Thomas Rosenoern

Cloud condensation nucleus activity of secondary organic aerosol particles mixed with sulfate

from 23 to 37 m gm � 3 . CCN analysis was performed for 80to 150-nm particles having variable organic-sulfate volume fractions, which were estimated from the diameter of the organic-sulfate particle relative to that of the seed as well as independently from mass spectra. Critical supersaturation, which increased for greater SOA volume fraction and �� ;

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.

The Dynamic Shape Factor of Sodium Chloride Nanoparticles as Regulated by Drying Rate

The influence of drying rate on the dynamic shape factor χ of NaCl particles was investigated. The drying rate at the efflorescence relative humidity (ERH) of 45% was controlled in a laminar flow tube and varied from 5.5 ± 0.9 to 101 ± 3 RH s–1 at ERH, where RH represents one percent unit of relative humidity. Dry particles having mobility diameters of 23–84 nm were studied, corresponding to aqueous particles of 37–129 nm at the RH (57%) prior to drying. At each mobility diameter and drying rate, the critical supersaturation of cloud-condensation activation was also measured. The mobility diameter and the critical supersaturation were combined in an analysis to determine the value of χ. The measured values varied from 1.02 to 1.26. For fixed particle diameter the χ value decreased with increasing drying rate. For fixed drying rate, a maximum occurred in χ between 35- and 40-nm dry mobility diameter, with a lower χ for both smaller and larger particles. The results of this study, in conjunction with the introduced apparatus for obtaining quantified drying rates, can allow the continued development of a more detailed understanding of the morphology of submicron salt particles, with the potential for the follow-on development of quantitative modeling of evaporation and crystal growth at these dimensions.

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.

The 1-by-3 Tandem Differential Mobility Analyzer for Measurement of the Irreversibility of the Hygroscopic Growth Factor

A new instrument, namely the 1 × 3 tandem differential mobility analyzer (1 × 3-TDMA), has been developed. Its primary measurement is the irreversibility of the hygroscopic growth factor of aerosol particles. The instrument uses the hysteresis of phase transitions to infer the solid or aqueous state of the particles. A first DMA passes particles of a selected electric mobility at relative humidity RH0. Exiting this DMA, the particles are split into three separate flows. The first flow is exposed to RH 0 → (RH 0 + δ ) → RH 0 in a deliquescence test before passing through a second DMA that is set to the same electric mobility as the first DMA. The second flow passes directly to a third DMA without change in RH, thereby serving as a reference arm. This DMA is also set to the same electric mobility as the first DMA. The transmission ratio of the 1 × 3-TDMA is defined as the particle concentration passing the deliquescence test divided by that passing through the reference arm. The transmission ratio is unity in the absence of deliquescence and zero when a phase transition occurs, at least for ideal instrument performance in application to a test aerosol of fully deliquesceable particles. For the third flow passing out of the first DMA, an efflorescence test is run by using the RH profile of RH 0 → (RH 0 − δ ) → RH 0 before passing through a fourth DMA. A full data set for the 1 × 3-TDMA is obtained by scanning RH0, typically from 20 to 85%. In the present paper, the 1 × 3-TDMA instrument is described, and laboratory data are presented for the phase transitions of externally mixed aerosols of aqueous and solid sodium chloride particles, aqueous and solid ammonium sulfate particles, and their mixtures, as well as a mixture of aqueous and solid sea salt particles. The observed transmission ratio is compared to a model analysis. The intent behind the development of this instrument is to deploy it for field measurements and use observations of the irreversibility of the growth factors of atmospheric particles as markers of their physical state.