Sources of aerosol nitrate to the Gulf of Aqaba: Evidence from δ 15N and δ 18O of nitrate and trace metal chemistry

Abstract

The nitrogen (N) and oxygen (O) isotopic composition (δ 15N and δ 18O) of water soluble aerosol nitrate was measured in aerosol samples collected in Eilat, Israel, from August 2003 to November 2004. During this period δ 15N values ranged from − 6.9‰ to + 1.9‰ and δ 18O from + 65.1‰ to + 84.9‰ and exhibited strong seasonal variability with higher average δ 15N values observed in the summer and higher δ 18O 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 NO 3 − 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 NO 3 − isotopic composition is considered; however, based on factor analysis, neither δ 15N nor δ 18O 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 δ 15N NO3 and δ 18O NO3 are related to both NO x source and transport processes as well as NO x chemical reactions in the atmosphere. The flux-weighted δ 15N of aerosol NO 3 − in this area averaged − 2.6‰ making aerosol deposition a substantial contributor of low δ 15N 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 NO 3 − from deep water, it nonetheless represents an important source of new N having a low δ 15N. Further, if this low δ 15N 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 δ 15N in the ocean. Indeed, in the Gulf of Aqaba, low upper water δ 15N NO3 values could be related to inputs of atmospheric NO 3 − as well as N-fixation.