Sources of aerosol nitrate to the Gulf of Aqaba: Evidence from δ15N and δ18O of nitrate and trace metal chemistry

The nitrogen (N) and oxygen (O) isotopic composition (δ¹⁵N and δ¹⁸O) of water soluble aerosol nitrate was measured in aerosol samples collected in Eilat, Israel, from August 2003 to November 2004. During this period δ¹⁵N values ranged from − 6.9‰ to + 1.9‰ and δ¹⁸O from + 65.1‰ to + 84.9‰ and exhibited strong seasonal variability with higher average δ¹⁵N values observed in the summer and higher δ¹⁸O values in the winter. Nitrate isotopic composition was compared with bulk chemical composition and extractable ion and trace metals on co-collected samples linking nitrate isotopic composition to various sources of aerosols to this region. Atmospheric processes impacting the isotopic signatures of nitrate were also considered.

Based on back trajectory analyses, the majority of NO₃⁻ came from air masses originating over the Mediterranean Sea (34%), Western Europe (20%) and the local Negev desert (19%), which contain a larger anthropogenic imprint compared to southern and eastern air masses which are dominated by mineral dust. The potential role of reactive mineral dust aerosols as a regulator of NO₃⁻ isotopic composition is considered; however, based on factor analysis, neither δ¹⁵N nor δ¹⁸O were associated with mineral dust components (such as Fe or Al), but rather with anthropogenic indicators such as Cu, Cd, P and Pb. Seasonality in primary NO_x cycling reactions driven by seasonal changes in solar radiation, relative humidity and temperature also influence the observed isotopic signatures. The isotope data, together with trace element analysis, suggests that seasonal variations in both δ¹⁵N_NO3 and δ¹⁸O_NO3 are related to both NO_x source and transport processes as well as NO_x chemical reactions in the atmosphere.

The flux-weighted δ¹⁵N of aerosol NO₃⁻ in this area averaged − 2.6‰ making aerosol deposition a substantial contributor of low δ¹⁵N nitrogen to the oligotrophic waters of the Gulf of Aqaba. Thus, while the flux of atmospheric N to oligotrophic marine systems is smaller than the upward flux of NO₃⁻ from deep water, it nonetheless represents an important source of new N having a low δ¹⁵N. Further, if this low δ¹⁵N signature is not considered, it could interfere with N-fixation estimates based on isotopic composition of dissolved nitrate or particulate organic nitrogen. Thus, atmospheric deposition should be constrained for accurate estimates of marine N-fixation when based on δ¹⁵N in the ocean. Indeed, in the Gulf of Aqaba, low upper water δ¹⁵N_NO3 values could be related to inputs of atmospheric NO₃⁻ as well as N-fixation.