Apportioning aerosol natural and 2 anthropogenic sources thorough 3 simultaneous aerosol size distributions and 4 chemical composition in the European high 5 Arctic

18 Institute of Marine Science, Consejo Superior de Investigaciones Científicas (CSIC), 19 Barcelona, Spain 20 National Centre for Atmospheric Science Division of Environmental Health & Risk 21 Management School of Geography, Earth & Environmental Sciences University of 22 Birmingham, Edgbaston, Birmingham, B15 2TT United Kingdom 23 Department of Environmental Science and Analytical Chemistry & Bolin Centre for 24 Climate Research, Stockholm University, Stockholm 10691, Sweden 25 Department of Chemistry, University of Florence, Via della Lastruccia 3, 50019, Sesto 26 Fiorentino, Florence, Italy 27 Institute of Atmospheric Sciences and Climate (CNR-ISAC),40129 Bologna, Italy 28 Korean Polar Research Institute, KOPRI, Republic of South Korea 29 Arctic Research Centre, iClimate, Department of Environmental Science, Aarhus 30 University, Roskilde 4000, Denmark 31


INTRODUCTION
The Arctic is a sensitive region affected by perturbations of the radiation budget, with complex feedback mechanisms resulting in a temperature increasing more than twice the global average since the 1980s (so-called ''Arctic amplification'', Cohen et al., 2014, Pithan andMauritsen, 2014).Aerosols are able to perturb the radiation balance of the Arctic environment in numerous ways (Carslaw et al., 2013).The contribution by aerosols to radiative forcing is considered a very important parameter, although still highly uncertain in a recent climate assessment (IPCC, 2014).In order to improve our knowledge in estimating direct and indirect climate effects, a better knowledge of the aerosols is an essential requisite, including their properties and seasonal variability, their sources, and the associated atmospheric reactions and transport processes.One of the main basic properties to characterize an aerosol is the size distribution.Atmospheric aerosol particles span over several orders of magnitude in diameter (Dp), from a few nanometer to hundreds of micrometers.Small particles, in particular nucleation mode (typically with Dp <10 nm) and Aitken mode (10 nm<Dp<100 nm) particles contribute little to total particulate mass in background air; however, they contribute significantly to surface area and Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-447Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 4 June 2018 c Author(s) 2018.CC BY 4.0 License.dominate particle number concentration (Dall´Osto et al., 2011).Measurements of Arctic aerosols have shown a strong annual cycle.The first full year of aerosol size distribution and chemical composition Arctic measurements was conducted at the Zeppelin station on Svalbard (Strom et al. 2003), showing a very strong seasonal dependence of the number mode particle size.Tunved et al. (2013) subsequently reported a qualitative and quantitative assessment of more than 10 yr of aerosol number size distribution data observed in the Arctic environment (Mt.Zeppelin, Svalbard); reporting that seasonal variation seems to be controlled by both dominant sources as well as meteorological conditions.This can be broadly summarised in three distinctly different periods: accumulation mode aerosol during the haze period (March-May), followed by a high concentration of small particles locally formed (June-August), leaving the rest of the year with a low concentration of accumulation mode particles and negligible abundance of ultrafine particles (September-February).Additional results from multi-year measurements reported similar conclusions using aerosol number size distributions collected at Tiksi (Asmi et al., 2016), Alert (Croft et al., 2016), Barrow (Lathem et al., 2013) and Villum Research Station -Station Nord (Nguyen et al., 2016).
Currently, the Arctic haze is not well represented within atmospheric models, mainly due to inadequate representation of scavenging processes different transport mechanisms and underestimation and an unknown number of aerosol sources.As regards natural aerosol sources, they have been emphasized to be much more important than transport from continental sources.Recently, the aerosol population was categorised via cluster analysis of aerosol size distributions taken at Mt Zeppelin (Svalbard, Dall´Osto et al., 2017a) during an 11 year record (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010) and at Station Nord (Greenland, Dall´Osto et al., 2018b) during a 7 year period (2010)(2011)(2012)(2013)(2014)(2015)(2016).Air mass trajectory analysis linked these frequent nucleation events to biogenic precursors released by open water and melting sea ice regions.Both studies reported a striking negative correlation (r = -0.89and -0.75, respectively) between sea ice extent and nucleation events.Given the likely decrease in future sea ice extend in the Arctic, it is likely that the impact of natural ultrafine Arctic aerosols will increase in future.However, it was stressed that further studies are needed given other new particle formation source regions and mechanisms exist, including an influence of emissions from seabird colonies (Croft et al., 2016;Weber et al., 1998) and intertidal zones (O´Dowd et al., 2002;Sipila et al., 2016).
