The preservation of long-range transported nitrate in snow at Summit, Greenland

The preservation of long-range transported nitrate in snow at Summit, Greenland

Jack Dibb1, Meredith Hastings2, Dorothy Fibiger3*, D. Chen4, L. Gregory Huey4

1. Earth Systems Research Center, University of New Hampshire; 2. Department of Earth, Environmental and Planetary Sciences, Brown University; 3. Department of Chemistry, Brown University (*now at NOAA/ESRL); 4. School of Earth and Atmospheric Sciences, Georgia Institute of Technology

The oxygen isotopes of NO3- show a strong, linear relationship. This relationship is

best explained as mixing among oxidants that influence the formation of NO3-. The

high end-member is most likely stratospheric O3 (δ18O = 100‰, Δ17O = 10‰), while the low end-member looks most like O2 (δ18O = 23.5‰, Δ17O = 0‰).

This cannot be explained with significant post-depositional processing of NO3- in the

surface snow. Photolytic loss of NO3- should not change the Δ17O, while enriching the

δ18O of NO3- left in the snow. Gas phase re-processing of NOx emitted from the snow

should yield snow NO3- that is associated with local oxidation processes. No

relationship is found with the expected range in δ18O-OH nor is any relationship found with atmospheric composition at Summit (right). Re-processing of NO3

- within snow grains (i.e. condensed phase) should yield NO3

- that would be isotopically linked to H2O isotopes (δ18O-H2O ranges seasonally from -25 to -45‰).

Thanks also to C. Corr, E. Scheuer, N. Chellman and R. Ho for help with sampling and analysis.

While there are no correlations between any atmospheric and surface snow NO3-

data, confirming the lack of local NO3- recycling, there are interesting “events” (in

the black rectangle) observed several times in both seasons in the isotopes of NO3

-. They involve δ15N suddenly increasing while, simultaneously, δ18O and Δ17O both decrease. It is unclear what causes these isolated events, but they may be related to times that the wind is carrying camp pollution into the sampling area. In addition, sampling on the polluted side of the camp indicates that camp pollution has low δ18O and Δ17O with a high δ15N.

A number of studies at Summit, Greenland [72°35’N, 38°25’W] have shown emission of NOx and HOx species from the snow, adding an important component to the local oxidizing environment. Additionally, halogen compounds, such as BrO and soluble Br- are frequently present at pptv levels above the snow and we expected that Br chemistry might have an important influence on NOx and HOx cycling at Summit.

Through both field seasons, there are no correlations found between gas phase species and isotopes of NO3

- in the snow (right). The relationship between δ18O and Δ17O-NO3

- in the snow cannot be explained by significant post-depositional loss or recycling of NOx/NO3

- (below). The isotopic composition of HNO3 in the air and NO3-

in the snow are distinct (bottom left). Combined, this suggests that 1) very little NO3-

is lost from surface snow via photolysis, 2) any locally produced HNO3 is not significantly influenced by Br chemistry, and 3) recycled HNO3 from snow-sourced NOx can only be a very small component of the snow nitrate budget.

The NO3- in snow at Summit is representative of (cloud to ground) long-

range transported NOy that arrives at Summit as NO3- and is largely

preserved in surface snow during photoactive periods.

Schematic description of different formation pathways for NO3

- in snow at Summit, Greenland

NOy

SNOW

ATMOSPHERE

NOx transport

reaction with local oxidants

NO3-

NOx

NO3- NO3

-NO3-

NO3- NO3

-

NO3-

δ15N-NO3- (‰) δ18O-NO3

- (‰) [NO3-] (μM) [HNO3] (pptv)

Surface snow NO3- and atmospheric HNO3

concentration, δ15N, and δ18O in May-June

- atmospheric measurements

h

17-May 27-May 6-Jun 16-Jun1

3

5

7101520253035

40

60

80

-9

-1

7

δ15N

-NO

3- (‰

)

δ18O

-NO

3- (‰

)

Δ17O

-NO

3- (‰

)

[NO

3- ] (μ

M)

0

20

40

60

0

5

10

010203040

0

200

400

[NO

3- ] (p

ptv)

[BrO

] (pp

tv)

[NO

y] (p

ptv)

[NO

] (pp

tv)

NO3-

NO3-

NO3-

airsnowairsnow airsnowairsnow snow air2010 2011 2010 2011 2010 2011 2010 2011

NO3- that is transported to Summit and deposited to the snow should carry isotopic

signatures (δ15N, δ18O, D17O) set by its source region and chemistry.

NO3- that is formed from NOx transported into Summit and locally oxidized will have an

oxygen isotopic composition (δ18O, Δ17O) that reflects local oxidants and a δ15N that represents the source region (and possible fractionations associated with chemistry).

NO3- that is photolyzed in surface snow can release NOx into the boundary layer that is

locally oxidized to re-form NO3-, and will contain a δ15N that reflects fractionation

associated with photolysis, a δ18O that reflects fractionation and the δ18O of local oxidants, and D17O that is unaffected by photolysis but will reflect local oxidation.

Surface snow NO3- Δ17O and δ18O from 2010 and 2011

Atmospheric and snow measurements from 2010

-40 -20 0 20 40 60 80 1000

5

10

15

20

25

30

35

40

δ18O-NO3- (‰)

Δ17O

-NO

3- (‰

)O3

O2H2O

OH

Fibiger et al., GRL, 2013

In both years, the δ15N is significantly lower in the air than that found in surface snow, which would be expected if the HNO3 was produced from snow-sourced NOx.

δ18O in the HNO3 varies -- significantly lower values in the air than in the snow in 2010, but significantly higher values in the air than in the snow in 2011.

The lower δ18O in 2010 occurs when BrO levels are notably higher than 2011 (avg ~3 pptv, see right), opposite of what we would expect if local formation of NO3

- influenced by Br chemistry was the dominant source of the measured HNO3.

- snow measurements

This work was supported by the National Science Foundation under grant 0909374 (Arctic Natural Sciences). D. Fibiger was also supported by the American Association of University Women.