The Differential Mobility Particle Sizer (DMPS) system comprises a custom-built twin differential mobility analyser (DMA) setup, including one Vienna-type medium DMA coupled to a TSI Condensation Particle Counter (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 merged. Both systems use a closed-loop setup. The instrument was inter-calibrated during an ACTRIS (https://www.actris.eu/, last access: 31 May 2019) workshop. Sizing and number concentrations are within 1 % and 5 % of the standard DMPS, respectively (Freud et al., 2017).

Aerosol size distribution in the diameter range from 10 to 470 nm using 54 channels was measured 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 dlog⁡Dj equivalent to 0.0312, where Dj indicates the instrumental class size. Further information can be found elsewhere (Lupi et al., 2016).

Scanning Mobility Particle Sizer (SMPS) data were collected in the period 2013–2015 in the size range of 9–915 nm in diameter. The SMPS is custom built with a Vienna-type medium column, and it used either a model TSI 3010 CPC or model TSI 7220 CPC. 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 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.

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 of 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 from a combination of multiple sources. The same aerosol category terminology was used in previous work; additional information can be found elsewhere (Dall'Osto et al., 2017a, b; Lange et al., 2018). Figure 3a (blue line) shows that the pristine category is associated with very low particle number concentrations (<100 particles cm-3). Average aerosol number concentrations across different sizes are shown in Fig. 3a, with two minor modes at 35 and 135 nm. The nucleation category (Fig. 3a, red line) shows average daily aerosol number size distributions peaking in the smallest detectable size at 10 nm. The name of this category – which will be used below to represent new particle formation events – stands for continuous gas–particle conversion occurring after the particle nucleation event. By contrast, Fig. 3a (green line) shows the average number size distribution with an ultrafine mode peaking at about 20–30 nm. We refer to this bursting category as 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 aerosol 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.

Figure 3b shows two main aerosol categories with a dominating aerosol mode peaking in the Aitken size range at about 30–60 nm. Whilst aerosol the nascent category possess a main mode at about 40 nm, the category nascent broad shows a much broader Aitken mode peaking at about 60 nm. The name of this category is meant to be associated with aerosol (of about 30–60 nm), mainly from growing aerosol of secondary origin, related to local and regional marine biogenic sources, occurring mainly during summer (Quinn et al., 2011; Tunved et al., 2013). By contrast, Fig. 3c shows three aerosol categories whose aerosol size distributions are all mainly located in size ranges larger than 100 nm. Main modes can be seen at 150 nm (accumulation_150 category), at 220 nm (accumulation_220 category) and in the largest detected SMPS modes at about 400–500 nm (coarse category).

The temporal frequency during the years 2013–2015 of the eight aerosol categories is presented in Table 1. The pristine category presents a remarkably similar occurrence among the three monitoring sites (12 %–14 %). The nucleation category is more frequent at the Svalbard sites (11 %–15 %) relative to the VRS site (8 %). A similar pattern can be seen for the bursting category. It is 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 fjord. The two Aitken categories (nascent and nascent broad) do not show such variability (7 %–21 %). By contrast, strong differences are seen in the accumulation-mode-dominated aerosol categories. For example, accumulation_150 is frequent at the ZEP site (19 %), whereas at the VRS site the dominating category is accumulation_220 (42 %), confirming a recent study specifically looking at the characterisation of distinct Arctic aerosol accumulation modes and their sources (Lange et al., 2018). Finally, the coarse aerosol category shows minor occurrence at all three sites (1 %–4 %).

