A case study of the usefulness of δ13C and δ15N for PM source apportionment under different meteorological conditions

In this study, we show the strengths and limitations of using the isotopes of Carbon (δ13C) and Nitrogen (δ15N) to identify their sources in atmospheric particulate matter (PM) under different meteorological conditions. Specifically, we show they are useful to identify a shift of C sources from mostly C3 plants/combustion to higher carbonate content, and from volatilization/biological to combustion sources for N, whereas they do not help identify biomass burning events. We collected particles with aerodynamic diameters <2.5 μm (PM2.5) and <10 μm (PM10) during May 2016 and late November 2016-early January 2017 in the city center of Naples. We measured total C and N, their δ13C and δ15N, major ions (NH4+, K+, Ca2+, Na+, Mg2+, NO3−, SO42−, C2O42−, Cl−) and analyzed wind direction, speed and back-trajectories of air masses. May was characterized by a shift from land-sea breeze to a synoptic system with winds from southwest. We found an increase of δ13C in PM10 during a dust resuspension event, suggesting an increase of the contribution from carbonate C from negligible to 18% of total C. δ15N in PM10, although significantly affected by isotope fractionation, showed a shift of total N from volatilization to combustion sources. In December, air masses mostly originated from the north, with no significant temporal pattern of wind speed and direction. We found days with elevated land-derived species (K+, NH4+, NO3−) typically linked to biomass burning, but without corresponding changes in δ13C and δ15N, highlighting limitations in the isotope tracer's sensitivity. We conclude that δ13C and δ15N can be effective, qualitative indicators of PM origin when integrated with chemical composition and air mass trajectory analysis. However, quantitative applications require well-constrained source signatures and careful consideration of fractionation effects.