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Atmospheric Environment 123 (2015) 166e170

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Atmospheric Environment

journal homepage: www.elsevier .com/locate/atmosenv

Images and properties of individual nucleated particles

Zolt�an N�emeth a, Mih�aly P�osfai b, Ilona Nyir}o-K�osa c, Pasi Aalto d, Markku Kulmala d,Imre Salma a, *

a Institute of Chemistry, E€otv€os University, H-1518 Budapest, P.O. Box 32, Hungaryb Department of Earth and Environmental Sciences, University of Pannonia, H-8200 Veszpr�em, P.O. Box 158, Hungaryc MTA-PE Air Chemistry Research Group, H-8200 Veszpr�em, P.O. Box 158, Hungaryd Department of Physics, University of Helsinki, FI-00014 Helsinki, P.O. Box 64, Finland

h i g h l i g h t s

� Collection of nucleated particles in the beginning of their growth process was realised.� Nucleated particles clearly differ from other particle types on TEM images.� Nucleated particles show homogeneous contrast and are very volatile.

a r t i c l e i n f o

Article history:Received 19 February 2015Accepted 19 October 2015Available online 2 November 2015

Keywords:Atmospheric nucleationElectron microscopy

* Corresponding author.E-mail address: salma@chem.elte.hu (I. Salma).

http://dx.doi.org/10.1016/j.atmosenv.2015.10.0511352-2310/© 2015 Elsevier Ltd. All rights reserved.

a b s t r a c t

Atmospheric aerosol particles were collected in Budapest, Hungary in AprileJune onto lacey Formvarsubstrates by using an electrostatic precipitator during the beginning phase of the particle growthprocess in ten nucleation and growth events. Median contribution of the nucleated particles - expressedas the concentration of particles with a diameter between 6 and 25 nm to the total particle numberconcentration e was 55%, and the median electrical mobility diameter of the particles was approxi-mately 20 nm. The sample was investigated using high-resolution transmission electron microscopy(TEM) and electron energy-loss spectroscopy. Major types of individual particles such as soot, sulphate/organic and tar ball particles were identified in the sample. In addition, particles with an optical diameterrange of 10e30 nm were also observed. They clearly differed from the other particle types, showedhomogeneous contrast in the bright-field TEM images, and evaporated within tens of seconds whenexposed to the electron beam. They were interpreted as representatives of freshly nucleated particles.

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

New atmospheric aerosol particles are produced by two distinctmechanisms: particulate emission or atmospheric nucleation.Shape and structure of particles originating from emission sources(e.g., soot aggregates, mineral dust or sea salt particles), and ofaccumulation-mode secondary aerosol particles formed bycondensation (e.g., particles consisting mostly of sulphate and/ororganics) were characterised previously (e.g., P�osfai and Buseck,2010). There is less information available on the shape and inter-nal structure of individual nucleated particles, despite theirimportance in key atmospheric processes (Kulmala et al., 2013). It

was estimated that the total particle number concentrations inregional background environments as well as in the global tropo-sphere are dominated by new particle formation processes(Spracklen et al., 2010). The freshly nucleated particles usually growin size, and if they reach equivalent diameters >50e100 nm, theycan act as cloud condensation nuclei (CCN) in aerosolecloud in-teractions (Andreae and Rosenfeld, 2008; Kerminen et al., 2012),and hence affect the aerosol indirect forcing of climate (Carslawet al., 2013). Global model simulations suggest that approximately50% of low-level cloud CCN at 0.2% supersaturation are derivedfrom nucleation (Merikanto et al., 2009). On local spatial scales, thenucleated particles also influence human health in cities (Salmaet al., 2014, 2015).

Nucleated particles are usually investigated by using on-lineexperimental methods (Kulmala et al., 2004, 2012). Several vari-ants of individual-particle (or aerosol) mass spectrometry (AMS)

Z. N�emeth et al. / Atmospheric Environment 123 (2015) 166e170 167

were shown to be valuable techniques for studying these particles.Collection of ultrafine (UF) aerosol particles enhanced by electro-static forces became also feasible (M€akel€a et al., 2002; Fierz et al.,2007). The collection performed in time intervals when the num-ber concentrations of nucleated particles prevail over the pre-existing aerosol concentrations results in aerosol samples thatcan be utilised to explore the properties of nucleated particles.Electron microscopy techniques are valuable for such studies. Themain goals of this paper are to present a collection method fornucleated particles, to show images of individual/single nucleatedparticles, to identify the morphological features that distinguishnucleated particles from the other UF particles, and to discuss someof their related properties.

