Water adsorption on ammounium sulfate and silica nanoparticles. Keywords: nanoparticles, hygroscopic growth, water monolayers INTRODUCTION

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1 Water adsorption on ammounium sulfate and silica nanoparticles Keskinen Helmi 1, Jaatinen Antti 1, Miettinen Pasi 1, Romakkaniemi Sami 1, Joutsensaari Jorma 1, Smith James N. 1, and Laaksonen Ari 1,3 1 Dept. of Physics and Mathematics, University of Eastern Finland, P.O. Box 167, 7011 Kuopio, Finland. National Center for Atmospheric Research, 80307, Boulder, CO USA 3 Finnish Meteorological Institute, 00101, Helsinki, Finland Keywords: nanoparticles, hygroscopic growth, water monolayers INTRODUCTION The atmosphere contains small nanoparticles (below 100 nm) with concentrations from few to hundreds of thousands per cubic centimeter. These nanoparticles are exposed to water vapor in atmosphere (typical relative humidity (RH) range from 30 to 100 %) that produces films on their surfaces. The thickness and structure of this film is determined by the relative humidity, interaction forces between the surface of the nanoparticle and the adsorbed water molecules (Thiel 1987). This water layer on nanoaerosol has major effect as it affect the scattering and absorbance of radiation. It can also have important consequences for heterogeneous chemistry in the atmosphere as the kinetics of many reactions is strongly dependent on the phase of aerosol particles (Knipping et al. 000). The sources of the atmospheric nanoparticles are natural, e.g., mineral dust, volcanic ash and sea spray (Satheesh and Krishna Moorthy, 005; Kanakidou et al., 005), or human made, e.g., aerosols produced in industrial processes, biomass burning and in fossil fuel combustion (Dubovik et al., 001). These particles consist of wide range of compounds, for example mineral dust is dominated by SiO and sea-salt by NaCl. Atmosphere consist also high amount of (NH 4 ) SO 4 particles which can actually be a result from natural and human made sulfur and ammonia sources. (NH 4 ) SO 4 aerosol has been very popular research target as it has ability deliquiscence into water droplets and thus greatly increases surface area and reflectivity of these particles. Water uptake of nanoparticles can be experimentally studied by various techniques e.g. thermogravimetric analysis (TGA) (Gustafsson et al., 005), physisorption technique (Ma et al., 010), X-ray photoemission and absorption spectroscopies (Verdaguer et al., 007), electrodynamic balance (EDB) (Cohen et al., 1987) and hygroscopic tandem differential mobility analyzer (H-TDMA) (Liu et al., 1978). As an example, recently, physisorption method was used to study water adsorption capacity of AlO3, NaCl, NH 4 NO 3, and (NH 4 ) SO 4 nanoparticles (over 100 nm) (Ma et al., 010). The water uptake and monolayer adsorption of the atmospherically relevant mineral/dust particles have also been studied by combined TGA and HTDMA measurements (Gustafsson et al., 005). H-TDMA is ideal for nanoparticles hygroscopic growth detection as it has capacity to measure the particle diameter (nano-dma from 3 nm) in dry and humid conditions. The hygroscopic growth factors can be then defined by the relation of particle size in humid and dry air. In fact, the H-TDMA studies at undersaturated conditions (RH = 90%) has been included to field campaigns in atmospheric aerosol studies all over the world for last decades (Swietlicki et al., 008). This study showed, that H-TDMA data is greatly facilitated by a classification of the observed hygroscopic growth factors into nearly hydrophobic, less-hygroscopic, more-hygroscopic and sea salt particles (Swietlicki et al., 008). However, when studying first monolayers adsorption on these nanoparticles the GF s need to be defined at lower RH s (e.g. Gustafsson et al. 005 and Romakkaniemi et al., 000). In fact, a water monolayer thickness is 0.19 nm and it would result the hygroscopic growth factor of on the particle having diameter of 100 nm and 1.04 on 10 nm, respectively. Therefore, the detecting of the monolayers based on the size change on the larger sizes is very tricky as even the minimum measurement uncertainities would shade the growth. However, at nanosizes (diameter = 8 15 nm) this is possible and have been done e.g. for salt particles such as NaCl and (NH 4 ) SO 4, where water monolayers adsorption were detected before these salt deliquescence relative humidity (DRH) (Romakkaniemi et al., 000).