It is becoming evident that coordinated field measurement studies of the atmospheric size distribution of aerosols are essential to elucidate the complex interactions between the cryosphere, atmosphere, ocean, and biosphere in different regions (Dall´Osto et al., 2018 a, b).In this regard, an emerging multi-year set of observed aerosol number size distributions in the diameter range of 10 to 500 nm from five sites around the Arctic Ocean (Alert, Villum Research Station -Station Nord, Zeppelin, Tiksi and Barrow) was recently assembled and analysed (Freud et al., 2017).Major accumulation mode aerosol sources were found over central Siberia and western Russia, and wet removal by snow or rain was found to be the main sink for accumulation-mode particles.It was argued that there is no single site that can be considered as fully representative for the entire Arctic region with respect to aerosol number concentrations and distributions.Following the pioneering study of Freud et al. (2017), the aim of this paper is to present a detail analysis of the main differences and similarities of the aerosol general features of the number size distributions between three different sites across the Arctic in the North Atlantic sector.Here, we use the stations named Gruvebadet (GRU), Zeppelin (ZEP) and Villum Research Station -Station Nord (VRS).The European Arctic is understood here as the part of the circumpolar Arctic located between Greenland and northwest Russia.Geographically, Greenland is part of the continent of North America.However, the island is politically and culturally water connection between the World Oceans and the Arctic.It is important to stress that the Svalbard archipelago is among the Arctic regions that has experienced the greatest temperature increase during the last three decades (Nordli et al., 2014); therefore comparing aerosol measurements simultaneously collected in Greenland and Svalbard is essential to better understand aerosol sources and processes that may affect our changing climate.Previous studies have focused on the characterization via air mass origin frequency and occurrence of different aerosol modes over time scales of the order of periods of weeks to years (Strom et al., 2003;Tunved et al., 2010;Nguyen et al., 2016;Lupi et al., 2016), but only using a single station as monitoring site.A brief comparison between ZEP and GRU was made in Lupi et al (2016), showing good agreement over a period of three months.However, to capture all scales of the variability of Arctic aerosols, it is important to merge intensive field campaigns and long-term measurements across different stations.Provision of the extensive resource-demanding equipment required is only possible by means of international collaborations such those created in the present work.A growing effort in understanding recent drastic changes in the Arctic climate has stimulated more measurements, and a growing number of monitoring sites and atmospheric measurements are taking place.Freud et al. (2017) for the first time assembled and analysed a multi-year set of observed aerosol number size distributions in the diameter range of 10 to 500 nm from five sites around the Arctic Ocean (Alert, Villum Research Station -Station Nord, Zeppelin, Tiksi and Barrow).Here, aerosol size distributions are analyzed by using k-means cluster analysis (Beddows et al., 2009) applied to a long term dataset composed of three years (2013)(2014)(2015)  from three stations (GRU, ZEP, VRS).This is the first time that the GRU site is used in a multi-year set of observed aerosol number size distributions.All size distributions are quality assured, and not filtered according to any other criteria.The cluster analysis herein applied uses the degree of similarity and difference between individual observations to define the groups and to assign group membership.By doing so, our clustering method provides a specific number of size distributions which can be compared across different time periods and across different monitoring sites (Beddows et al., 2009;Dall´Osto et al., 2018b).Whilst a number of intensive field studies have been focusing on single site datasets (Tunved et al., 2004;Dall´Osto et al., 2017a, Dall´Osto et al., 2018b), cluster analyses of multi-site year-long particle size distributions measurements are scarce (Freud et al., 2017;Dall´Osto et al., 2018b).It is important to stress in this study the only aim was to compare the three stations by apportioning different aerosol categories and possible source associations.Future studies will look at transport, both vertical (i.e. between VRS and GRU/VRS) and horizontal (i.e between GRU and ZEP) of both anthropogenic and natural aerosols.

Site Description
Ultrafine aerosol size distributions were measured at three different sites.Fig. 1 (a,b) shows the location and the sea ice coverage across the whole of 2015 taken as an example.
The first measurement site is situated at 78º 580N and 11º 530E on the Zeppelin Mountain in the Ny-Alesund community on Svalbard.The Zeppelin (ZEP) station is located 474m above sea level, it is easily accessible yet practically unaffected by local sources.Compared to stations closer to sea level the Zeppelin station is less affected by local particle production occurring in the surf zone, and to local air flow phenomena such as katabatic winds (Strom et al., 2003).The ZEP station is part of ACTRIS Data Centre (ACTRIS DC, developed through the EU project Aerosols, Clouds, and Trace gases Research InfraStructure Network -URI: http://www.actris.eu-within the EC 7th Framework Programme under "Research Infrastructures for Atmospheric Research"), part of the Global Atmosphere Watch (GAW) programme; and it is likely the longest Arctic aerosol size distribution dataset existing (Strom et al., 2003, Tunved et al., 2010;Freud et al., 2017).
