1. The preservation of long-range transported nitrate in snow at Summit, Greenland Jack Dibb 1, Meredith Hastings 2, Dorothy Fibiger 3*, D. Chen 4, L. Gregory Huey 4 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 NO 3 - show a strong, linear relationship. This relationship is best explained as mixing among oxidants that influence the formation of NO 3 -. The high end-member is most likely stratospheric O 3 (δ 18 O = 100‰, Δ 17 O = 10‰), while the low end-member looks most like O 2 (δ 18 O = 23.5‰, Δ 17 O = 0‰).  This cannot be explained with significant post-depositional processing of NO 3 - in the surface snow. Photolytic loss of NO 3 - should not change the Δ 17 O, while enriching the δ 18 O of NO 3 - left in the snow. Gas phase re-processing of NO x emitted from the snow should yield snow NO 3 - that is associated with local oxidation processes. No relationship is found with the expected range in δ 18 O-OH nor is any relationship found with atmospheric composition at Summit (right). Re-processing of NO 3 - within snow grains (i.e. condensed phase) should yield NO 3 - that would be isotopically linked to H 2 O isotopes (δ 18 O-H 2 O 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 NO 3 - data, confirming the lack of local NO 3 - recycling, there are interesting “events” (in the black rectangle) observed several times in both seasons in the isotopes of NO 3 -. They involve δ 15 N suddenly increasing while, simultaneously, δ 18 O and Δ 17 O 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 δ 18 O and Δ 17 O with a high δ 15 N.  A number of studies at Summit, Greenland [72°35’N, 38°25’W] have shown emission of NO x and HO x 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 NO x and HO x cycling at Summit.  Through both field seasons, there are no correlations found between gas phase species and isotopes of NO 3 - in the snow (right). The relationship between δ 18 O and Δ 17 O-NO 3 - in the snow cannot be explained by significant post-depositional loss or recycling of NO x /NO 3 - (below). The isotopic composition of HNO 3 in the air and NO 3 - in the snow are distinct (bottom left). Combined, this suggests that 1) very little NO 3 - is lost from surface snow via photolysis, 2) any locally produced HNO 3 is not significantly influenced by Br chemistry, and 3) recycled HNO 3 from snow-sourced NO x can only be a very small component of the snow nitrate budget.  The NO 3 - in snow at Summit is representative of (cloud to ground) long- range transported NO y that arrives at Summit as NO 3 - and is largely preserved in surface snow during photoactive periods. Schematic description of different formation pathways for NO 3 - in snow at Summit, Greenland NO y SNOW ATMOSPHERE NO x transport reaction with local oxidants NO 3 - NO x NO 3 - δ 15 N-NO 3 - (‰)δ 18 O-NO 3 - (‰)[NO 3 - ] (μM)[HNO 3 ] (pptv) Surface snow NO 3 - and atmospheric HNO 3 concentration, δ 15 N, and δ 18 O in May-June - atmospheric measurements h δ 15 N-NO 3 - (‰) δ 18 O-NO 3 - (‰) Δ 17 O-NO 3 - (‰) [NO 3 - ] (μM) [NO 3 - ] (pptv) [BrO] (pptv) [NO y ] (pptv) [NO] (pptv) NO 3 - airsnowairsnowairsnowairsnow air 20102011201020112010201120102011 NO 3 - that is transported to Summit and deposited to the snow should carry isotopic signatures (  15 N,  18 O,  17 O) set by its source region and chemistry. NO 3 - that is formed from NO x transported into Summit and locally oxidized will have an oxygen isotopic composition (  18 O, Δ 17 O) that reflects local oxidants and a  15 N that represents the source region (and possible fractionations associated with chemistry). NO 3 - that is photolyzed in surface snow can release NO x into the boundary layer that is locally oxidized to re-form NO 3 -, and will contain a  15 N that reflects fractionation associated with photolysis, a  18 O that reflects fractionation and the  18 O of local oxidants, and  17 O that is unaffected by photolysis but will reflect local oxidation. Surface snow NO 3 - Δ 17 O and δ 18 O from 2010 and 2011 Atmospheric and snow measurements from 2010 O3O3 O2O2 H2OH2O OH Fibiger et al., GRL, 2013  In both years, the δ 15 N is significantly lower in the air than that found in surface snow, which would be expected if the HNO 3 was produced from snow- sourced NO x.  δ 18 O in the HNO 3 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 δ 18 O 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 NO 3 - influenced by Br chemistry was the dominant source of the measured HNO 3. - 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.