2. Methods

Aerosol samples were collected by an electrostatic precipitatorsimilar to that described by Fierz et al. (2007). Its sampling regionconsists of a vertical, electrically conducting metal rod with adiameter of 5 mm. The sampling substrate is placed on the top-plate of the rod, and it is positioned by a cap made of Teflon.Lacey Formvar film coated with amorphous carbon on a 200-meshcopper grid disk with a diameter of 3 mm (Ted Pella Inc., USA) wasused as the sampling substrate. The sampled air flows around therod with a flow rate of 4 L min�1. A negative high voltage of 10 kV isapplied to the substrate. Positively charged particles are collectedon the substrate by electrostatic forces. The collection efficiency isthe largest for the UF particles, and it approaches 100% for 10-nmparticles (Fierz et al., 2007). No electrical charging was applied tothe aerosol at the inlet.

The collections were accomplished during the beginningphase of particle growth process. The sampling substrate wasexposed to particles ten times in ten nucleation and growthevents to increase the number of particles collected on the grid.Overview data on the collection intervals are shown in Table 1.The total sampling time and sampled air volume were 21 h and1.0 m3, respectively. A differential mobility particle sizer (DMPS)was operated in parallel with the sampler in order to determinethe start and end times of the collections as well as to monitorthe continuity of the growth process during the collection in-tervals. The system operates in an electrical mobility diameterrange from 6 to 1000 nm in 30 size channels. The diameters referto the dry state of the particles. The measurements were per-formed with a time resolution of approximately 10 min. TheDMPS system and method fulfil the recommendations of theinternational technical standards (Wiedensohler et al., 2012). Thecollections and on-line measurements were performed at theBudapest Platform for Aerosol Research and Training (BpART)

Table 1Starting date in 2014 and duration of the aerosol collections with the concentrations of pconcentration ratio of N6e25 to N6e1000, air temperature and relative humidity for the co

Sampling interval Starting date and time Duration [h:mm] N6e25

1 24-04 11:46 2:30 14,4712 27-04 11:31 3:04 11,4643 29-04 11:54 0:55 10,7244 30-04 12:29 2:00 73805 06-05 11:58 1:54 36566 07-05 09:56 2:48 18,7207 19-05 11:29 2:06 65508 22-05 11:45 2:03 62569 02-06 10:52 1:49 808410 11-06 11:23 2:09 12,871Median 2:04 9404

located in an open area (latitude 47� 280 29.86500 N, longitude 19�

030 44.69300 E, altitude 117 m above mean sea level) near the riverDanube in central Budapest, Hungary from March to June 2014.The nucleation frequency in Budapest exhibits maximum duringthe spring season (Salma et al., 2011). The new particle formationand growth events observed in Budapest often extend horizon-tally over large areas of the Carpathian Basin (N�emeth and Salma,2014), and therefore, the sample likely represents a larger regionin Central Europe.

The aerosol sample was investigated by transmission electronmicroscopy (TEM) and electron energy-loss spectroscopy (EELS)using a JEM-3010 high-resolution analytical transmission electronmicroscope (JEOL, Japan). The instrument was operated at 300-kVaccelerating voltage and had a point-to-point resolution of 0.17 nm.We recorded conventional bright-field and high-resolution imagesusing a Gatan Orius CCD camera, and obtained three-windowelectron energy-loss elemental maps using a Gatan GIF system.

3. Results and discussion

3.1. Particles collected

Starting date, collection time and particle number concentra-tions in different size fractions for the ten collection time intervalstogether with some meteorological values are also shown inTable 1. It can be seen that the total particle number concentrationswere usually above the yearly median of 11.8 � 103 cm�3 for thesame location (Salma et al., 2011), as expected for the nucleationand particle growth intervals. Freshly nucleated particles as rep-resented by the concentration of particles with a diameter between6 and 25 nm (N6�25) usually exhibited the largest contribution tothe total particle number concentrations, with a median of 55%.This confirms our aim that the particle number concentrations inthe air during the collections were dominated by nucleated parti-cles; hence, the particles collected on the substrate mostly origi-nated from nucleation events. The meteorological data show thatordinary atmospheric conditions were present during thecollections.

