Photoemission of Iodide from Aqueous Aerosol Particle Surfaces.

The photoemission of iodide from aqueous aerosol particle surfaces measures the surface concentration of iodide in predominantly supersaturated NaCl aerosol particles. Using the Langmuir model to describe the adsorption to the surface of aqueous iodide anions, the standard Gibbs free energy of adsorption is -15 kJ/mol in these systems. The presence of charged surfactants on the particle surfaces changes the adsorption behavior of iodide. The addition of sodium docecylsulfate (SDS) reduces the coverage of iodide, consistent with a competitive adsorption scenario. For surfaces coated with C12-, C14-, or C16-trimethylammonium chloride, the addition of iodide results in the formation of iodide-surfactant ion pairs at the surface with enhanced photoemission. The adsorption free energy for iodide in these systems is -21 kJ/mol. The results demonstrate the surface enhancement of iodide in supersaturated, atmospherically relevant conditions and demonstrate important differences between single-salt solutions and mixtures in the limit of high concentration.

Uptake of Pyrene onto Fatty Acid Coated NaCl Aerosol Particles.

Photoelectric charging experiments monitor the uptake of pyrene onto NaCl aerosol particles coated with either oleic acid or myristic acid. In both cases, thin coatings produce a small net decrease in pyrene uptake. In the larger coverage limit, the uptake of the myristic acid coated particles remains nearly constant whereas the oleic acid coated particles exhibit greater uptake rates than the bare NaCl particles. Fitting the results with a multilayer kinetic model yields uptake rate coefficients as well as parameters that describe the distribution of organic molecules on the aerosol particle surface. The model accounts for the decrease in uptake associated with thin coatings of oleic acid through a concomitant reduction in surface area. The adsorption rate constants for the myristic and oleic acid coated surfaces are 50 and 80 times faster, respectively, than for NaCl. The desorption rates for pyrene on the fatty acid surfaces are faster, as well. For myristic acid coatings, the fast desorption (over 400 times the rate of desorption from NaCl) results in slower net adsorption, whereas for oleic acid (approximately 12 times the desorption rate from NaCl), the net uptake rate increases with coverage. The results also suggest that both myristic acid and oleic acid spread incompletely on the aerosol surfaces under the conditions of these experiments. In the optimized kinetic model, the fatty acids cover approximately 50% of the surface when the nominal coating thickness is approximately 6 nm. The surface is over 90% covered with a nominal coating thickness of 20 nm, which is approximately 10% of particle diameter in these experiments. Very thin oleic acid coatings reduce the surface area of particles consistent with the preferential coverage of highly corrugated or porous regions.

Effects of NO(y) aging on the dehydration dynamics of model sea spray aerosol.

The reactions of NO(y) species in the atmosphere with sea spray aerosol replace halogen anions with nitrate. These experiments show the effect of increasing the nitrate content of model sea spray aerosol particles on the morphology changes and the phase transitions driven by changes in relative humidity (RH). The components of the model particles include H2O, Na+, Mg2+, Cl-, NO3-, and SO4(2-). Tandem differential mobility analyzer (TDMA) measurements yield the water content and efflorescence relative humidity (ERH) of these particles, and probe molecule spectroscopic measurements reveal subsequent phase transitions and partially characterize the salt composition on the surface of dry particles. The results show three effects of increasing the nitrate composition: decreasing the EFH (46 to 29%), production of a metastable aqueous layer on the surface of effloresced particles, and decreasing the sulfate content near the surface of dry particles. For the mixtures studied here, the initial crystallization event forms a core of NaCl. For particles that contain a substantial metastable aqueous layer following efflorescence, probe molecule spectroscopy shows a second crystallization at a lower RH. This subsequent phase transition is likely the formation of Na2SO4. Homogeneous nucleation theory (HNT) using a semiempirical formulation predicts the ERH of all mixtures within 2.0% RH, with a mean absolute deviation of 1.0%. The calculations suggest that structures associated with highly concentrated or supersaturated magnesium ions strongly affect the interfacial tension between the NaCl crystal nucleus and the droplet from which it forms.

Uptake of pyrene by NaCl, NaNO3, and MgCl2 aerosol particles.