2 Here, the aim is to study the hygroscopic growth factors and water monolayers adsorption on nanoparticles at subsaturated water vapor conditions by nano-h-tdma. The studied particulate compounds are (NH 4 ) SO 4 and fumed-sio within size range from 8 to 50 nm. METHODS The nanoparticles are generated from salt-solution or suspension by an aerosol generator (Model 3076, TSI Inc., USA). The (NH 4 ) SO 4 (Sigma Aldrich, %) /water (de-ionized) solution concentration was set to 1 wt %. Solid content in the water suspension of SiO (Degussa, Aerosil, d BET = 7 nm) was set to 0.06 wt %. The produced aerosol was fed to diffusion drier (porous tube surrounded by silica gel) result relative humidity below 5 % (RH sensor, Rotronic). Hygroscopic growth is measured using the nano-htdma (Hygroscopicity Tandem Differential Mobility Analyzer). Here, the produced nanoparticles are first size-selected by nano-dma1 (Model 3085, TSI Inc., USA) based on its electrical mobility. DMA1 has sheath and sample flows at 10 and 1 l/min, respectively. These nanoparticles with known size are first pre-humidified (Relative Humidity (RH) = %) in nafion tube (Perma Pure, USA). The residence time in pre-humidifier is 5 s. The size-selected particles distribution is measured by nd nano-dma (Model 3085, TSI Inc., USA) and water CPC (Condensation Particle Counter; Model 3786, TSI Inc., USA). DMA has sheath and sample flows 5 and 1 l/min, respectively. The nd nano-dma sheath air is humidified to same RH as in the pre-humidifier. The humidification system for pre-humidifier and sheath air is consisted of mass flow controller (Brooks and DIN, ISE, Inc., USA) controlling the relative humidity by humidified flow trough only water vapour permeable core-text tubing placed in water bath (T = 30 ºC) and dry air. The RH and T of the measured aerosol are measured before and after DMAs and pre-humidifier. This entire set-up is placed in the temperature isolated room (accuracy in the set-up ± 0. 5 ºC.). The aerosol and sheath air RH is kept same within accuracy ± 1 %. The RH, T, selected sizes and DMPS data are monitored, controlled and preanalyzed by using standard DMA data inversion algorithm (Reischl, 1991; Knutson and Whitby, 1975) in Visual Basic Program (like in Joutsensaari et al., 001). The produced nanoparticles size distributions before size-selection is measured by DMA and CPC (max size range nm) (DMA1 bypassed). The hygroscopic growth factor is defined as the ratio of the humidified to dry diameter. In the data evaluation possible diffusion losses are taken account as the growth factors are corrected by measured dry particle size: GF= d dry /d RH, (1) where d dry is measured geometric mean diameter for dry particles (RH below 5 %) and d RH is the geometric mean diameter at humid conditions. The water monolayers numbers (N mlr ) was calculated from: where l ml is 0.19 nm which is one water monolayer thickness. N ml = (d RH - d dry )/l ml, () RESULTS AND DISSCUSSION We carried out first the experiments with (NH 4 )SO nanoparticles. The atomization of (NH 4 )SO solution produced nanoparticles with the geometric mean diameter (GMD) of 45 ± 0.4 nm, geometric standard deviation (GSD) 1.8 ± 0.0 within total number concentration 0.5 * 10 6 #/ cm 3 from which the sizes of 10, 15, 0, 30 and 50 nm were selected. Figure 1 shows the growth factors (GF) as a function of particle size

3 at 50 90% RH. At higher RH (81-90%) the GFs increase with increased RH and particle size due the Kelvin effect. For 30 nm particles at RH = 80% the obtained GF = 1.36 showed a good agreement with earlier study by Hämeri et al. 000 (GF = 1.4). The growth factors here at RH =90 % for 10 to 30 nm are in same range as the more hygroscopic ones observed during the nucleation events while the sulphuric acid concentration were substantially high (Swietlicki et al., 010). The growth factors at lower RHs at 7-78% before the salt particle deliquescence RH (DRH) show the opposite trend as the growth factor decreases with increased particle size. One explanation to this trend could be water adsorption before the DRH (Romakkaniemi