The Gruvebadet (GRU) observatory is located in the proximity of the village of Ny-Alesund (78º 55 N, 11º 56 E) in the island archipelago of Svalbard.The observatory is 67 m above sea level, located south-east of the main buildings of the village.The instrument collected aerosol size distribution usually from the end of March to the beginning of September.It is located at about 2Km distant from the ZEP station, at about 350m lower altitude.About 800Km away from Svalbard we have the Station Nord Villum research station (VRS).Located at 81° 36' N, 16° 40' W the station is situated in the most north-eastern part of Greenland, on the coast of the Fram Strait.The sampling took place about 2 km south-west of the main facilities of the Station Nord military camp in two different sampling stations as measurements were shifted in summer 2015 from the original hut called "Flygers hut" to the new air observatory, 300 m west of "Flygers hut".The sampling locations were located upwind from the station for most of the time.Detailed descriptions of the site and analysis of predominant wind directions are available elsewhere (Nguyen et al., 2016;Nguyen et al., 2013) Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-447Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 4 June 2018 c Author(s) 2018.CC BY 4.0 License.

ZEP DMPS
The Differential Mobility Particle Sizer (DMPS) system comprises a custom-built twin DMA (differential mobility analyser) setup including one Vienna-type medium DMA coupled to a TSI CPC 3010 covering sizes between 25 and 800 nm and a Vienna-type short DMA coupled with a TSI CPC 3772, effectively covering sizes between 5 and 60 nm.The number size distributions from the two systems are transferred to a common size grid and then merged.Both systems use a closed-loop setup.The instrument has been intercalibrated during an ACTRIS (www.actris.eu)workshop.Sizing and number concentrations are within 1 and 5% from the standard DMPS, respectively (Freud et al., 2017).

GRU SMPS
Aerosol size distribution in the diameter range from 10.4 to 469.8 nm using 54 channels was evaluated with a commercial Scanning Mobility Particle Sizer SMPS TSI 3034, (Hogrefe et al. 2006), with a time resolution of 10 min and particle size with a resolution of dlogDj equivalent to 0.0312, where Dj indicates the instrumental class size.Further information can be found elsewhere (Lupi et al., 2016).

Data analysis and additional chemical and physical supporting data
SMPS data from the three different stations were merged and only days where measurements were taken simultaneously at the three stations were considered in this analysis, resulting in 584 total days.Additional chemical and physical data were included in this study, in order to better describe the aerosol particle types sampled.PM 10 sampling was performed at GRU station by a TECORA Skypost sequential sampler equipped with a PM 10 sampling head operating following the EN 12341 European protocol.Aerosol samples were collected daily on Teflon (PALL Gelman) filters from March to September 2013-2015, in total 385 daily samples were analysed.MSA was determined by ion chromatography on the aqueous extract obtained from one half of each filter (Becagli et al., 2016).Gaseous NH 3 and SO 2 data, and inorganic aerosol species (Na, Mg, Cl, K, sulphate, nitrate, ammonium) at the ZEP monitoring site were obtained at daily resolution at the NILU website data for the period 2013-2015 (total days 650).Concentrations of Cloud Condensation Nuclei (CCN) were measured continuously using a commercial available Droplet Measurement Technology (DMT) CCN counter at the ZEP station.In this study we selected CCN values when supersaturation is 0.4%.In total, 723 days of sampling were obtained at hourly resolution for the years 2013-2015.The size distribution data were averaged over 24 hours using the start and end time of the chemical measurements.

Average monthly size distributions
The monthly averaged aerosol size distributions at the three sites are presented in Fig. 2.
