On the origin of the occasional springtime nitrate concentration maximum in Greenland snow

Abstract

An occasional spring nitrate concentration maximum was observed in ice cores from Central Greenland but its origin is unknown. This study performs a case study on its origin by measuring concentration and isotopic composition of nitrate (δ 15 N, δ 18 O and Δ 17 O) in a snowpit from Summit, Greenland covering three years of snow accumulation. A nitrate concentration maximum was found in the spring of 2005. Isotopic data of nitrate combined with photochemical calculations suggest that the presence of this spring maximum was linked to a significantly weakened stratospheric ozone (O 3) layer. This weakened O 3 layer resulted in elevated UV-B (Ultraviolet B) radiation on the snow surface, where the productions of OH and NO x from the photolysis of their precursors were enhanced. A concentration maximum was then observed as the result of enhanced local nitrate production due primarily to the increased OH concentrations, as indicated by decreases in δ 18 O and Δ 17 O of nitrate associated with the spring maximum. We further examined the nitrate concentration record from a shallow ice core covering the period from 1772 to 2006 and compared this record to satellite observations of springtime O 3 column density data from 1979 to 2006. We found 19 spring nitrate maxima after the 1950s. After 1979, all spring concentration maxima appeared with O 3 column density near or below the 1979–2006 average. We hypothesize that the presence of the spring nitrate concentration maximum is largely associated with and may be determined by the interannual variability of O 3 column density, under the condition of elevated local NO x abundance at Summit after the 1950s resulting from enhanced anthropogenic nitrate deposition, though other factor(s) may dominate in some years. Isotopic data covering additional years of low O 3 column density are needed to further examine this hypothesis.