Median particle number concentrations for each DMPS sizechannel were obtained from the measured normalised concentra-tion data for the whole collection time, and these were utilised forderiving the median particle number size distribution (Fig. 1). Theresults show that the particle number median mobility diameterwas approximately 20 nm, which is much smaller than 42 nm, avalue typically derived for the same urban location (Salma et al.,2014). This implies that the collected nucleated particles wereindeed in the beginning phase of their growth process.

articles in the size fraction from 6 to 25 nm (N6e25), of total particles (N6e1000), thellection intervals. Median values of the data sets are indicated in the last row.

[cm�3] N6e1000 [cm�3] N6e25/N6e1000 [%] T [�C] RH [%]

22,367 65 23 5615,504 71 21 4915,428 48 21 5817,823 43 23 488801 40 20 48

28,876 63 20 6011,040 57 24 5412,506 50 28 4611,207 71 22 5224,302 52 33 4215,466 55 22 51

Fig. 1. Median particle number size distribution of atmospheric aerosol particles forthe collection time interval.

Z. N�emeth et al. / Atmospheric Environment 123 (2015) 166e170168

3.2. Particle types

TEM images of atmospheric aerosol particles collected on thefibres of the lacey substrate are shown in Fig. 2. Major types ofreadily identifiable particles include nanosphere-soot (ns-soot)aggregates (Buseck et al., 2014), particles containing internallymixed sulphate and organic compounds in various proportions, andtar-ball particles. Some coarse and refractory particles, probablymostly mineral dust and anthropogenic primary particles were alsopresent but these were not analysed. Nanosphere-soot aggregates(Fig. 2) are associations of spherules with diameters between 10and 50 nm in branching or compact clusters. The spherules are builtof curved, graphene-like layers wrapped into spheres (P�osfai et al.,2003; Buseck et al., 2014). An example of their distinctive internal

Fig. 2. Bright-field TEM image showing a representative area of the TEM grid. Aerosolparticles are attached to the fibres of the lacey substrate. The black arrows mark sul-phate/organic particles (some with nanosphere-soot inclusions), the white arrowspoint to nanosphere-soot aggregates, and a spherical tar-ball particle is present in thelower left part. The insert in the lower right part shows a high-resolution TEM image ofan exceptionally small nanosphere-soot particle; the stacks of wavy lines are producedby graphene-like layers that are viewed edge-on in certain parts of the particle.

structure is presented in the high-resolution image in Fig. 2. The ns-soot particles did not show changes in the vacuum or under theelectron beam. Interestingly, even very fine soot particles occur inthe images that appear to be composed only of one or two incipientnanospheres. Aggregation of soot with sulphate is also evident.Internal mixtures of ammonium sulphate and organic compoundswere observed as well (Fig. 2). One of the easiest ways to identifysulphate particles is by their sensitivity to the electron beam. Theirmottled texture results from the decomposition of ammoniumsulphate in the electron beam (P�osfai et al., 2004; P�osfai andBuseck, 2010). Eventually, the sulphate component sublimatesand only S-bearing carbonaceous residues are left behind. Tar-ballparticles (P�osfai et al., 2003, 2004) can be also unambiguouslyrecognised (Fig. 2). They have almost perfect spherical shapes andmedian in-vacuum optical diameters between approximately 100and 500 nm. In contrast to ns-soot particles, the tar balls do notpossess an internal structure. These particles are typically notaggregated with other particles.

In addition to the above particle types, which had been observedand described previously, we identified a new particle class thatwere interpreted as nucleated particles captured in their growthstage.