Simultaneously collected data are presented for the whole years (2013)(2014)(2015), but only for 8 months (March-October) considering all three sites, due to GRU not having data coverage during winter months (November through February).The average size distributions at ZEP and VRS are broadly similar during the months of January and February (2a-b), with low particle number concentrations and a broad accumulation mode, although larger at the ZEP site (about 250nm) than at the VRS one (about 180nm).The months of March and April (Fig. 2c-d) present similar size distributions among the three stations, showing a main large accumulation mode peak at about 190nm, likely associated with the Arctic haze occurring mainly during these months.It is worth noting that higher ultrafine particle number concentrations are seen in these two months relative to Jan-Feb (Fig. 2a-b).During the month of May (Fig. 2e) a clear increase of ultrafine particles can be seen at the Svalbard sites (GRU, ZEP) due to local new particle formation.The increased occurrence of new particle formation events (NPF) in May was found to correspond to the increasing concentration of biogenic aerosol in the Svalbard sites (Becagli et al., 2016;Dall´Osto et al., 2017a).Interestingly, the VRS site does not show this enrichment, likely due to the fact that sea ice is still covering most of the areas near the north-east corner of Greenland.In contrast, during the summer months of June-August, higher concentrations of ultrafine particles can be seen progressively at all sites.The changes in sources, sinks and processes associated with colder autumn months (Tunved et al., 2013;Freud et al., 2017) progressively shifts the aerosol modes seen at about 20-40nm (September, Fig. 2i) to a bimodal-like aerosol distribution seen in October (Fig. 2j) with two main aerosol modes at about 50nm and 150nm, respectively.The remaining winter months show low particle number concentrations, and data are available for ZEP and VRS only.The data herein presented so far help us to compare the three monitoring sites.As expected, whilst the sites at GRU and ZEP are broadly similar, the VRS site located in Greenland seems to have fewer new particle events with a shorter summer frequency.In order to fully elucidate the chemical and physical processes affecting the aerosol size distributions, we use statistical tools to reduce the complexity of these SMPS datasets.

K-means clustering analysis
Approximately 25,000 aerosol size distributions obtained at one hour resolution at the three monitoring sites were averaged to daily resolution and then normalised by their vector-length and cluster analysed (Beddows et al., 2009).The standard procedure used (Beddows at al., 2014) including the Cluster Tendency test provided a calculated a Hopkins Index of 0.20 (Beddows et al., 2009).The method used minimize the sum of squared distances between all points and the cluster centre, allowing identification of homogeneous groups by minimizing the clustering error defined as the sum of the squared Euclidean distances between each dataset point and the corresponding cluster center.
The complexity of the dataset is reduced allowing characterization of the data according to the temporal and spatial trends of the clusters.In order to choose the optimum number of clusters the Dunn-Index (DI) identified dense and well separated clusters, it provided a clear maximum for 8 clusters, some of which belonged only to specific times of the day, specific mechanisms as well as specific seasons.The eight K-means clusters obtained exhibited frequencies which varied between 1% and 42% (Table 1), without main clusters

Aerosol categories and occurrence
An aerosol K-means cluster can be interpreted as a particle size spectrum which is determined by a superposition of individual sources and processes.Therefore, the name associated with each cluster aims only to reflect a main feature associated with the particle size spectrum.It is not possible to associate a single source or process, given that each cluster results mainly from a combination of multiple sources.Fig. 3a (blue line) shows that the pristine category is associated with very low particle number concentrations (<100 particles cm -3 ). Figure 3a shows average aerosol number concentrations across different sizes, with two minor modes at 35nm and 135nm.The nucleation category (Fig. 3a, red line) shows average daily aerosol number size distributions peaking in the smallest detectable size at 10nm.The name of this category -which will be used below to represent new particle formation events -stands for continuous gas-to-particle growth occurring after the particle nucleation event.By contrast, Figure 3a (green line) shows the average number size distribution with an ultrafine mode peaking at about 20-30nm.We refer this bursting category to a population that bursts and begin to exist or develop.Contrary to the nucleation category, this one fails to grow to larger sizes.The origins of this particle type can be multiple, including new particle formation with limited growth (so called "apple" new particle formation events), or open ocean nucleation, an Arctic ultrafine primary origin can also not be ruled out.
Fig. 3b shows two main aerosol categories with a dominating aerosol mode peaking in the Aitken size range at about 40-60nm.Whilst aerosol category nascent possess a main mode at about 40nm, the category nascent broad shows a much broader Aitken mode peaking at about 60nm.By contrast, Fig. 3c shows three aerosol categories whose aerosol size distributions are all mainly distributed in size ranges larger than 100nm (accumulation mode dominating.Main modes can be seen at 150nm (category accumulation_150), at 220nm (category accumulation_220) and in the largest detected modes at about 400-500nm (category coarse).
The temporal frequency during the years 2013-2015 of the eight aerosol categories is presented in Table 1.The category pristine presents a remarkably similar occurrence among the three monitoring sites (12-14%).The nucleation category seems more frequent at the Svalbard sites (11-15%) relative to the VRS site (8%).Similar patterns can be seen for the bursting category, also more frequent at GRU-ZEP (14-21%) relative to VRS (8%).
