The Arctic region is warming considerably faster than the global average, a phenomenon known as Arctic amplification. Svalbard, a Norwegian archipelago located between 74 and 81∘ N, has experienced some of the greatest observed regional temperature increases throughout the past 3 decades . The impacts of Arctic amplification can be observed in a multitude of parameters , including most notably, reductions in Arctic summer sea ice . Diminishing sea ice has been proposed as the leading mechanism to explain the sensitivity of the region in terms of enhanced sea surface temperature increases . However, numerous other mechanisms have been studied, including changes in cloud cover , transportation of heat from the mid-latitudes , increases in the total water vapour in the Arctic atmosphere , and sulfate aerosol reductions in Europe . The exact relative importance of each of them is debated . Arctic amplification may be linked to mid-latitude weather , with increased Rossby wave amplitude and the appearance of atmospheric circulation anomalies . Changes in air circulation patterns within the Svalbard region have been observed and linked to changes in the Arctic Oscillation; the changes include increased anticyclonic advection from the south during winter and from the north during summer .

Aerosols exert a considerable influence on the Arctic climate . The reports that in combination with clouds, aerosols continue to contribute the largest uncertainty to our understanding of changes to the Earth's energy budget. Aerosols can influence the Earth's climate directly through aerosol–radiation interactions. Arctic aerosols have a net cooling effect on the region , whereby aerosol particles reflect shortwave radiation. With the significant decrease in sulfate aerosol in the past few decades , research into aerosol pollution in the Arctic has been gaining renewed interest as a means of explaining the increased rate of warming within the Arctic region . The small amount of solar radiation that the Arctic receives means the region is fairly sensitive to any perturbations in radiative fluxes . In addition, aerosol particles may act as a layer of light absorbing material over a highly reflective ice/snow surface . Changes in aerosol concentrations are also likely to have an impact on cloud condensation nuclei (CCN) concentrations, given the two are well correlated . The strong seasonal cycles in Arctic aerosol properties have been well documented . Observations from in situ ground-based sites have helped to describe this annual cycle. The Zeppelin Observatory (ZEP), Svalbard, experiences a typical Arctic annual cycle; there are peaks in accumulation-mode concentrations in late spring and early winter in accordance with the Arctic haze phenomenon. The appearance of the Arctic haze phenomenon is a result of the combined effects from the increased south-to-north transportation of aerosol and a decline in removal efficiency during northward transport . Summer, with more frequent cloudiness and low-intensity precipitation events within a retracted polar front, is nearly free of anthropogenic aerosol influence , with the exception of well-defined events related to biomass burning and forest fires . Summertime experiences increased concentrations of smaller nucleation and Aitken-mode particles, formed in situ . The light scattering properties at ZEP have been shown to reflect these changes in aerosol composition, with a scattering peak in the late winter and early spring . In addition, optical properties at ZEP signal an aerosol composition composed of smaller aerosol in the spring and larger aerosol in the late summer. More details on the seasonality of in situ optical properties can be found in and .

Long-term measurements are vital to understanding changes related to the Arctic and thus to the Earth's climate. There is a need for long-term measurements to help reduce the uncertainties surrounding the impacts of aerosol . Ground-based in situ measurement sites offer the chance to examine the long-term trends associated with aerosol optical properties. However, there are numerous difficulties in maintaining continuous and long-term observations, leading to a shortage of multidecadal time series. It is, therefore, essential to use the available data sets to examine trends and research the underlying mechanisms behind changes in aerosol properties. Aerosol optical properties experience a high degree of natural variability at different temporal resolutions and therefore pose difficulties in determining the underlying long-term trends. Sites in polar regions exhibit the largest number of statistically significant positive trends in particle light scattering , with multiple Arctic sites experiencing alternating trend slopes based on the duration of the trend (most notably in Utqiaġvik, formerly Barrow, Alaska). Interestingly, most Arctic sites do not reflect the overall decreasing trends in scattering coefficients observed throughout Europe and North America . Thus, it is important to study why these differences are present and the mechanisms underlying these trends. Exploring the reasons for these trends is a central topic of this study and separates it from most previous long-term trend analyses. This study further differs from in that a longer data set is used and that the data are prescreened with respect to ambient relative humidity. The trends in aerosol measurements cited in this study are all conducted under dry conditions, as opposed to ambient conditions. Dry conditions, where the relative humidity (RH) is controlled, help to minimise the effects of water uptake, known as hygroscopicity. It should be noted that the hygroscopic effect on particle light scattering is more pronounced in the Arctic, compared with measurements taken in other global regions .