et al., 001). Figure a shows the water monolayer (0.19 nm) number versus initial particle diameter at RH= 78 %. Even when GF decreases with particle diameter the monolayer number stays quite constant in range 5 to 7, whilst in the study by Hämeri et al. (000) the 0 and 50 nm particles were already at their DRH at RH= 78 % while Biskos et al. (006) did not detect full monolayers before the DRH. Within the theory based on the FHH (Frenkel, Halsey, and Hill) and BET (Brunauer, Emmett, and Teller) the monolayer number should slightly decrease within particle size (Romakkaniemi et al., 000) because of Kelvin effect. In Figure b we can observe that no full monolayers were formed at RH = 50% and the monolayer number slight decreases at RH = 7 and 78 % until the particle size reach 15 nm. However, at 10 nm the monolayers number was increased from the one obtained at 15 nm (Figure b). This GF and monolayer number increase at the smallest selected size (10 nm) could be result from salt or water impurities (e.g. NaCl). Growth factor 1,8 1,6 1,4 1, RH= 90 % 81 % 78 % 7 % 50 % 1,0 Figure 1. Hygroscopic growth factor versus initial particle diameter at RH= 50, 7, 78, 81, 90 %. At higher RH (81-90%) the GFs increase with increased RH and particle size due the Kelvin effect at lower RH the trend is opposite RH= 78 % RH= 78 % RH =78 % RH = 7 % RH= 50 % Monolayer # 1 Monolayer # (a) This study (RH =RH ) sheat aero Hämeri et al., 000 (RH sheat =RH aero + 3 %) Biskos et al., 006 (RH sheat = RH aero ) Figure. (a) Monolayer number versus initial particle diameter at RH= 78 %, and (b) at 78, 7 and 50 %. (b)

4 The fumed SiO -suspension atomization before size selection produced the size distribution with GMD = 7 ± 0.7 nm, GSD =1.8 ± 0.06 and total number concentration 1 * 10 4 ± 0.1 #/cm 3 (Figure 3). The distribution before and after H-TDMA measurement was staying same, which indicates that suspension was rather stabile. However, most probably also larger agglomerates were produced but not measured as the size range within used set-up (nano-dma) ended at 141 nm. The fumed silica primary particle size was 7 nm (measured by manufacturer (BET-analysisi), Aerosil, Degussa). The fumed SiO consist of the primary particles, aggregates and agglomerates and its primary particle size is known to be distributed within close to the self-preserving size distribution with GSD = 1.45 (Heine and Pratsinis, 007). Thus, it is expected that the produced SiO nanoparticles agglomerate fraction is increasing with particle size..5e+4.0e+4 Fumed SiO Before measurement After measurement dn/dlogdp 1.5e+4 1.0e+4 5.0e Figure 3. Number size distribution of produced fumed-sio nanoparticles measured by DMA and CPC stayed rather stabile during the experiments. The fumed-sio particles with diameter 8, 10, 15, 0, 30 and 50 nm were selected for the hygroscopicity measurements. Figure 4 shows the growth factors (GF) as a function of RH for each selected size. SiO is hydrophilic and do not solve to the water, so only monolayers formation should affect on these GFs. The growth factors (GF ) at RH =90 % for size range 8 to 0 nm would be classified as more hygroscopic ones and at 30 to 50 nm (GF = ) as less hygroscopic ones in comparison to Swietlicki et al. 008 GFs review obtained during nucleation events. Mineral dust composition is dominated by SiO and Gustafsson et al. 005 studied the Arizona test dust particles (~ 60 nm) hygroscopic growth factors. They obtained GFs from to 1.08 at RH range from 0 to 80 %. However, the results interpretation is now very complicated. Firstly, in our study the produced SiO particles are most probably agglomerates at least at larger sizes. Secondly, the exact size depended morphology and composition is also unknown for reference studies. However, agglomerate structure can reconstruct during the humidification which has been observed to decrease the growth factor for nanosized soot agglomerates (Zhang et al., 008). Thus, here, the observed clear drop in growth factors from particle diameter 0 to 30 nm (Figure 4) is most probably caused by agglomerates reconstruction.

