Nitrate sources and formation of rainwater constrained by dual isotopes in Southeast Asia: example from Singapore

Emission of reactive nitrogen species has a major impact on atmospheric chemistry, ecosystem and human health. The origin and formation mechanisms of wet-deposited nitrate are not well understood in Southeast Asia (SEA). In this study, we measured stable isotopes of nitrate (δ15N and δ18O) and chemical compositions of daily rainwater from May 2015 to July 2017 in Singapore. Our results showed that δ15N–NO3− and δ18O–NO3− varied seasonally with higher values during the Inter-monsoon period (April–May and October–November) than during Northeast (December–March) and Southwest monsoon (June–September). Bayesian mixing modeling, which took account of the isotope fractionation, indicated that traffic emission (47 ± 32%) and lightning (19 ± 20%) contributed the most to NO3− with increased traffic contribution (55 ± 37%) in the Northeast monsoon and lightning (24 ± 23%) during the Inter-monsoon period. Biomass burning and coal combustion, likely from transboundary transport, contributed ∼25% of nitrate in the rainwater. Monte Carlo simulation of δ18O–NO3− indicated that oxidation process by hydroxyl radical contributed 65 ± 14% of NO3−, with the rest from hydrolysis of N2O5. Wind speed had large effect on δ18O–NO3− variations in the atmosphere with more involvement of hydroxyl radical reactions when wind speed increased. Our study highlights the key role of isotopic fractionation in nitrate source apportionment, and the influence of meteorological conditions on nitrate formation processes in SEA.