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Abstract. Even though the Arctic is remote, aerosol properties observed there arestrongly influenced by anthropogenic emissions from outside the Arctic. Thisis particularly true for the so-called Arctic haze season (January throughApril). In summer (June through September), when atmospheric transportpatterns change, and precipitation is more frequent, local Arctic sources,i.e., natural sources of aerosols and precursors, play an important role.Over the last few decades, significant reductions in anthropogenic emissionshave taken place. At the same time a large body of literature shows evidencethat the Arctic is undergoing fundamental environmental changes due toclimate forcing, leading to enhanced emissions by natural processes that mayimpact aerosol properties. In this study, we analyze 9 aerosol chemical species and 4 particleoptical properties from 10 Arctic observatories (Alert, Kevo, Pallas,Summit, Thule, Tiksi, Barrow/Utqiaġvik, Villum, and Gruvebadet and ZeppelinObservatory – both at Ny-Ålesund Research Station) to understand changesin anthropogenic and natural aerosol contributions. Variables includeequivalent black carbon, particulate sulfate, nitrate, ammonium,methanesulfonic acid, sodium, iron, calcium and potassium, as well asscattering and absorption coefficients, single scattering albedo andscattering Ångström exponent. First, annual cycles are investigated, which despite anthropogenic emissionreductions still show the Arctic haze phenomenon. Second, long-term trendsare studied using the Mann–Kendall Theil–Sen slope method. We find in total41 significant trends over full station records, i.e., spanning more than adecade, compared to 26 significant decadal trends. The majority ofsignificantly declining trends is from anthropogenic tracers and occurredduring the haze period, driven by emission changes between 1990 and 2000.For the summer period, no uniform picture of trends has emerged. Twenty-sixpercent of trends, i.e., 19 out of 73, are significant, and of those 5 arepositive and 14 are negative. Negative trends include not only anthropogenictracers such as equivalent black carbon at Kevo, but also natural indicatorssuch as methanesulfonic acid and non-sea-salt calcium at Alert. Positivetrends are observed for sulfate at Gruvebadet. No clear evidence of a significant change in the natural aerosolcontribution can be observed yet. However, testing the sensitivity of theMann–Kendall Theil–Sen method, we find that monotonic changes of around 5 % yr−1 in an aerosol property are needed to detect a significanttrend within one decade. This highlights that long-term efforts well beyonda decade are needed to capture smaller changes. It is particularly importantto understand the ongoing natural changes in the Arctic, where interannualvariability can be high, such as with forest fire emissions and theirinfluence on the aerosol population. To investigate the climate-change-induced influence on the aerosolpopulation and the resulting climate feedback, long-term observations oftracers more specific to natural sources are needed, as well as of particlemicrophysical properties such as size distributions, which can be used toidentify changes in particle populations which are not well captured bymass-oriented methods such as bulk chemical composition.

Pan-Arctic seasonal cycles and long-term trends of aerosol properties from 10 observatories