Photoelectric charging experiments measure heterogeneous uptake coefficients for pyrene on model marine aerosol particles, including NaCl, NaNO(3), and MgCl(2). The analysis employs a multilayer kinetic model that contains adsorption and desorption rate constants for the bare aerosol surface and for pyrene-coated surfaces. First coating the aerosol particles with a pyrene layer and following the desorption using both t-DMA and photoelectric charging yields the desorption rate constants. Separate experiments monitor the increase in surface coverage of initially bare aerosol particles after exposure to pyrene vapor in a sliding-injector flow tube. Analyzing these data using the multilayer model constrained by the measured desorption rate constants yields the adsorption rate constants. The calculated initial heterogeneous uptake coefficient, γ(0)(295 K), is 1.1 × 10(-3) for NaCl, 6.6 × 10(-4) for NaNO(3), and 6.0 × 10(-4) for MgCl(2). The results suggest that a free energy barrier controls the uptake rate rather than kinematics.

Surface morphology and phase transitions in mixed NaCl/MgSO4 aerosol particles.

Probe molecule spectroscopy characterizes the surface environment of mixed NaCl/MgSO(4) (0.01-50 wt % MgSO(4)) aerosol particles as a model for marine aerosol. Two complementary measurements, the probe's excited state spectroscopy and photoionization efficiency, measure the electronic properties of the particle surface and monitor phase changes that are driven by changes in relative humidity (RH). The results illustrate that over a wide range of composition, these particles have a layered structure with NaCl in the core and primarily hydrated MgSO(4) at the surface. Modeling the spectroscopic data reveals that the surface layer is not a uniform shell and that the coumarin 314 probe molecules partition selectively to the MgSO(4) domains. The surface layer has a pi* value of 1.7, indicative of a very high interfacial polarity. In cases where MgSO(4) is a minor component (< or = 10 wt %), the NaCl component crystallizes at 44% RH, consistent with the single salt NaCl result. Deliquescence-mode experiments with these particles show that the MgSO(4) component forms a solution at 42% RH, prior to the full deliquescence of the particle. For mixed particles with 50 wt % MgSO(4), the crystallization of NaCl occurs at 35% RH, and the predeliquescence of MgSO(4) occurs at 38% RH owing to the contribution of MgCl(2) in the surface layer. A model surfactant, SDS, slightly lowers the RH of the NaCl formation to approximately 42% and leads to the formation of a thin soap film that persists to low values of RH.

Linear solvation energy parameters for model tropospheric aerosol surfaces.

Excited-state absorption spectra for several coumarin derivatives adsorbed to aerosol particles provide linear solvation energy (LSE) relationships for the aerosol surfaces. This study focuses on NaCl and (NH4)2SO(4) particles as models for tropospheric aerosol. We investigate several others, including NH(4)Cl, NaBr, KI, Na(2)SO(4), NaNO(3), Al(2)O3, and CaCO(3), to establish trends and understand the factors that control polarity for surfaces. The Kamlet-Taft dipolarity/polarizability parameter, pi*, for these particles ranges from 0.73 to 1.69. The values are high compared to most homogeneous molecular solvents and are attributable to ion-dipole forces, especially at defect sites. We also find that the smaller values of pi* (1.01 for (NH4)2SO(4) and 0.73 for NH(4)Cl) correlate with appreciable hydrogen bond donor acidity in the surface (alpha = 0.23 and 1.06, respectively). Strong hydrogen bonds with the surface lead to a drop in overall polarity either by making interaction with very polar defect sites less likely or orienting the probe molecule away from the surface. Adsorbed water layers mainly alter the alpha value of the surface, but can have indirect effects on pi* by changing the interaction of the adsorbed molecule with the surface.

Phase transitions and surface morphology of surfactant-coated aerosol particles.

