Canadian forest fires, Icelandic volcanoes and increased local dust observed in six shallow Greenland firn cores

. Greenland ice cores provide information about past climate. Few impurity records covering the past 2 decades exist from Greenland. Here we present results from six ﬁrn cores obtained during a 426 km long northern Greenland traverse made in 2015 between the NEEM and the EGRIP deep-drilling stations situated on the western side and eastern side of the Greenland ice sheet, respectively. The cores (9 to 14 m long) are analyzed for chemical impurities and cover time spans of 18 to 53 years ( ± 3 years) depending on local snow accumulation that decreases from west to east. The high temporal resolution allows for annual layers and seasons to be resolved. Insoluble dust, ammonium, and calcium concentrations in the six ﬁrn cores overlap, and the seasonal cycles are also similar in timing and magnitude across sites, while peroxide (H 2 O 2 ) and conductivity both have spatial variations, H 2 O 2 driven by the accumulation pattern, and conductivity likely inﬂuenced by sea salt. Overall, we determine a rather constant dust ﬂux over the period, but in the data from recent years (1998–2015) we identify an increase

Insoluble dust was determined using an Abakus laser particle detector with a LDS-23/25bs sensor type (Klotz, Simonsen et al., 2018) and the conductivity using a bench-top conductivity meter (3082 with micro flow cell 829, Amber Science).The measurements of ammonium, hydrogen peroxide and calcium is based upon fluorescence methods (Kaufmann et al., 2008;Bigler et al., 2011) using photomultiplier detectors (PMT-FL, FIAlab instruments) and spectrofluorimeters set to specific (Sigg 5 et al., 1994a).Acid was determined as described in Kjaer et al. 2016a using a dye-based method.In Table S1 and S2   The NH4 + and Ca 2+ was calibrated using 3 standards made from a multi-element stock solution, acid was calibrated with a twopoint calibration made from a 0.1M HCl (Fluka), and the H2O2 was calibrated based on a two-point calibration using a Peroxide stock solution (30% H2O2).The standards concentration preparation scheme is presented in table S3.

S2 Data on a depth scale
Below is shown from top the calibrated data of peroxide, Calcium, Insoluble Dust, Ammonium, Conductivity and Acidity on a depth scale together with in red vertical lines the annual layers chosen for the six individual firn cores.right for the period 2000 AD and onwards to make comparable estimates between sites.See also Figure 3 for a whiskers plot of the data.

S3 -Tables of correlations
Below tables of Pearson correlations between the records are presented.The correlations are made on both annual means and monthly mean records.The maximum available overlap between years have been used.

S4-Additional analysis of the data
Conductivity in the 6 shallow cores are presented in Figure S8.
Table S5 presents the correlation of the excess ammonium between the 5 sites for which ammonium was analyzed.
In Figure S6 the particle dust fluxes are presented with time for small particles 1.25-2.9µm, medium particles 2.9-8.13µm and large particles 8.13-10.5 µm and Table S7 presents a similar analysis as that of Table 2, but for the longer period 1988-5 2015 covered by cores T2015A2 through T2015-A6).
S1-The Copenhagen CFA set up for the analysis of the shallow firn coresIn FigureS1a flow chart of the CFA setup used to analyse the firn cores is presented.

Figure S1 :
Figure S1: Copenhagen CFA (Bigler et al., 2011) set up used to measure the 6 firn cores.Flowchart of the Copenhagen continuous flow analysis system setup with a melt head (MH) melting the ice sample, melting speed encoder (MS), melting weight (MW), Coin added for firn analysis, debubbler (DB) selection and injection valves (V) for standard solutions (STD) and blanks (MQ), waste (W), flow meter (FM), peristaltic pumps (P), triangular debubbler (D), buffer and reagents, reaction coils, Accurel membrane debubblers (Ac), absorption (A), fluorescence (F), discrete sampling line (S), conductivity detector (σ), particle sensor (Dust) and a vacuum degassing unit.Arrows indicate flow directions.Flow rates within the pump system (mL min -1 ), temperatures, reaction coil lengths and detector light wavelengths are shown in the flowchart.
a full list of reagents and instrumentation used is presented.

Figure S2 :
Figure S2: T2015-A1 data on a depth scale together with chosen annual layers (red vertical lines) used for generating the 5 age scale.

Figure S3 :
Figure S3: T2015-A2 data on a depth scale together with chosen annual layers (red vertical lines) used for generating the age scale.

Figure S4 :
Figure S4: T2015-A3 data on a depth scale together with chosen annual layers (red vertical lines) used for generating the age scale.

Figure S5 :
Figure S5: T2015-A4 data on a depth scale together with chosen annual layers (red vertical lines) used for generating the age scale.

Figure S6 :
Figure S6: T2015-A5 data on a depth scale together with chosen annual layers (red vertical lines) used for generating the age scale.Note for this core ammonium was not determined.

Figure S7 :
Figure S7: T2015-A6 data on a depth scale together with chosen annual layers (red vertical lines) used for generating the age scale.