Do N-isotopes in atmospheric nitrate deposition reflect air pollution levels?

Dry and wet deposition of atmospheric reactive nitrogen compounds mostly originate from anthropogenic NH3 and NOX sources. Regarding land-borne pollutants, coastal environments usually have a lower pollution level than terrestrial/urban areas, which have a greater anthropogenic imprint. To investigate this spatial characteristic, we measured NO3− and NH4+ deposition and N isotopes of NO3− (δ15N-NO3−) in 94 and 88 wet and dry deposition samples, respectively, at a coastal (List on Sylt) and a terrestrial/urban site (Geesthacht) in Germany from May 2012 to May 2013. A higher total N deposition rate was observed in Geesthacht (10.4 vs. 8.9 kg N ha−1 yr−1) due to higher NH4+ deposition, which can be explained by more agricultural influence. Surprisingly, overall NO3− fluxes were higher at the coastal site than at the terrestrial/urban site. We assume that sea-salt aerosols and the increased influence of NOX emissions from ships in most recent times compensate the higher terrestrial/urban pollution level and thus lead to higher NO3− fluxes in dry and comparable fluxes in wet deposition at the coastal site, despite a much lower impact of land-based sources. In line with this, overall mean N isotopes values of NO3− show higher values in List than in Geesthacht in dry (+3.1 vs. +1.9‰) as well as in wet deposition (−0.1 vs. −1.0‰). This surprising result can mainly be attributed to an emerging source of NOX, ship emissions, which have a distinctly higher impact at the coastal site. The usage of heavy oil and possibly new technologies in marine engines, which emit more enriched 15N in comparison to older engines, caused the spatial isotopic differences.