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How can sources of nitrate in precipitation be distinguished, particularly between polluted urban air and other sources?

Our precipitation samples show high nitrate, but the catchment has mixed influences: urban traffic, distant farmland, and natural soils. We need to move beyond simple concentration data to defensible source apportionment. I understand isotope ratios are key, but I'm unsure how to interpret the δ¹?N and δ¹?O signatures to untangle these overlapping sources effectively.

 

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By Abhay R Answered 2 years ago

You're right to focus on dual-isotope plots (δ¹?N vs. δ¹?O). From my work on atmospheric deposition, I've seen that different sources occupy distinct realms: vehicle NOx tends to have a low, clustered δ¹?N signature; soil emissions have a wider, often higher range; and fertilizer nitrate has a very low δ¹?N. The δ¹?O helps separate atmospheric processes. I would recommend analyzing co-existing ions like sulfate (for fossil fuel markers) and ammonium. Then, use a Bayesian mixing model (like SIAR or MixSIAR) with your local source end-members to quantify proportional contributions. It's rarely one source, but a mix the model can resolve

 

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