Abstract
Abstract. Ice core nitrate concentrations peak in the summer in both Greenland and Antarctica. Two nitrate concentration peaks in one annual layer have been observed some years in ice cores in Greenland from samples dating post-1900, with the additional nitrate peak occurring in the spring. The origin of the spring nitrate peak was hypothesized to be pollution transport from the mid-latitudes in the industrial era. We performed a case study on the origin of a spring nitrate peak in 2005 measured from a snowpit at Summit, Greenland, covering 3 years of snow accumulation. The effect of long-range transport of nitrate on this spring peak was excluded by using sulfate as a pollution tracer. The isotopic composition of nitrate (δ15N, δ18O and Δ17O) combined with photochemical calculations suggest that the occurrence of this spring peak is linked to a significantly weakened stratospheric ozone (O3) layer. The weakened O3 layer resulted in elevated UVB (ultraviolet-B) radiation on the snow surface, where the production of OH and NOx from the photolysis of their precursors was enhanced. Elevated NOx and OH concentrations resulted in enhanced nitrate production mainly through the NO2 + OH formation pathway, as indicated by decreases in δ18O and Δ17O of nitrate associated with the spring peak. We further examined the nitrate concentration record from a shallow ice core covering the period from 1772 to 2006 and found 19 years with double nitrate peaks after the 1950s. Out of these 19 years, 14 of the secondary nitrate peaks were accompanied by sulfate peaks, suggesting long-range transport of nitrate as their source. In the other 5 years, low springtime O3 column density was observed, suggesting enhanced local production of nitrate as their source. The results suggest that, in addition to direct transport of nitrate from polluted regions, enhanced local photochemistry can also lead to a spring nitrate peak. The enhanced local photochemistry is probably associated with the interannual variability of O3 column density in the Arctic, which leads to elevated surface UV radiation in some years. In this scenario, enhanced photochemistry caused increased local nitrate production under the condition of elevated local NOx abundance in the industrial era.
Highlights
Knowledge of the abundance and variability of reactive nitrogen oxides (NOx = NO + NO2) is valuable because of the critical role that NOx plays in determining the oxidative capacity of the atmosphere
Spring nitrate peaks have been observed in Greenland ice core records beginning 1900 and were hypothesized to originate from pollution transport from the mid-latitudes in the industrial era (Burkhart et al, 2006; Whitlow et al, 1992; Yang et al, 1995)
The specific mechanism leading to the additional peak is unclear, as it could be from direct transport of nitrate or transport of NOx precursors followed by enhanced local photochemistry
Summary
Knowledge of the abundance and variability of reactive nitrogen oxides (NOx = NO + NO2) is valuable because of the critical role that NOx plays in determining the oxidative capacity of the atmosphere. The oxidative capacity of the atmosphere is determined by the tropospheric abundance of hydrogen oxide radicals (HOx = OH + HO2) and O3 and largely controls the residence times of pollutants (e.g., CO). L. Geng et al.: The spring nitrate peak in Greenland snow and greenhouse gases (e.g., CH4). NOx is emitted from a variety of sources including fossil fuel combustion, biomass burning, soil emissions and lightning (Logan, 1983). NOx cycles rapidly between NO and NO2 according to: NO + O3 → NO2 + O2,
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