C43B 670

Black Carbon and other Absorbing Impurities in Northwestern Greenland

Jack E. Dibb1, Chris Polashenski2, 3, and Zoe Courville2, 3

1 Earth Systems Research Center EOS UNH, Durham, NH2 CRREL, Hanover, NH

3 Thayer School of Engineering, Dartmouth College, Hanover, NH

1500

2000

2500

3000

Summit PitBenson 4-325Benson 4-275Benson 4-225Benson 4-175Benson 4-150Benson4-100Benson 4-50Benson 4-0Benson 2-200Benson 2-175Benson 2-125Benson 2-70Benson 2-20Benson 1-50Benson 1-10Benson 1A-20

R 1R 2R 3R 4R 5R 6R 7

Elevationmean albedo 550-570mean albedo 1090-1110

Elevation m

Albedo0.83

1.00

0.67

0

500

1000

1500

2000

2500

3000

3500BC ng/L Ca nmol/L

Concentrations

Top 3 cm

0

0.5

1

1.5

2

2.5BC ng/cm2

Ca nmol/cm2

Snow g/cm2

Inventories

Top 3 cm

0

2

4

6

8

10

Summit PitBenson 4-325Benson 4-275Benson 4-225Benson 4-175Benson 4-150Benson4-100Benson 4-50Benson 4-0

Benson 2-200Benson 2-175Benson 2-125Benson 2-70Benson 2-20Benson 1-50Benson 1-10Benson 1A-20

R 1 R 2 R 3 R 4 R 5 R 6 R 7

BC ng/cm2

Ca nmol/cm2

Snow g/cm2

Inventories

Top 2 months

0

1000

2000

3000

4000

5000

6000

7000NO3 nmol/LSO4 nmol/L

Na nmol/L

Concentrations

Top 3 cm

0

1

2

3

4

5

6

7NO3 nmol/cm2

SO4 nmol/cm2

Na nmol/cm2

Inventories

Top 3 cm

0

5

10

15

20

25

30

35

Summit PitBenson 4-325Benson 4-275Benson 4-225Benson 4-175Benson 4-150Benson4-100Benson 4-50Benson 4-0

Benson 2-200Benson 2-175Benson 2-125Benson 2-70Benson 2-20Benson 1-50Benson 1-10Benson 1A-20

R 1 R 2 R 3 R 4 R 5 R 6 R 7

NO3 nmol/cm2

SO4 nmol/cm2

Na nmol/cm2

Inventories

Top 2 months

A snowmobile traverse was conducted from Summit Station nearly to Thule between 7 May and 4 June, 2013. At 26 locations, many along the Benson traverse route, snowpits were sampled from the surface to 30 cm depth for chemical and microphysical characterization, and surface albedo was measured at multiple places surrounding each pit. In each pit the depth to the prominent 2012 melt surface was determined and found to range from just 19 cm at station R2 up to 163 cm at Station Benson 1A-20. We focus here on the top 3 cm (likely to be the most important for albedo) but also present the inventories of snow and select chemical impurities that had accumulated at each site in the ~ 2 months prior to sampling.

BC and Ca2+ (dust tracer) are expected to be the dominant absorbing impurities in Greenland snow, we also show NO3

- and SO4

= as tracers of anthropogenic emissions, NH4+ as a

wildfire tracer, and Na+ as a seasalt indicator. All of the tracers except Na+ show elevated concentrations and inventories at the stations near the summit, and also close to Thule. Seasalt is enhanced at the stations at lower elevations. Particularly strong gradients are seen when crossing divides from south to north (compare sites 1-50 and 2-20 just north to sites 1-10 and 1A-20 just south of ridge crest, and note also the low inventories of impurities in R1-3 driven largely by low snow accumulation east of the ridge).

The concentration of BC is not strongly correlated with the ionic tracers of pollution or biomass burning emissions, at least for snow that fell in May 2013. We expect a stronger contribution of BC from fires in summer, and will look for that in samples collected in 2014. Strongest relationships between BC and any of the ionic tracers were seen for Ca2+ and Mg2+, tracers of dust.

Albedo in the UV and Visible ranges of the spectra ranged from 0.9 to ~1.0 at all of the sites, while the albedo in the IR had a slightly wider range (0.71 to 0.84). Highest albedo values tended to be at sites near 2500 m elevation but the trend with elevation was weak.

Albedo did show small decreases with increasing BC and Ca2+ concentrations and inventories. Slopes are steeper and the correlations stronger for Ca2+, suggesting that during the spring absorption by dust in snow is stronger than that due to BC. It is reassuring that essentially no trends are found between albedo and non-absorbing impurities (NO3

- shown as example).

However, impurities are not expected to strongly impact albedo in the IR yet the correlations between both BC and Ca2+ versus albedo in the 1090-1110 nm band are similar to those in the UV and visible bands. We also measured snow specific surface area and collected stereology samples in all pits, and will be looking at the relationships between snow microphysics, impurities, and albedo in the next phase of analysis.

Acknowledgments: we greatly appreciate the outstanding assistance Mike and Nate Stewart provided to Chris on the traverse. We also thank the Government of Greenland for allowing us to conduct this research on the ice sheet and in the national park. Essential logistical support was provided by CPS and the 109 th NY ANG. This research was supported by the National Science Foundation Division of Polar Programs.