3.3. Properties of individual nucleated particles

Individual nucleated particles occurred as single, non-aggregated particles with a geometric size smaller than or com-parable to the sizes of spherules in the ns-soot aggregates. Theiroptical diameters in TEM ranged from 12 to 50 nmwith a median ofapproximately 27 nm (Fig. 3). The particles showed homogeneousand weak contrast relative to the background substrate, whichsuggests that they were thin and were film-like or disk-shapedobjects on the collection surface (Fig. 4a). Although the thicknessof the particles is difficult to estimate from their two-dimensionalprojections, some of the particles appear to be attached to the fi-bres of the lacey filmwith their smaller dimension (with the heightof the disk) perpendicular to the electron beam. Two such particlesare marked by arrows in Fig. 4a. Based on themeasured dimensionsof these particles, we estimated that the maximum thickness of thedisks (nucleated particles) is 15 nm. Assuming disk-shaped parti-cles with a diameter of 27 nm and a thickness of 10 nm, the mediandiameter of a volume-equivalent sphere is calculated to be 22 nm,which is in agreement with the measured median mobility diam-eter of 20 nm.

A surprising feature of the distribution of these particles on thegrid is that they occur in relatively large numbers only in certain

Fig. 3. Particle number size distribution of nucleated particles as determined from theTEM images.

Fig. 4. Bright-field TEM images obtained from a sample area containing a swarm of nucleated particles (encircled) before (a) and after (b) exposure to a strong electron beam. Theswarm of small particles in the centre and the upper part of the image completely disappeared, while the nanosphere-soot particle in the left has not changed and the sulphate/organic particle in the upper right mostly decomposed in the beam. The arrows mark nucleated particles that are discussed in the text.

Z. N�emeth et al. / Atmospheric Environment 123 (2015) 166e170 169

limited regions of the substrate, as if they had been collected in“swarms”. This uneven distribution on the TEM grids can well be acollection artefact that resulted from minor inhomogeneities inaerodynamics within the sampler that were not strong enough toaffect the distribution of larger, Aitken- and accumulation-modeparticles. Another possible - albeit less likely - explanation is thatfreshly nucleated particles occurred in swarms in the air. It is notedthat a similar distribution of newly formed secondary organicparticles can be observed in the scanning electron microscopy(SEM) images obtained by Virtanen et al. (2010) as well.

The nucleation particles apparently survive the high-vacuumconditions within the TEM column; however, they are volatilewhen exposed to the electron beam. Several nucleated particlestogether with a soot and a sulphate/organic particle are shown inFig. 4a. After exposure to an intense electron beam for a few sec-onds, the particles completely disappeared without leaving anyvisible residues (Fig. 4b), while the soot particle remained un-changed and the sulphate/organic particle lost its sulphate content.After the disappearance of the nucleated particles, elemental mapswere obtained from the same region using three-window electronenergy-loss imaging. Carbon, O and S maps were collected (Fig. 5,panels a, b and c) but none of them suggested higher concentra-tions of these elements at the positions where the original particleshad been present. The lack of any signal in the elemental maps canbe related to both the extremely small amount of material in theoriginal particles and the fact that most of their mass sublimated in

Fig. 5. Electron energy-filtered images showing the distribution of C, O and S in the same sawith minor oxygen. In addition to C, some O and S (in heterogeneous distribution) occurs ienriched in all three elements. However, no enrichment of any of the three elements can b

the electron beam. Thus, unfortunately, the elemental maps did notprovide information on the compositions of nucleated particles.

4. Conclusions

In the present paper, TEM images of freshly nucleated particlesin the beginning stage of their growth process are presented. Theseparticles can be clearly distinguished from the other types of fine orultrafine particles. They withstood high-vacuum conditions in theTEM but were volatile under the electron beam. The volatility andsmall size of nucleated particles strongly limited the informationthat could be obtained about them using conventional electronmicroscopy techniques. In spite of this disadvantage, TEM is stilluseful for observing such particles and to trace their coagulationand growth processes. Obtaining elemental compositions, espe-cially the C/S ratios in nucleated particles would be very importantfor an understanding of their formation mechanism and physicalproperties. Such data may be acquired from individual particles byusing a combination of advanced scanning transmission electronmicroscopy (STEM) techniques including low-voltage imaging,cryo-TEM and high-resolution EELS in a state-of-the-art, aberra-tion-corrected electron microscope. These improvements indicatethe further possible steps that are required in the studies dedicatedto individual freshly nucleated particles.

mple region as shown in Fig. 4. The supporting substrate is mainly composed of carbon,n the nanosphere-soot particle, whereas the residue of the sulphate/organic particle ise observed at the former sites of the nucleated particles.