Interestingly, the bursting shows high occurrence at GRU (21%), perhaps reflecting some processes occurring near sea level across the fiord.The two Aitken categories (nascent and nascent broad) do not show much variability (7-21%).By contrast, strong differences are seen in the accumulation dominating mode aerosol categories.For example, accumulation_150 is frequent at the ZEP site (19%), whereas at the VRS site the category dominating is accumulation_220 (42%), confirming a recent study specific on characterization of distinct Arctic aerosol accumulation modes and their sources (Lange et  al., 2018).Finally, aerosol category coarse shows minor occurrence at all three sites (1-4%).

Annual behaviour
The pristine category did not present a clear annual seasonality at the ZEP and VRS sites, although at the GRU site it occurred mainly during spring months (Fig. 4a).The nucleation category clearly showed high strong occurrence during summer months at the VRS site.
By contrast, at the Svalbard sites (GRU, ZEP) two main periods dominate in May and in August (Fig. 4b).Similar trends can be seen for the bursting category (Fig. 4c).Whilst at the VRS site this category shows occurrence similar to the nucleation category (Fig. 4b), the Svalbard sites (GRU, ZEP) mainly occur during spring months.As previously discussed (Dall´Osto et al., 2017a, Dall´Osto et al., 2018) the lack of gaseous precursors during spring may be the limiting factors.The two Aitken dominant mode aerosol categories (Nascent and Nascent broad) show very similar temporal trends peaking mainly during summer months at all three stations (Fig. 4d, e).Previous studies already discussed freshly and locally produced aerosol particles dominating the Arctic summer, driven by an increase in both biological activity and photochemistry as well as limited long range transport from mid latitudes (Ström et al., 2009).Therefore, particles are not growing more than into a pronounced Aitken mode in summer month, particularly in July and August (Tunved et al., 2013, Dall'Osto et al., 2017a).The two accumulation dominant aerosol mode categories mainly occur during wintertime.The accumulation_150 peaks mainly during the months of February-April (maximum in March) confirming its association with the Arctic haze phenomenon (Fig. 4f) in all three stations.By contrast, the larger accumulation_220 mode category occurs during all autumn and winter months, including October-December (Fig. 4g).Finally, the last minor cluster (coarse) does not show any clear trend due to its low frequency (Fig. 4h).The overall annual temporal frequency can be summarised in Fig, 5, where broader categories can be seen.It is well known that the Arctic atmosphere is heavily impacted by transport of air pollution from lower latitudes in spring compared to in summer (Heidam et al., 2004;Law and Stohl, 2007).The continentderived winter and spring aerosols, known as Arctic haze, reach their maximum number concentration during late spring, approximately in April (Tunved et al., 2013;Nguyen et al. 2016) A recent intercomparison of particle number size distributions from several Arctic stations by Freud et al. (2017) suggests differences between the studied stations regarding cluster frequency of occurrence throughout the year.The most prominent differences were observed between the stations at Barrow and Zeppelin, but the GRU site was not consider in their analysis.

Association of aerosol categories with chemical and physical parameters
Different chemical species of natural and anthropogenic origin may contribute to the Arctic aerosol (Tunved et al., 2013;Hirdman et al., 2010).SO 2 in the Arctic has both anthropogenic and natural sources (Barriel et al., 1986), but in our study it is shown mainly occurring with large accumulation mode during wintertime (Fig. 6a).Combustion-derived particles can be transported to the Arctic and can be accompanied by aging of the aerosol through condensational processes.Our study confirm previous ones where SO 2 was shown to parallel with black carbon both at VRS and ZEP (Nguyen et al., 2013;Massling et al., 2015, Dall´Osto et al., 2017a).By contrast, we find the highest concentrations of ammonia associated with the nucleation category.Interestingly, also the two Aitken dominating mode categories (nascent and nascent broad) show high concentrations of ammonia (Fig. 6b).Ammonia can impact increasing rates of new particle formation and growth via stabilization of sulphuric acid clusters (Kirkby et al., 2011).There is growing interest to better constrain the location, population, and ammonia emissions of the Arctic.
Zooplankton excretion and bacterial remineralization of phytoplankton-derived organic matter is believed to be a dominant source in the marine environment (Carpenter et al., 2012) although there remains considerable uncertainty (Lin et al., 2016).The melting of sea ice is also a significant source of ammonium (Tovar-Sanchez et al., 2010), with protein-like compounds accumulating in the sea-ice interface (Galgani et al., 2016), similar processes are also seen in Antarctic sea ice (Dall´Osto et al., 2017b).There is evidence that coastal seabird colonies are sources of NH 3 in the summertime Arctic (Wentworth et al., 2016), although there is still uncertainty (Riddich et al., 2012).Recently, ammonia from seabirds was found to be a key factor contributing to bursts of newly formed coastal particles at Alert, Canada (Croft et al., 2016).However, regions of open water and melting sea ice were found to drive new particle formation in North East Greenland (Dall´Osto et al., 2018b), such events seem not to be related to bird colonies from coastal zones.