This study aims to show long-term trends in Arctic aerosol optical properties, in combination with back trajectory analysis and satellite-derived sea ice data, to help better understand the changing processes controlling the optical properties of Arctic aerosol. Connecting back trajectory data with aerosol optical measurements helps to assess the potential aerosol sources and types. Not only does this study explore long-term trends in aerosol optical data, but it provides an analysis of the mechanisms influencing these changes. The influence on Arctic aerosol of sea ice retreat remains poorly constrained . One particular motivation for this study concerns the retreat in Arctic summer sea ice, which has seen significant reductions throughout the same period. The retreat of Arctic sea ice has the potential to induce changes to the aerosol composition; model results suggest an increase in sea salt aerosol (SSA) emissions from the loss of summer ice . The aims of this study are to answer the following two research questions: (a) at what rate have key aerosol optical properties changed at Svalbard, in the Arctic, during the last 2 decades? (b) Can changes in long-term aerosol optical properties be explained by meteorological parameters and/or changes in sea ice coverage?

Section  focuses on long-term trends; trends in aerosol light scattering properties are presented in Sect. , and the trends in key meteorological parameters from the back trajectory analysis and the changes in source types and regions are presented in Sect. . The trends are calculated and presented based on both seasonal and daily medians. The seasonality of the trends is also included. The final part of the results section, Sect. , combines the aerosol and back trajectory parameters.

Table  presents an overview of the main findings. α decreases for all seasons, with a clear NW and SW increase throughout the year and slight decreases in NE and SE in the autumn and winter.

Increasing concentrations of coarse-mode particles relative to fine-mode particles reduce α . Hence, the large decreasing trend in α (Fig. d) is suggestive of a significant shift to aerosol dominated more by coarse-mode particles. At the same time, increasing trends in σsp and σbsp are observed. Accumulation-mode particles are responsible for the majority of light scattering; however, as shown in Fig. S4 (in the Supplement) at ZEP coarse-mode particles still contribute a significant amount to the overall light scattering. The relative increase in coarse-mode particles can, therefore, explain the increasing trend in σsp (Fig. a) as well. Smaller particles, on the other hand, exhibit increased values in b . b is more sensitive to the changes to the distribution of particles at the smaller end of the accumulation mode . There is a small negative trend in b (Fig. ); nonetheless, a small relative decrease in b could potentially mean substantial changes at the lower end of the accumulation mode. The trends reported are consistent in sign and significance with those in . It should be mentioned though that gaps in the data could affect the calculated magnitude of the trends. Previous studies have used different averaging and pre-whitening methods to calculate the trends and their respective statistical significance e.g.. However, by applying different statistical methods on both daily and seasonal averaging times, a better indication of the robustness of the derived trends is achieved.

Sea spray aerosol (SSA) is the most plausible type of coarse-mode aerosol to be encountered at ZEP. It is produced at the ocean surface through the process of wind-induced bubble bursting and the tearing of wave crests . The production and transportation of SSA are linked to surface wind speeds , with increasing sea salt mass associated with increasing wind speeds . SSA is identified by smaller α values, typically less than 0.5 , and can contribute to the total amount of scattering. The clear oceanic influence on α (see Fig. ) suggests that the relative influx of coarse-mode particles is marine in origin and most likely SSA as opposed to coarse-mode-dominated dust from land sources.

Within this work, we show that the increased presence of SSA at ZEP is the result of changes in air circulation patterns, as opposed to the retreat in Arctic sea ice. There is a clear increase in the contribution of SW air masses, in particular in autumn, which coincides with the largest trend in α. The trends in σsp and α in summer can be explained by a greater influx of SSA arriving from the NW, over the Arctic sea ice, during the summer. Despite the continued increase in the expanse of open water in the Arctic Ocean, and the potential for increased SSA production, the effects of retreating sea ice on observations at ZEP were shown to be negligible (Fig. S5 in the Supplement). This study suggests that the growing contribution from western air masses leads to more SSA being transported to ZEP (Fig. ). The overall effect is that air masses reaching ZEP are becoming more marine dominated. The largest increases in the expanse of summertime open water, within the Arctic Ocean, concern the Beaufort and Chukchi seas. These areas are a considerable distance from ZEP; thus the residence times back trajectories spend over these regions are minimal compared to the combined area that all the back trajectories cover. However, more work is required to ascertain the exact annual movement of the polar front and whether its location excludes ZEP from any changes in emissions induced by the melting summer sea ice.

The back trajectory analysis offers a plausible partial explanation of the changes in aerosol optical properties observed at ZEP. However, it should be noted that there is unlikely to be any single overarching factor determining the observed trends. Accompanying the changes in air circulation patterns, there is a positive trend in wind speeds along the back trajectories (Figs.  and ). The inverse relationship between wind speeds and α suggests that higher wind speeds allow larger particles to be produced and transported to the observatory.

