Investigating primary marine aerosol properties: CCN activity of sea salt and mixed inorganic-organic particles.

Sea spray particles ejected as a result of bubbles bursting from artificial seawater containing salt and organic matter in a stainless steel tank were sampled for size distribution, morphology, and cloud condensation nucleus (CCN) activity. Bubbles were generated either by aeration through a diffuser or by water jet impingement on the seawater surface. Three objectives were addressed in this study. First, CCN activities of NaCl and two types of artificial sea salt containing only inorganic components were measured to establish a baseline for further measurements of mixed organic-inorganic particles. Second, the effect of varying bubble residence time in the bulk seawater solution on particle size and CCN activity was investigated and was found to be insignificant for the organic compounds studied. Finally, CCN activities of particles produced from jet impingement were compared with those produced from diffuser aeration. Analyses indicate a considerable amount of organic enrichment in the jet-produced particles relative to the bulk seawater composition when sodium laurate, an organic surfactant, is present in the seawater. In this case, the production of a thick foam layer during impingement may explain the difference in activation and supports hypotheses that particle production from the two methods of generating bubbles is not equal.

Hygroscopic growth of multicomponent aerosol particles influenced by several cycles of relative humidity.

Infrared aerosol flow tube experiments were performed for mixtures of ammonium, sulfate, and hydrogen ions at 293 K. The impact of the cycling of relative humidity (RH) on the crystals formed and on the hygroscopic growth was evaluated. Submicron particles having an extent of neutralization (X) between 0.60 and 0.75 were the focus, with special emphasis on the composition of aqueous letovicite (NH4)3H(SO4)2 (X = 0.75) because of its unique behavior. Aqueous letovicite particles crystallized initially as an external mixture of solid particles, forming pure particles of letovicite (NH4)3H(SO4)2(s) (LET) in some cases and internally mixed particles of ammonium sulfate ((NH4)2SO4(s);AS) and ammonium bisulfate (NH4HSO4(s); AHS) in other cases. Cycling between 3% and 48% RH increased the fraction of LET particles in the aerosol population, moving in the direction of the more thermodynamically favored species. However, some internally mixed particles remained even after multiple cycles, possibly indicative of a memory effect of AS as a heterogeneous nucleus for AHS. For all compositions studied, the RH of first water uptake and the magnitude of water uptake at higher RH were compared to model predictions. As expected, the more acidic particles (X = 0.60 and 0.65) took up water at the eutonic RH (37%) of mixed AHS/LET particles, but not as expected, both solids dissolved completely, arguing for an increased water solubility possibly attributable to nanocrystalline materials. Particles of X = 0.70 took up water above 41% RH, suggesting a particle morphology of an outer coating of AHS that prevents water uptake at the lower eutonic RH values of mixed AHS/LET and AHS/AS particles. Particles of X = 0.75 took up water as expected for an externally mixed particle population of LET and AS/AHS particles, although the fraction of each type in the population depended on the RH history. These results show that the hysteresis effect for some particles depends on a multi-node RH history. The implication for atmospheric particles is that the crystals present therein as well as particle morphology, water content, and extent of internal/external mixing might continue to evolve during multiple atmospheric cycles of RH.