The association with selected chemical components measured in aerosols collected at GRU and ZEP are shown in Fig. 7.It is important to stress that the aerosol chemical composition shown are derived from PM 10 measurements, and thus does not necessarily reflect the chemical composition of the aerosol covered by the size distribution analysis herein presented and discussed.Nevertheless, the comparison may help in apportioning aerosol sources and processes.Fig 7 (a-c) shows similar trend for three chemical elements (Cl, Na, Mg).Mechanically generated sea salt particles are normally found in the coarser size fraction which points to a marine source for Na, Mg and Cl.Indeed, the highest concentrations are seen for the coarse category (about 350 ng m -3 , 300 ng m -3 and 40 ng m -3 for Cl, Na and Mg, respectively), followed for categories accumulation_150,  et al., 2000;Grythe et al., 2014).Potassium can be associated with sea salt, although K-rich particles are often also attributed to biomass burning (Hudson et al., 2004;Moroni et al., 2017), correlating with gas-phase measurements of acetonitrile, a good biomass-burning tracer.Indeed, accumulation aerosol categories show high concentrations of potassium (about 25-30 ng m -3 ), but the trend is not observed for the pristine class.Aerosols not only originate from primary sources like sea spray and biomass burning, but can also be formed via secondary processes, through chemical transformation of gas-phase species in the atmosphere, most notably sulphur dioxide, oxides of nitrogen, and volatile organic compounds (Harrison et al., 1999).Non-sea-salt sulphate (nss-SO 4 ) is a mixed source tracer with a large anthropogenic fossil and biomass fuel component and a smaller biogenic marine component.Aerosol nitrate is predominantly anthropogenic and arises from the oxidation of NOx from combustion processes associated with vehicles and industrial activity.A considerable proportion of the these acidic nitric and sulphuric aerosols are neutralized in the atmosphere by NH 3 (Asman et al., 1998).The two categories with the highest concentrations of sulphate, nitrate and ammonium are found to be accumulation_150 and accumulation _220 (about 500 ng m -3 , 120 ng m -3 and 65 ng m -3 , respectively) suggesting as expected these two are composed of a number of primary and secondary components of anthropogenic origin.It is interesting to note that ammonium is only partly neutralising the Arctic aerosols (in average with one-third) and in the Arctic, the aerosols are highly acidic.
Overall, the lowest aerosol mass concentrations seen in Fig. 7 (a-e) are seen for the nucleation, nascent and nascent broad categories.This is not surprising, because the occurrence of new particle formation events and growth to the Aitken mode is mainly controlled not only by the precursor gaseous presence but also by pre-existing particle concentrations (Kulmala et al., 2001).Indeed, these events are often found under low aerosol concentration conditions in remote areas (Tunved et al., 2013).The low aerosol mass concentrations associated with these recently formed categories still allow us to draw some important conclusions about the possible sources forming these new particles in the Arctic.An opposite trend relative to the previously discussed chemical aerosol markers can be seen in Fig. 7h showing methane sulphonate (MSA) concentrations sampled at the GRU monitoring site.The highest concentrations can be seen for the categories bursting, nucleation, nascent and nascent broad.MSA is formed via oxidation of dimethyl sulfide (DMS), a gas produced by marine phytoplankton (Gali et al., 2015).It is the most abundant form of biogenic sulphur released from the ocean (Lovelock et al., 1972;Stefels et al., 2007).Previous studies show that the emission of oceanic DMS may impact aerosol formation in the Arctic atmosphere (Levasseur et al., 2013;Becagli et al., 2016, Dall´Osto et al., 2017a).Recent study at the ZEP size shows that during summer, the impact of the anthropogenic sources upon sulphate is lower (42%), with a contribution comparable to that coming from biogenic emissions (35%) (Udisti et al., 2016).The association of MSA not only with the nucleation but also with the bursting category suggests that secondary processes may drive both categories, somehow pointing to a lower ultrafine primary origin association with this particle type.However, it is important to stress that high uncertainty regarding the mechanism of aerosol production in the Arcticespecially from leads and open pack ice -still remains (Leck et al., 2002).The interactions between the surface layer of the ocean and the atmosphere are highly variable and ecosystem interactions are more important than any single biological variable.For relation to phytoplankton biomass.The DMS production capacity of the Greenland Sea was estimated to be a factor of three greater than that of the Barents Sea, whereas the phytoplankton biomass in the Barents Sea was more than two fold greater than that in the Greenland Sea, stressing the occurrence of a greater abundance of DMS-producing phytoplankton in the Greenland Sea than in the Barents Sea during the phytoplankton bloom periods.