The polar front advances in the winter to include more industrial emissions coming from Eurasia (as showed by the increased NE contribution in Fig. S6 in the Supplement). In the summertime, the polar front retreats, and ZEP is found more often outside of the polar front receiving mainly marine air masses from the Atlantic Ocean . The increased contribution from SW air masses in the autumn and winter (Fig. ) over the 18 years suggests that the time the polar front is residing north of ZEP is increasing; this further signals the observatory's shift from a polar to a marine site. Previous studies have shown that transport through the North Atlantic has become more frequent in the last decades . The long-term increasing trend in the contribution from air masses from the SW in winter and NW in summer is consistent with the findings in , who have shown that in the winter troposphere, the frequency in wind from north-westerly directions is reduced, and, instead, there is much more frequent wind from southern directions.

The overall changes in air circulation patterns can be tied to regional shifts in weather phenomena. It is well known that long-term trends in the North Atlantic Oscillation (NAO) influence long-term trends in Arctic pollution . The Arctic Oscillation (AO) can also be used to describe the same phenomenon as the NAO. The AO index has been shown to be shifted towards more negative anomalies in recent years , which is consistent with the findings in this study. A negative AO increases the amount of meridional transport (south-to-north or vice versa) ; the Icelandic low pressure system and the Azores high pressure system are weakened, restricting westerly flow and enabling an increased amount of meridional transport over the North Atlantic.

Other potential explanations for the shift towards large particles exist. The following alternatives are addressed as to whether these trends are the result of any other hypotheses: (1) increased influence of mineral dust in the Arctic; (2) decreases in new particle formation (NPF); (3) decreases in long-range transport and/or local emissions of anthropogenic pollution; or (4) decreases in the scavenging of larger particles due to changes in the rates of wet deposition.

Mineral dust, arising from deglaciation in the Arctic, is another coarse-mode aerosol with the potential to be observed at ZEP. However, it is well established that SSA is the most abundant coarse-mode aerosol at ZEP . The clear open water dependency of α (see Fig. ) suggests the changes are a result of marine influences, thus negating (1). Ultrafine particles (< 100 nm) produced via NPF are considered to contribute a negligible amount to the total aerosol light scattering. Furthermore, NPF events at ZEP have been shown to be anti-correlated with sea ice extent ; thus (2) is not considered. Anthropogenic pollution, which is either transported up to the Arctic or produced locally, is typically dominated by accumulation-mode aerosol particles and thus exhibits higher α values. If (3) is assumed and also that there is no increase in the contribution from coarse-mode aerosol, it is difficult to explain the observed increasing trend in σsp. The measurements in this study, however, cannot be used to demonstrate a reduction in anthropogenic pollution at ZEP. However, previous studies have reported reductions in elemental black carbon (BC) at ZEP , notable given the increase in BC emission inventories in the past decade . also present reductions in measured total sulfate concentrations at ZEP. It should be noted that conclude that about 4.9 % and 0.3 % of the reductions in BC and sulfate respectively at ZEP can be explained by circulation changes within the time period (1990–2009). Higher median α values for the air masses arriving from the north-east and south-east are observed (Fig. S7 in the Supplement). These air masses, where a considerable amount of Eurasian pollution originates from, have decreased in their relative contribution. The influence of wet deposition on Arctic aerosol requires a thorough analysis. However, the trend in precipitation from arriving back trajectories is increasing (Fig. c), as wetter air masses from the south-west arrive at the observatory. The increase in precipitation suggests that (4) is unlikely to be the reason for the shift to more coarse-mode particles. show that NPF events are linked to the retreat in Arctic sea ice; ultrafine particles (< 100 nm) are shown to be associated with air masses travelling over open water and sea ice. In this study, SSA is linked to air masses traversing over open water; however, the overall mechanism that is ascribed to the observed aerosol trends is changes in air mass contributions.

Aerosol composition at ZEP has undergone significant changes over the last 2 decades, manifesting in a clear shift of aerosol light scattering properties. The statistically significant increasing trend in σsp of ∼ 2.6 % yr-1 to 2.9 % yr-1, combined with a statistically significant decreasing trend in α of around -4.9 % yr-1 to -6.5 % yr-1, demonstrates that aerosol observed at ZEP has become more dominated by coarse-mode particles over time, which in turn contribute a greater proportion to the scattering of light. There is a clear open water dependency on α, suggesting that the increasing trend is a result of marine influences leading to increased transport of SSA to the site. The strong decrease of Arctic sea ice over the last decades, leading to more open water, is, however, not the main reason for the increased contribution of SSA. Here, we demonstrate that the growing marine influence originates from the increasing time back trajectories spend over the open water, particularly south-west of Svalbard. As such, the changes in air circulation patterns have resulted in a characteristic shift of ZEP, from an Arctic towards a more marine-dominated site.

ZEP is a site that requires detailed analysis, given the significance and magnitude of the observed trends. The results in this study suggest that climate-related changes are influencing the transportation of aerosol particles in and to the Arctic region, as well as the processing and sources of particles. It is important to note that the results are site dependent and that no general conclusions for the entire Arctic can be made.