Probe molecule spectroscopy and hygroscopic growth curves characterize the morphology of surfactant-coated aerosol particles as a function of relative humidity (RH). This study focuses on particles composed of either potassium iodide or sodium chloride and sodium dodecyl sulfate (SDS). At high RH, these mixed particles assume a reverse micelle type structure, and at low RH, they comprise a solid core of either KI or NaCl coated with SDS and water. The deliquescence relative humidity (DRH) and efflorescence relative humidity (ERH) of the inorganic fraction of the mixed particles are very similar to those of the pure salts. The surface polarity and morphology sampled by the coumarin 314 probe molecule ranges from that of a water-organic interface to that of an ionic surface and depends strongly on the RH and the amount of SDS. When the SDS coverage of the droplet just prior to efflorescence reaches approximately one monolayer, a thin soap film persists on the surface to values of RH much lower than the ERH. Both the electronic spectroscopy and photoelectric charging efficiency show a separate efflorescence for this layer at RH < 5%. The spectroscopy further reveals that there is a hysteresis associated with this low RH phase transition for both KI and NaCl cores.

Probe molecule spectroscopy of NaCl aerosol particle surfaces.

Excited-state absorption spectroscopy and ionization threshold measurements for coumarin 314 (C314) adsorbed to the surface of NaCl aerosols characterize the chemical environment of the particle surface as a function of relative humidity (RH). An atmospheric pressure flow of aerosol passes through an ionization cell where two-photon laser ionization of the adsorbed molecules produces a net charge on the particle. Monitoring this charge as a function of the laser wavelength produces either the absorption spectrum of the S(1) <-- S(0) transition or the ionization threshold. The wavelength of maximum absorption for the S(1) excited-state shifts from 448 nm for RH < 5% to 441 nm for RH = 60%, indicating that adsorbed water decreases the polarity of the surface. Similarly, the ionization threshold increases from 5.10 to 5.27 eV over a similar range of RH. The decrease in polarity is attributable to changes in the local electric field experienced by C314, which is on the order of 1 x 10(7) V/cm, and is correlated with changes in the surface topography. Using a continuum model, we estimate the contributions to the measured thresholds of the polarization response of the surface ions and the electric field and calculate an effective dielectric constant for the adsorbed water film. For a multilayer water coverage (RH = 65%), the effective dielectric constant is approximately 2.4. These results demonstrate that the changes in surface topography with adsorbed water are as important as direct water-solute interactions in determining the solvent character of the surface.

Depth profiling of heterogeneously mixed aerosol particles using single-particle mass spectrometry.

Infrared laser evaporation of single aerosol particles in a vacuum followed by vacuum ultraviolet (VUV) laser ionization and time-of-flight mass spectroscopy of the resulting vapor provides a depth profile of the particle's composition. Analyzing glycerol particles coated with 60-150-nm coatings of oleic acid using either a CO2 laser or a tunable optical parametric oscillator as an evaporation laser results in mass spectra that depend on the IR laser power. Low infrared laser powers incompletely vaporize particles and preferentially probe the composition of the surface layers of the particle, but high laser powers evaporate the entire particle and produce spectra representative of the particle's total composition. In the limit of low laser power, the fraction of oleic acid in the mass spectra is as much as 50 times greater than the fraction of oleic acid in the particle, providing a surface-layer-specific characterization. The OPO laser provides even more surface specificity, producing an [oleic acid]/[glycerol] ratio as much as four times larger (for a 60-nm coating) than that obtained using the CO2 laser. The infrared laser power required to sample the core of the particle increases with the thickness of the coating and is sensitive to changes in the coating thickness on the order of 10 nm. In contrast to these intuitively appealing results, high CO2 laser powers (approximately 90 mJ/pulse) produce mass spectra that, at short delays between the CO2 and VUV lasers, show enrichment of the core material rather than the coating. Likewise, tuning the OPO to frequencies that are resonant with the core material but transparent to the coating also results in selective detection of the core. The results suggest that a shattering mechanism dominates the vaporization dynamics in these situations.

Thermal vaporization-vacuum ultraviolet laser ionization time-of-flight mass spectrometry of single aerosol particles.

Single aerosol particles of ethylene glycol and oleic acid are vaporized on a heater at temperatures between 500 and 700 K, and the resulting vapor plume is ionized by a 10.5-eV vacuum ultraviolet (VUV) laser. The mass spectra are compared to those obtained by CO2 laser vaporization followed by VUV laser ionization. The relative intensities of the parent and fragment ion peaks are remarkably similar for the two modes of vaporization. A Maxwell-Boltzmann distribution of speeds accurately describes the dependence of the signal as a function of the VUV laser pulse timing. The signal levels obtained with this design are sufficient to obtain good-quality mass spectra.