Z. N�emeth et al. / Atmospheric Environment 123 (2015) 166e170170

Acknowledgement

Financial support by the Hungarian Scientific Research Fund(contract K84091) is appreciated.

References

Andreae, M.O., Rosenfeld, D., 2008. Aerosol-cloud-precipitation interactions. Part 1.The nature and sources of cloud-active aerosols. Earth Sci. Rev. 89, 13e41.

Buseck, P.R., Adachi, K., Gelencs�er, A., Tompa, �E., P�osfai, M., 2014. Ns-soot: amaterial-based term for strongly light-absorbing carbonaceous particles.Aerosol Sci. Technol. 48, 777e788.

Carslaw, K.S., Lee, L.A., Reddington, C.L., Pringle, K.J., Rap, A., Forster, P.M.,Mann, G.W., Spracklen, D.V., Woodhouse, M.T., Regayre, L.A., Pierce, J.R., 2013.Large contribution of natural aerosols to uncertainty in indirect forcing. Nature503, 67e71.

Fierz, M., Kaegi, R., Burtscher, H., 2007. Theoretical and experimental evaluation of aportable electrostatic TEM sampler. Aerosol Sci. Technol. 41, 520e528.

Kerminen, V.-M., Paramonov, M., Anttila, T., Riipinen, I., Fountoukis, C.,Korhonen, H., Asmi, E., Laakso, L., Lihavainen, H., Swietlicki, E., Svenningsson, B.,Asmi, A., Pandis, S.N., Kulmala, M., Pet€aj€a, T., 2012. Cloud condensation nucleiproduction associated with atmospheric nucleation: a synthesis based onexisting literature and new results. Atmos. Chem. Phys. 12, 12037e12059.

Kulmala, M., Vehkam€aki, H., Pet€aj€a, T., Dal Maso, M., Lauri, A., Kerminen, V.,Birmili, W., McMurry, P., 2004. Formation and growth rates of ultrafine atmo-spheric particles: a review of observations. J. Aerosol Sci. 35, 143e176.

Kulmala, M., Pet€aj€a, T., Nieminen, T., Sipila, M., Manninen, H.E., Lehtipalo, K., DalMaso, M., Aalto, P.P., Junninen, H., Paasonen, P., Riipinen, I., Lehtinen, K.E.J.,Laaksonen, A., Kerminen, V.M., 2012. Measurement of the nucleation of atmo-spheric aerosol particles. Nat. Protoc. 7, 1651e1667.

Kulmala, M., Kontkanen, J., Junninen, H., Lehtipalo, K., Manninen, H.E., Nieminen, T.,Pet€aj€a, T., Sipil€a, M., Schobesberger, S., Rantala, P., Franchin, A., Jokinen, T.,J€arvinen, E., €Aij€al€a, M., Kangasluoma, J., Hakala, J., Aalto, P.P., Paasonen, P.,Mikkil€a, J., Vanhanen, J., Aalto, J., Hakola, H., Makkonen, U., Ruuskanen, T.,Mauldin III, R.L., Duplissy, J., Vehkam€aki, H., B€ack, J., Kortelainen, A., Riipinen, I.,Kurt�en, T., Johnston, M.V., Smith, J.N., Ehn, M., Mentel, T.F., Lehtinen, K.E.J.,Laaksonen, A., Kerminen, V.-M., Worsnop, D.R., 2013. Direct observations ofatmospheric aerosol nucleation. Science 339, 943e946.

Merikanto, J., Spracklen, D.V., Mann, G.W., Pickering, S.J., Carslaw, K.S., 2009. Impactof nucleation on global CCN. Atmos. Chem. Phys. 9, 8601e8616.