The chemical nature and origin of the fine particulate matter over Arctic regions, and especially of its organic fraction, are still largely unknown (Kawamura et al., 1996a, b;Leaitch et al., 2018).Water-soluble dicarboxylic acids and related polar compounds, including oxocarboxylic acids and α-dicarbonyls are ubiquitously found from the ground surface to the free troposphere (Decesari et al., 2006;Kawamura and Bikkina, 2016).
Primary sources include fossil fuel combustion and burning of biomass and biofuels.
Secondary sources include production via photooxidation of volatile organic compounds (VOCs) and unsaturated fatty acids (UFAs) derived from anthropogenic and biogenic sources.VOC sources include wildfire, emissions from snow, ocean, sea ice, boreal forest and tundra (Tunved et al., 2006;Carpenter et al., 2012, Kos et al., 2014;Haque et al., 2016, Mungall et al., 2017).For this study, we were able to compare our SMPS aerosol categorization with two organic chemical species measured at daily time resolution at the GRU monitoring sites.Results are shown in Figure 8.A clear association can be seen for oxalic and pyruvic acids, anti-correlating between them.Broadly, in the remote marine atmosphere, pyruvic acid may be produced by photochemical oxidation of isoprene and other biogenic volatile organic compounds (BVOCs) emitted from marine biota, which are finally oxidized to produce oxalic acid (Carlton et al., 2007;Carpenter et al., 2012;Bikkina et al., 2014).Oxalic acid is often found as the most abundant water-soluble organic compound, and in-cloud processing is recognized as its major production pathway (Yu et al., 2005).Figure 8 further supports our studies suggesting that the aerosol categories defined by low mass concentrations and high ultrafine sub-50nm particles are associated with rather local secondary processes from marine VOC sources.Recent studies have found that lower organic mass (OM) concentrations but higher ratios of OM to non-sea-salt sulfate mass concentrations accompany smaller particles during the summer (Leitch et al., 2018), stressing the importance of marine Arctic organic components responsible for the ultrafine aerosol population.
We conclude this section by reporting the calculated average CCN concentrations corresponding to each aerosol category.CCN number concentrations influence cloud microphysical and radiative properties, and consequently the aerosol indirect radiative forcing (IPCC, 2014).The variability of even low concentrations of CCN is important in the Arctic, an environment where cloud formation -and hence cloud forcing -is limited by the CCN availability (Mauritsen et al., 2011).Figure 9 shows that the two accumulation categories (accumulation_150 and accumulation_220) are associated with the highest CCN concentrations (about 125 particles cm -3 ) with also the highest ratio of CCN over N.
Usually, ultrafine particles smaller than 100nm in diameter are considered too small to activate to cloud droplets.However, Leaitch et al. (2016) concluded that 20-100 nm particles from Arctic natural sources can have a broad impact on the range of cloud droplet number concentrations (CDNC) in clean environments, affirming a large uncertainty in estimating a baseline for the cloud albedo effect.Changes in pressure and temperature may not be efficient to generate the required supersaturations needed to activate smaller particles, with the Kelvin effect acting as limiting factor (Browse et al., 2014;Leaitch et al., 2013).However, the low concentrations of accumulation mode aerosols often found in the Arctic may lower water vapour uptake rates during droplet formation, and the resulting increase supersaturation may enable smaller particles to become cloud droplets.The (accumulation_150, 13-42%; accumulation_220, 8-19%) and coarse sea spray aerosols (coarse, 1-4%).During winter months, mass concentrations of atmospheric aerosols in the Arctic are higher compared to summer.Broadly, this is due to differences in the transport of anthropogenic particles and wet scavenging (Stohl, 2006); local boundary layer height, stability and stratification also play a role (Brooks et al., 2017).By contrast, total aerosol number concentrations in the Arctic are often found to be similar throughout the period of March-September (Tunved et al., 2013).However, the number concentrations in spring (March-April) are almost exclusively governed by accumulation mode aerosols peaking at 150nm, while the summer concentrations are associated with elevated numbers of Aitken mode particles and frequent new particle formation events.The main findings of this work follow: • The three monitoring sites experience very pristine low particle number concentrations only 12-14% of the time.
• New particle formation and growth and bursts of sub-30nm particles are detected 8-21% of the time.The lower frequencies detected at the Greenland site (VRS 8%) relative to the Svalbard sites (ZEP-GRU, 11-21%) are likely due to the former site being surrounded by the ice stream from the Arctic Ocean and isolated from open ocean and melting sea ice regions emitting biogenic gas precursors.The Aitken mode aerosol categories dominate the summer time periods at all sites (19-35%), but at VRS one has a shorter summer season due to longer sea ice coverage and 14º degrees lower yearly average temperature compared to the stations at Svalbard.
