A B S T R A C T Arctic regions are generally nutrient limited, receiving an extensive part of their bio-available nitrogen from the deposition of atmospheric reactive nitrogen. Reactive nitrogen oxides, as nitric acid (HNO 3 ) and nitrate aerosols (p-NO 3 ), can either be washed out from the atmosphere by precipitation or dry deposited, dissolving to nitrate (NO À 3 ). During winter, NO À 3 is accumulated in the snowpack and released as a pulse during spring melt. Quantification of NO À 3 deposition is essential to assess impacts on Arctic terrestrial ecology and for ice core interpretations. However, the individual importance of wet and dry deposition is poorly quantified in the high Arctic regions where in-situ measurements are demanding. In this study, three different methods are employed to quantify NO and p-NO 3 using atmospheric concentrations and stability observations, resulting in a total combined nitrate dry deposition of Á10.7691.26 mg m (2 . The model indicates that deposition primarily occurs via HNO 3 with only a minor contribution by p-NO 3 . Modelled median deposition velocities largely explain this difference: 0.63 cm s (1 for HNO 3 while p-NO 3 was 0.0025 and 0.16 cm s (1 for particle sizes 0.7 and 7 mm, respectively.Overall, the three methods are within two standard errors agreement, attributing an average 14% (total range of 2Á44%) of the total nitrate deposition to dry deposition. Dry deposition events were identified in association with elevated atmospheric concentrations, corroborating recent studies that identified episodes of rapid pollution transport and deposition to the Arctic.

A 20-year dataset of weekly precipitation observations in Ny-Ålesund, Svalbard, was analysed to assess atmospheric wet deposition of nitrogen. Mean annual total nitrogen deposition was 74 mg N/(m2 yr) but exhibited large interannual variability and was dominated by highly episodic “strong” events, probably caused by rapid transport from European sources. The majority (90%) of precipitation samples were defined as “weak” (<2 mg N/m2) and contributed an annual baseline of ~17 mg N/(m2 yr), whereas 10% of precipitation samples were defined as “strong” (>2 mg N/m2) and additionally contributed up to 225 mg N/(m2 yr). Nitrate deposition largely occurred in samples within the solid-precipitation season (16 September–2 June), and ammonium deposition occurred equally in both solid and liquid seasons. Trends of reactive nitrogen emissions from Europe are uncertain, and increasing cyclonic activity over the North Atlantic caused by a changing climate might lead to more strong deposition events in Svalbard.

The snowpack acts as a sink for atmospheric reactive nitrogen, but several postdeposition pathways have been reported to alter the concentration and isotopic composition of snow nitrate with implications for atmospheric boundary layer chemistry, ice core records, and terrestrial ecology following snow melt. Careful daily sampling of surface snow during winter (11-15 February 2010) and springtime (9 April to 5 May 2010) near Ny-Ålesund, Svalbard reveals a complex pattern of processes within the snowpack. Dry deposition was found to dominate over postdeposition losses, with a net nitrate deposition rate of (0.6 ± 0.2) μmol m À2 d À1 to homogeneous surface snow. At Ny-Ålesund, such surface dry deposition can either solely result from long-range atmospheric transport of oxidized nitrogen or include the redeposition of photolytic/bacterial emission originating from deeper snow layers. Our data further confirm that polar basin air masses bring 15 N-depleted nitrate to Svalbard, while high nitrate δ( 18 O) values only occur in connection with ozone-depleted air, and show that these signatures are reflected in the deposited nitrate. Such ozone-depleted air is attributed to active halogen chemistry in the air masses advected to the site. However, here the Ny-Ålesund surface snow was shown to have an active role in the halogen dynamics for this region, as indicated by declining bromide concentrations and increasing nitrate δ( 18 O), during high BrO (low-ozone) events. The data also indicate that the snowpack BrO-NO x cycling continued in postevent periods, when ambient ozone and BrO levels recovered.