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 early summer months (Fig. 4a). The nucleation category clearly showed high occurrence during summer months at the VRS site. By contrast, at the Svalbard sites (GRU, ZEP) aerosol concentrations 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), at the Svalbard sites (GRU, ZEP), it mainly occurs during May–July. As previously discussed (Dall'Osto et al., 2017a, 2018), the lack of gaseous precursors during spring may be the limiting factors for the formation of new particles and/or due to the large numbers of pre-existing particles transported from mid-latitudes. The two Aitken-mode-dominated 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 further than into a pronounced Aitken mode in summer months, particularly in July and August (Tunved et al., 2013; Dall'Osto et al., 2017a). The accumulation_150 category peaks mainly during the months of February–April, confirming its association with the Arctic haze phenomenon (Fig. 4f) at all three stations. By contrast, the larger accumulation_220 mode category occurs during all autumn and winter months at ZEP, including October–December (Fig. 4g). Finally, the coarse category does not show any clear trend due to its low frequency (Fig. 4h). The overall annual frequency is summarised in Fig. 5, in which the aerosol classes are shown. It is well known that the Arctic atmosphere is more heavily impacted by the transport of air pollution from lower latitudes in spring compared to in summer (Heidam et al., 2004; Law and Stohl, 2007). The continent-derived 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). We would like to remind the reader at this stage that the recent inter-comparison 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 Utqiaġvik (formerly Barrow) and Zeppelin, but the GRU site was not considered in their analysis.

Different chemical species of natural and anthropogenic origin may contribute to the Arctic aerosol (Tunved et al., 2013; Hirdman et al., 2010). In this section we compare – where possible – the aerosol size distribution categories herein apportioned with the chemical and physical parameters available in selected Arctic stations. A limitation of this study is that chemical and physical parameters were not simultaneously collected at the three stations for the entire period of study (2013–2015). Nevertheless, this section adds value to the work by presenting chemical and physical parameters when available. SO2 in the Arctic has both anthropogenic and natural sources (Barriel, 1986), but in our study it mainly occurs with accumulation-mode aerosols during wintertime (Fig. 6a; ZEP site only). Combustion-derived particles can be transported to the Arctic and experience ageing of the aerosol through condensational processes. Our study confirms previous findings, where SO2 was shown to correlate 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-mode-dominated categories (nascent and nascent broad) show high concentrations of ammonia (Fig. 6b, ZEP site only). Ammonia can increase rates of new particle formation and growth via stabilisation of sulfuric acid clusters (Kirkby et al., 2011). There is growing interest to better constrain the ammonia emissions of the Arctic. Zooplankton excretion and bacterial remineralisation of phytoplankton-derived organic matter are believed to be a dominant source in the marine environment (Carpenter et al., 2012), although considerable uncertainty remains (Lin et al., 2016). The melting of sea ice is also a significant source of ammonium (Tovar-Sánchez et al., 2010), with protein-like compounds accumulating at the sea–ice interface (Galgani et al., 2016). Similar processes have also been seen in Antarctic sea ice (Dall'Osto et al., 2017b). There is evidence that coastal seabird colonies are sources of NH3 in the summertime Arctic (Wentworth et al., 2016), although this is still uncertain (Riddick 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). These new particle formation events did not seem to be related to coastal zone bird colonies.

The association of size distribution categories with selected aerosol chemical components measured at GRU and ZEP is shown in Fig. 7. The aerosol chemical composition shown is derived from PM10 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 to apportion aerosol sources and processes. Figure 7a–c show similar trends for three chemical elements (Cl, Na, Mg). Mechanically generated sea salt particles are normally found in the coarser size fraction, indicating a marine source for Na, Mg and Cl. Indeed, the highest concentrations are seen for the coarse category (about 350, 300 and 40 ng m-3 for Cl, Na and Mg, respectively), followed by accumulation_150, accumulation_220 and pristine categories. Sea spray aerosol (SSA) is generated by bubble bursting due to surface winds. The contribution of SSA to the global aerosol burden is multiple times larger than that of anthropogenic aerosols (Raes 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 acetonitrile, a good biomass-burning tracer. Indeed, accumulation-mode aerosol categories show high concentrations of potassium (about 25–30 ng m-3), but the trend is not observed for the pristine category, likely more associated with biogenic Arctic activity. Non-sea-salt sulfate (nss-SO4) is a mixed-source tracer with a large anthropogenic fossil and biomass fuel component. At the same time nss-SO4 is also formed in large quantities from the atmospheric oxidation of dimethyl sulfide (DMS); this is further elaborated below. 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 acidic nitric and sulfuric aerosols are neutralised in the atmosphere by NH3. The two categories with the highest concentrations of sulfate, nitrate and ammonium are found to be accumulation_150 and accumulation_220 (about 500, 120 and 65 ng m-3, respectively), suggesting that these two categories are composed of a number of combined primary and secondary components of anthropogenic origin. It is interesting to note that ammonium only partly neutralises the Arctic aerosols (on average by one-third). Therefore, the aerosols are highly acidic.

