Pan-Arctic seasonal cycles and long-term trends of aerosol properties from 10 observatories
Authors: Schmale Julia , Sharma Sangeeta , Decesari Stefano , Pernov Jakob , Massling Andreas , Hansson Hans-Christen , von Salzen Knut , Skov Henrik , Andrews Elisabeth , Quinn Patricia K. , Upchurch Lucia M. , Eleftheriadis Konstantinos , Traversi Rita , Gilardoni Stefania , Mazzola Mauro , Laing James , Hopke Philip 
Published: March 2022

Abstract. Even though the Arctic is remote, aerosol properties observed there are
strongly influenced by anthropogenic emissions from outside the Arctic. This
is particularly true for the so-called Arctic haze season (January through
April). In summer (June through September), when atmospheric transport
patterns 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 emissions
have taken place. At the same time a large body of literature shows evidence
that the Arctic is undergoing fundamental environmental changes due to
climate forcing, leading to enhanced emissions by natural processes that may
impact aerosol properties. In this study, we analyze 9 aerosol chemical species and 4 particle
optical properties from 10 Arctic observatories (Alert, Kevo, Pallas,
Summit, Thule, Tiksi, Barrow/Utqiaġvik, Villum, and Gruvebadet and Zeppelin
Observatory – both at Ny-Ålesund Research Station) to understand changes
in anthropogenic and natural aerosol contributions. Variables include
equivalent black carbon, particulate sulfate, nitrate, ammonium,
methanesulfonic acid, sodium, iron, calcium and potassium, as well as
scattering and absorption coefficients, single scattering albedo and
scattering Ångström exponent. First, annual cycles are investigated, which despite anthropogenic emission
reductions still show the Arctic haze phenomenon. Second, long-term trends
are studied using the Mann–Kendall Theil–Sen slope method. We find in total
41 significant trends over full station records, i.e., spanning more than a
decade, compared to 26 significant decadal trends. The majority of
significantly declining trends is from anthropogenic tracers and occurred
during the haze period, driven by emission changes between 1990 and 2000.
For the summer period, no uniform picture of trends has emerged. Twenty-six
percent of trends, i.e., 19 out of 73, are significant, and of those 5 are
positive and 14 are negative. Negative trends include not only anthropogenic
tracers such as equivalent black carbon at Kevo, but also natural indicators
such as methanesulfonic acid and non-sea-salt calcium at Alert. Positive
trends are observed for sulfate at Gruvebadet. No clear evidence of a significant change in the natural aerosol
contribution can be observed yet. However, testing the sensitivity of the
Mann–Kendall Theil–Sen method, we find that monotonic changes of around 5 % yr−1 in an aerosol property are needed to detect a significant
trend within one decade. This highlights that long-term efforts well beyond
a decade are needed to capture smaller changes. It is particularly important
to understand the ongoing natural changes in the Arctic, where interannual
variability can be high, such as with forest fire emissions and their
influence on the aerosol population. To investigate the climate-change-induced influence on the aerosol
population and the resulting climate feedback, long-term observations of
tracers more specific to natural sources are needed, as well as of particle
microphysical properties such as size distributions, which can be used to
identify changes in particle populations which are not well captured by
mass-oriented methods such as bulk chemical composition.