• Two types of accumulation mode aerosols are found, one associated with the Arctic haze peaking in March-April (monomodal at about 150nm) and one seen during the winter months (monomodal at about 220nm).The site in Greenland (VRS) is exposed to accumulation mode aerosols longer than the one at Svalbard.This is likely due to different transport pathways into the polar dome, a boundary which separates cold air in the Arctic from the relatively warm air in midlatitude regions (Stohl, 2006).
The aerosol size distributions data herein compared from three different stations were intercompared for the first time.The study builds additional knowledge to the findings presented by Freud et al. (2017), with a focal point on the new particle formation phenomena as observed in the Arctic environment.This important exercise had to be carried out, and the expected results -although not striking -set the ground for important future studies.In the future, a decrease in sea ice coverage across the Arctic Ocean may increase the annual primary production (Arrigo et al., 2008), and may alter species composition of phytoplankton (Fujiwara et al., 2014).Hence, the emissions of biogenic sulphur gases that are aerosol precursors and hence affect aerosol growth and formation would increase change in summer.In this regard, the location of the monitoring sites at Svalbard and Greenland are ideal to study aerosol formation and transport across the two different regions.The two stations are separated by the Greenland Sea, a highly productive region with a great abundance of DMS-producing phytoplankton (Park et al., 2018).As the DMS production capacity of the ocean depends critically on the phytoplankton species composition and the complex food web mechanisms (Stefels et al., 2007), multidisciplinary studies across these regions are warranted.The recent transformations in the Arctic and their global causes and consequences have put international cooperation in the Arctic Council at the forefront of research in governance (Knecht et al., 2016).Larger atmospheric chemistry and physics datasets are being collected by a number of countries, and this work highlights the benefit that can be gained from international cooperation.Given that the present work has validated the quality of the presented aerosol size distributions, these data will be used again to address specific questions, including vertical transport (i.e. the two sites at the Svalbard) and horizontal transport (i.e.Arctic aerosol transport from Greenland to Svalbard regions).The significant costs associated with these types of coordinated international collaborations can provide far more information than individual sites operating on their own.This may help to understand better the complex interactions and feedbacks between the aerosol, the clouds, the longwave and shortwave radiation, the ocean dynamics, the biota and the environment (Browse et al., 2014).Special concern is arising also from increasing navigability in the rapidly melting Arctic Ocean with expanding community re-supply, fishing, tourism, fossil fuel exploitation and cargo trading, which is projected to cause a large increase in emissions by 2050 (Melia et al., 2016).Future studies looking simultaneously at different Arctic monitoring sites will reduce the uncertainties in future projections of Arctic climate changes and its implications for our planet (Koivurova et al., 2012;Byers, 2013;Conde Perez et al., 2016) Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-447Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 4 June 2018 c Author(s) 2018.CC BY 4.0 License.
Mobility Particle Sizer (SMPS) data were collected in the period 2013-2015 in the size range of 9-915 nm in diameter.The SMPS used either a condensation particle counter (CPC) model TSI 3010 or model TSI 7220.To ensure correct functioning, volumetric flow rates, temperatures and relative humidity (RH) of the aerosol-and sheath flow were monitored, as well as inlet ambient pressure.No additional drying was Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-447Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 4 June 2018 c Author(s) 2018.CC BY 4.0 License.performed, as the transition from the low ambient temperatures outside of the huts (-45 to +15 °C, yearly average -15 °C) to the heated inside (>20 °C) generally provides sufficient decrease in RH.
Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-447Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 4 June 2018 c Author(s) 2018.CC BY 4.0 License.dominating the overall population.The individual clusters could be distributed into three main groups named nucleation, Aitken and accumulation categories.This additional categorisation was based not only upon their similar size distributions among each other (see Fig. 3a-d) but also by considering strong similarities between other chemical and physical parameters presented in the next sections.The reduction to the three moregeneric classifications was based on our data interpretation, and the average aerosol size distributions of each aerosol category are presented in Fig. 3: (a) pristine and nucleation mode particle types; (b) Aitken mode dominating particle type and (c) accumulation mode dominating particle type, and presented together in Fig. 3 (d).
Figure 5. Annual variation of the frequency of the monthly cluster count for the three stations (VRS, ZEP, GRU) summarized in sub-aerosol main categories.

Table 1 . Occurrence of the K-means cluster analysis featuring the eight aerosol categories detected at the three monitoring sites. At the bottom of the table reported are general aerosol size distribution modes representing as sum of selected aerosol categories.
. Our study supports international environmental cooperation concerning the Arctic region.LIST OF TABLES