Overall, the lowest aerosol mass concentrations seen in Fig. 7a–e are the nucleation, nascent and nascent broad categories. This is not surprising because the occurrence of NPF events and growth in the Aitken mode is mainly controlled not only by the presence of precursor gases 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 important conclusions about the possible sources forming these new particles. An opposite trend relative to the previously discussed chemical aerosol markers can be seen in Fig. 7h, showing methane sulfonic acid (MSA) concentrations sampled at the GRU monitoring site. The highest concentrations can be seen for the bursting, nucleation, nascent and nascent broad categories. MSA is formed via oxidation of DMS, a gas produced by marine phytoplankton (Gali et al., 2015). DMS is the most abundant form of biogenic sulfur 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). A recent study at the ZEP size shows that during summer, the impact of the anthropogenic sources upon sulfate 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. However, it is important to stress that high uncertainty regarding the mechanism of aerosol production in the Arctic – especially 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 example, Park et al. (2018) discussed atmospheric DMS in the Arctic Ocean and its relation to phytoplankton biomass. The DMS production capacity of the Greenland Sea was estimated to be a factor of 3 greater than that of the Barents Sea, whereas the phytoplankton biomass in the Barents Sea was more than 2-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, 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 the production of volatile organic compounds (VOCs) via photooxidation and unsaturated fatty acids (UFAs) derived from anthropogenic and biogenic sources. VOC sources include wildfires and 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 categorisation with two organic chemical species measured at daily time resolution at the GRU monitoring sites. Results are shown in Fig. 8. A clear anti-correlation can be seen for oxalic and pyruvic acid. 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 oxidised to produce oxalic acid (Carlton et al., 2007; Carpenter et al., 2012; Bikkina et al., 2014). Oxalic acid is often found to be the most abundant water-soluble organic compound in aerosols, and in-cloud processing is recognised as its major production pathway (Yu et al., 2005). Figure 8 further supports our hypothesis that the aerosol categories defined by low mass concentrations and numerous ultrafine sub-50 nm 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), illustrating that marine Arctic organic components are responsible for the ultrafine aerosol population.

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 (ZEP site only) shows that the two accumulation categories (accumulation_150 and accumulation_220) are associated with the highest CCN concentrations (about 125 cm-3) as well as the highest ratio of CCN over N. Usually, ultrafine particles smaller than 100 nm 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 (CDNCs) 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 enough to generate the required supersaturations needed to activate smaller particles (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 increased supersaturation may enable smaller particles to become cloud droplets. The nascent and nascent broad categories also show associations with high CCN concentrations, despite the much lower average size distributions (Fig. 3d). Natural sources indeed have a significant impact on particle number over summer. Therefore, these natural sources facilitate aerosol activation to cloud droplets and thus cloud formation. Pristine, bursting and nucleation categories show very low associated CCN concentrations (about 50–75 cm-3), with only about 30 % of the total N being activated. In the previously mentioned study by Dall'Osto et al. (2017a) it is also shown that the new particle formation (NPF) events and the growth of these particles to a larger size can affect the CCN number concentration, reporting an increase of the CCN number concentration (measured at a supersaturation of 0.4 %) of 21 %, which is linked to NPF events. Low-level clouds are one of the major factors controlling the radiative balance in the Arctic. Further multidisciplinary studies are needed in order to understand the processes that determine cloud properties by which particles actually form cloud droplets under various conditions.