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Surface Composition of Free Mixed NaCl/NaSO Nanoscale Aerosols Probed by X-ray Photoelectron Spectroscopy Egill Antonsson, Christopher Raschpichler, Burkhard Langer, Dmitry Marchenko, and Eckart Ruehl J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b00615 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on March 6, 2018
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The Journal of Physical Chemistry
Surface Composition of Free Mixed NaCl/Na2SO4 Nanoscale Aerosols Probed by X-ray Photoelectron Spectroscopy
E. Antonsson, C. Raschpichler, B. Langer, D. Marchenko,a and E. Rühl Physical Chemistry, Freie Universität Berlin, Takustr. 3, 14195 Berlin, Germany
Abstract The local chemical surface composition of unsupported mixed solid NaCl/Na2SO4 aerosols (d~70 nm) is studied by X-ray photoelectron spectroscopy. The solid aerosols are generated by drying of aqueous droplets containing mixtures of the two salts in different mole fractions. The mole fraction of these salts is found to deviate at the solid aerosol surface significantly from the initial droplet composition. The minority species in the droplets are found to be enhanced at the surface of the solid mixed aerosols. This surface enhancement is rationalized in terms of the nucleation/crystallization process, where the salts evidently do not co-crystallize, rather than each salt forms pure crystal moieties. Characteristic variations of the surface ion concentration as a function the mole fraction of the salts in the initial droplet are observed in the nanometer size regime. This is unlike core-shell architectures previously found in mixed micron salt aerosols, indicating that aerosol models derived from micron sized aerosols are evidently not fully reliable to describe the surface composition of nano-sized aerosols. Furthermore, surface enhancement of the minority component in mixed NaCl/Na2SO4 aerosols is also different from previous results on surface segregation of mixed NaCl/NaBr aerosols, where one of the anionic species is surface segregated for all mole fractions, which was explained in terms of the ability of the involved salts to co-crystallize and forming solid solutions. The present results rather indicate that mixed NaCl/Na2SO4 aerosols do not co-crystallize. Electron microscopy of deposited mixed salt aerosols reveals mostly a cubic structure of pure NaCl aerosols, whereas mixed salt aerosols are found to show a grainy structure composed of multiple small crystals which supports the present findings obtained from photoelectron spectroscopy.
a
present address: Helmholtz-Zentrum Berlin, Albert-Einstein Str. 15, 12489 Berlin, Germany
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Introduction Aerosols are ubiquitous in the atmosphere and have both, natural and man-made sources. They have a broad range of sizes, shapes, chemical composition, and states of aggregation which vary spatially and temporally.1, 2 Among the most abundant aerosols in the atmosphere are sea-salt aerosols, which are created by wind action in the marine boundary layer.3 The local chemical concentration at the aerosol surface is crucial to their role in atmospheric chemistry as the interaction of the aerosols with the gas phase surroundings takes place at the aerosol/gas interface. The architecture of multicomponent aerosols also influences their optical properties, where e.g. core-shell particles absorb and scatter sunlight differently than homogeneously mixed spheres.4 Their optical properties are of crucial importance for the radiation budget of the climate via direct effects, such as scattering and absorption of sunlight,5 as well as indirect effects, such as their contributions to serve as cloud condensation nuclei.6 Sea salt aerosols are initially created as solution droplets. In the atmosphere these droplets may form solid aerosols or exist in deliquescent metastable states depending on the relative humidity of the surroundings. Their optical properties depend critically on the phase changes they may undergo, as both the particle size and its refractive index are changed.2 Single component aerosols show efflorescence and deliquescence at characteristic values of relative humidity of the surroundings, known as the deliquescence and efflorescence relative humidity points.7 However, the behavior of multi-component aerosols can be more complex.8, 9
The response of the droplets to changes in the relative humidity of the surrounding
atmosphere with respect to phase transitions are dependent on the size and the chemical composition of the particles.2, 10 In order to study the intrinsic properties of aerosols as a function of time under conditions that are free from any influence from a substrate or surrounding matrix, single aerosols have been investigated in traps or particle beams. Trapped aerosols allow studying the intrinsic properties of single aerosol particles over extended periods of time.11, 12, 13, 14 Narrow beams of isolated aerosols generated by aerodynamic focusing offer another opportunity to investigate aerosols without contact with a substrate. Beams of free aerosols have advantages over methods, in which the aerosols are e.g. deposited on substrates prior to the experiments. As an example, the nucleation of aqueous aerosols deposited droplets have revealed nucleation to commence at the droplet/substrate interface.15 Depositing aerosols on a substrate appears to be a drawback from an atmospheric chemistry point of view, as the droplet/substrate interface is absent in atmospheric aerosols, which are emerged in gas phase surroundings. Furthermore, radiation-induced changes to the sample over time can be 2 ACS Paragon Plus Environment
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The Journal of Physical Chemistry
avoided, which is of particular importance in photoionization studies of non-conducting materials where sample charging occurs during the experiment.16 In addition, charging of particles is shown to change significantly the nucleation properties of microparticles.14 Instead, a continuous beam of aerosols constantly injects pristine particles into the interaction region with ionizing radiation, so that radiation damage as well as uncontrolled radiation-induced particle charging is avoided. Specifically, free nanoparticles in a beam have been studied in the soft X-ray regime17,
18, 19
as well as in the VUV range, including
scattering of VUV radiation20 and photoemission.21 Photoionization studies on free nanoparticles using intense femtosecond laser pulses has also been reported using freeelectron-lasers in the vacuum ultraviolet (VUV)22 and X-ray regimes,23 as well as in the optical regime.24 NaCl/Na2SO4 nanoparticles are a suitable model system for marine aerosols formed by sea spray, as chlorine and sulfate are the two most abundant anions in sea water, while Na+ is the most abundant cation.25 This motivates the choice of NaCl and Na2SO4 for the present study. Photoelectron spectroscopy is used to probe the local surface structure due to the high surface sensitivity of this approach, which is a consequence of the limited escape depth of photoelectrons in solid aerosols. This allows for the determination of the surface composition, as opposed to the overall composition of the aerosols, which is an important property from the atmospheric chemistry point of view. Experimental The salts used in this work, NaCl and Na2SO4, have a stated purity of >99.5% and >99%, respectively (Manufacturer: Roth). The salts are used without further purification and are dissolved in MilliQ water.
The procedure to generate a beam of nanoscale aerosols has been described in detail elsewhere.17 Briefly, mixed NaCl/Na2SO4 aerosols are prepared by spraying a 1 g/L aqueous solution into a nitrogen atmosphere. The resulting droplets, which contain the dissolved salts, are subsequently dried by a silica gel diffusion dryer. The relative humidity at the dryer outlet is ca. 20%. This results in evaporation of the solvent and crystallization of the salts which yields solid mixed salt aerosols. The aerosols are transferred into high vacuum through a differential pumping stage and an aerodynamic lens system,26 which generates a collimated beam of free nanoparticles with a full width at half maximum (FWHM) of
