration was 20–50 times higher at the tower base than at the tower top, whereas total iodine was only twice as high. The I/Cl ratio was a factor of 3–30 higher for samples collected at the tower top. Concentration per cubic meter vs particle size curves for all three halogens were considerably different for samples collected at the two elevations. This vertical gradient shows the large differences between spray particles from waves breaking on the shore and spray particles from waves breaking at sea. Great care must be exercized in selecting coastal sampling sites for the collection of atmospheric sea salt particles which are intended to be representative of particles present in the atmosphere over the open ocean. DOI: 10.1111/j.2153-3490.1971.tb00586.x