5 Growth factor nm 10 nm 15 nm 0 nm 30 nm 50 nm Fumed SiO RH % Figure 4 Hygroscopic growth factor versus RH at selected dry particle diameters 8, 10, 15, 0, 30 and 50 nm for fumed SiO nm 10 nm Verdaguer et al. 007 (on flat SiO surface) Fumed SiO Monolayers, # Figure 5 Monolayer number versus RH at selected dry particle diameters 8 (spheres), 10 (triangles) nm for fumed SiO and for flat SiO surface (adapted from Verdaguer et al., 007). RH % Figure 5 shows the water monolayer (0.19 nm) number versus RH at selected dry particle diameters 8 and 10 nm for fumed SiO. As expected, the monolyaers number increase with RH and particle size. In fact, SiO is hydrophilic and interact strongly with the water (Saliba et al., 001). It is proposed that first - 3 monolayers on SiO interact with surface via hydrogen bonding with silanol groups (Verdaguer et al., 007). The monolayers numbers defined by physisorption methods of water adsorption on flat SiO surfaces (Figure 5, (broken line); Verdaguer et al., 007) are very close the ones observed here (Figure 5, spheres and triangles). This may suggest that the Kelvin effect have minimal influence for the monolayer adsorption on the strongly hydrophilic surfaces. However, this need to be further reproduced and explored, especially, the SiO particles exact size-selected morphology needs to be solved. CONCLUSIONS These preliminary experiments showed that for (NH 4 ) SO 4 the GF increases within particle size after DRH and decreases within particle size slightly below DRH. They also showed that there are few 5 to 7 monolayers formed before DRH. For SiO nanoparticles the GFs dropped strongly at 30 nm, which is most probably caused by agglomerates restructuring. For 8 and 10 nm SiO particles monolayer number agreed with the one measured for flat SiO surfaces. These hydrophilic surfaces had at least water monolayers at RH = 50 90%. These experiments will be further developed by size-selective

6 morphology/composition and explored in near future to several atmospherically relevant nanoparticles (e.g. Arizona dust, and surfactants). ACKNOWLEDGEMENTS This work was supported by the academy of Finland (Center of Excellence, No and decision numbers 13466, 11830), Kone foundation and Saastamoinen foundation. REFERENCES Biskos et al. (006) Prompt deliquescence and efflorescence of aerosol nanoparticles, Atmos. Chem. Phys. 6, Cohen, M. D., Flagan, R. C. and Seinfeld, J. H. (1987) Studies of concentrated electrolyte solutions using the electrodynamic balance. : water activities for mixed-electrolyte solutions. J. Phys. Chem. 91, Dubovik et al. (00) Variability of absorption and optical properties of key aerosol types observed in worldwide locations, J. Atmos. Sci. 59, Gustafsson et al., (005) Water uptake on mineral surfaces, Atmos. Chem. Phys. 5, Hameri et al. (000) Hygroscopic growth of ultrafine ammonium sulphate aerosol measured using an ultrafine tandem differential mobility analyzer, J. Geophys. Res.-Atmos. 105, Heine M. C. and S. E. Pratsinis, (005) Ind. Eng. Chem. Res. 44, 6. Joutsensaari et al. (001). A novel tandem differential mobility analyzer with organic vapour treatment of aerosol particles. Atmos. Chem. Phys. 1, Kanakidou et al. (005), Organic aerosol and global climate modelling: a review, Atmos. Chem. Phys., 5, Knipping et al. (000) Experiments and Simulations of Ion-Enhanced Interfacial Chemistry on Aqueous NaCl Aerosols, Science, 88, 301. Knutson, E. O. and Whitby, K. T. (1975) Aerosol classification by electric mobility: apparatus, theory and applications, J. Aerosol Sci., 6, Liu et al. (1978) Aerosol mobility chromatograph newdetector for sulphuric-acid aerosols. Atmos. Environ. 1, Ma et al.(010), The Utilization of Physisorption Analyzer for Studying the Hygroscopic Properties of Atmospheric Relevant Particles, J. Phys. Chem. A, 114, Reischl, G. P. (1991) Measurement of ambient aerosols by the different mobility analyzer method: concepts and realization criteria for the size range between and 500 nm, Aerosol Sci. Technol., 14, 5 4. Romakkaniemi, et al. (001) Adsorption of water on 8-15 nm NaCl and (NH 4 ) () SO 4 aerosols measured using an ultrafine tandem differential mobility analyzer, J. Phys. Chem. A 105, Saliba et al., (001) Reaction of gaseous nitric oxide with nitric acid on silica surfaces in the presence of water at room temperature, J. Phys. Chem. A, 105, Satheesh, S.K. and Krishna Moorthy, K. (005) Radiative effects of natural aerosols: a review, Atm. Environ., 39, Swietlicki et al., (008) Hygroscopic properties of submicrometer atmospheric aerosol particles measured with H-TDMA instruments in various environments a review, Tellus, 60B, Thiel, P. A. and T.E. Madey, (1987) The interaction of water with solid-surfaces fundamental-aspects. Surf. Sci. Rep. 7, 11. Verdaguer et al., (007) Growth and structure of water on SiO films on Si investigated by Kelvin probe microscopy and in situ X-ray Spectroscopies, Langmuir, 3,