M€akel€a, J.M., Hoffmann, T., Holzke, C., V€akev€a, M., Suni, T., Mattila, T., Aalto, P.P.,Tapper, U., Kauppinen, E.I., O'Dowd, C.D., 2002. Biogenic iodine emissions andidentification of end-products in coastal ultrafine particles during nucleation

bursts. J. Geophys. Res. 107, 8110. http://dx.doi.org/10.1029/2001JD000580.N�emeth, Z., Salma, I., 2014. Spatial extension of nucleating air masses in the Car-

pathian Basin. Atmos. Chem. Phys. 14, 8841e8848.P�osfai, M., Simonics, R., Li, J., Hobbs, P.V., Buseck, P.R., 2003. Individual aerosol

particles from biomass burning in southern Africa: 1. Compositions and sizedistributions of carbonaceous particles. J. Geophys. Res. 108, 8483. http://dx.doi.org/10.1029/2002JD002291.

P�osfai, M., Gelencs�er, A., Simonics, R., Arat�o, K., Li, J., Hobbs, P.V., Buseck, P.R., 2004.Atmospheric tar balls: particles from biomass and biofuel burning. J. Geophys.Res. 109, D06213. http://dx.doi.org/10.1029/2003JD004169.

P�osfai, M., Buseck, P.R., 2010. Nature and climate effects of individual troposphericaerosol particles. Annu. Rev. Earth Planet. Sci. 38, 17e43.

Salma, I., Bors�os, T., Weidinger, T., Aalto, P., Hussein, T., Dal Maso, M., Kulmala, M.,2011. Production, growth and properties of ultrafine atmospheric aerosol par-ticles in an urban environment. Atmos. Chem. Phys. 11, 1339e1353.

Salma, I., Bors�os, T., N�emeth, Z., Weidinger, T., Aalto, P., Kulmala, M., 2014.Comparative study of ultrafine atmospheric aerosol within a city. Atmos. En-viron. 92, 154e161.

Salma, I., Füri, P., N�emeth, Z., Farkas, �A., Bal�ash�azy, I., Hofmann, W., Farkas, �A., 2015.Lung burden and deposition distribution of inhaled atmospheric urban ultra-fine particles as the first step in their health risk assessment. Atmos. Environ.104, 39e49.

Spracklen, D.V., Carslaw, K.S., Merikanto, J., Mann, G.W., Reddington, C.L.,Pickering, S., Ogren, J.A., Andrews, E., Baltensperger, U., Weingartner, E., Boy, M.,Kulmala, M., Laakso, L., Lihavainen, H., Kivekas, N., Komppula, M.,Mihalopoulos, N., Kouvarakis, G., Jennings, S.G., O'Dowd, C., Birmili, W.,Wiedensohler, A., Weller, R., Gras, J., Laj, P., Sellegri, K., Bonn, B., Krej�cí, R.,Laaksonen, A., Hamed, A., Minikin, A., Harrison, R.M., Talbot, R., Sun, J., 2010.Explaining global surface aerosol number concentrations in terms of primaryemissions and particle formation. Atmos. Chem. Phys. 10, 4775e4793.

Virtanen, A., Joutsensaari, J., Koop, T., Kannosto, J., Yli-Piril€a, P., Leskinen, J.,M€akel€a, J.M., Holopainen, J.K., P€oschl, U., Kulmala, M., 2010. An amorphous solidstate of biogenic secondary organic aerosol particles. Nature 467, 824e827.

Wiedensohler, A., Birmili, W., Nowak, A., Sonntag, A., Weinhold, K., Merkel, M.,Wehner, B., Tuch, T., Pfeifer, S., Fiebig, M., Fj€araa, A.M., Asmi, E., Sellegri, K.,Depuy, R., Venzac, H., Villani, P., Laj, P., Aalto, P., Ogren, J.A., Swietlicki, E.,Williams, P., Roldin, P., Quincey, P., Hüglin, C., Fierz-Schmidhauser, R., Gysel, M.,Weingartner, E., Riccobono, F., Santos, S., Grüning, C., Faloon, K., Beddows, D.,Harrison, R., Monahan, C., Jennings, S.G., O'Dowd, C.D., Marinoni, A., Horn, H.-G.,Keck, L., Jiang, J., Scheckman, J., McMurry, P.H., Deng, Z., Zhao, C.S.,Moerman, M., Henzing, B., de Leeuw, G., L€oschau, G., Bastian, S., 2012. Mobilityparticle size spectrometers: harmonization of technical standards and datastructure to facilitate high quality long-term observations of atmospheric par-ticle number size distributions. Atmos. Meas. Tech. 5, 657e685.