Multiple lines of evidence help identify the sources and formation mechanisms of Nitrogen and Carbon in Particulate Matter sampled in the historical center of Naples (Italy)

The aim of this study is to investigate Particulate Matter (PM) sources and mechanisms of formations over the city of Naples (Italy) and their seasonal and day-to-day variations.

We have sampled fine particles with diameter < 2.5 μm (PM_2.5) and < 10 μm (PM₁₀) daily on pre-cleaned (700 °C for 2 h) quartz filters, during the months of May and November 2016-January 2017, on top of the historical building complex in Largo San Marcellino, Naples. We have measured the concentrations of total N/C together with their isotopic composition (δ¹⁵N and δ¹³C). We have also measured the concentration of major ions and interpreted the results with data of gaseous compounds, as well as consideration of the meteorology, using data and state of the art models of atmospheric circulation (Hysplit). Our point was to show that the uncertainty associated with quantification of sources contribution with an apportionment model decreases when the model is constrained with information derived from different methods.

Seasonal differences: the results show that the concentrations of total PM₁₀/PM_2.5, N/C measured in autumn are more variable than those measured in spring. This is related to a different wind regime, whereby in spring air masses mostly originated from West and South (the “clean” Mediterranean sea), whereas in autumn the wind blew air from North (over the highly urbanized and “dirty” European continent). This interpretation is supported by the concentration of major ions showing more scattered values in autumn for species typically originating from land (K⁺, NH₄⁺, NO₃^-), with high values on the 9^th and the 26-27^th of December and the 2^nd of January 2017. However, neither the monthly mean δ¹⁵N and δ¹³C, nor the daily values corresponding to the spikes show significant changes, suggesting that the isotopic composition of total N/C has limited power in identifying changes of mean monthly sources or for the spikes. 

Day-to-day variations: a significant change of the main species measured is found around the middle of May. This event is associated with a change in weather pattern going from a typical land-sea breeze wind regime (typically causing poor air circulation and stagnation of air masses) to an intense synoptic with winds originating mostly from South/South-West (the sea). Correspondingly, there is a peak in the concentration of major ions originating mostly from land (NO₃^-, SO₄^2-, Ca₂⁺, C₂O₄^2-, K⁺) towards the end of the land-sea breeze regime (9-11^th May), followed (10-15^th May) by an increase of the concentration of major ions originating mostly from the sea (Na⁺, Mg²⁺, Cl^-). The entire period (9-14^th) is characterized by a concurrent variation of total N, C, δ¹⁵N and δ¹³C. While the changes of δ¹⁵N are caused mainly by isotope fractionations, associated with the dissociation of NH₄Cl producing NH₃ and HCl, the changes of δ¹³C are caused mostly by a change of the source of total C, associated with carbonate (CO₃^2-) apportion.

We conclude that the concentrations and isotopic compositions of N/C in PM are useful tools only when coupled with other tools like the analysis of the meteorology and the concentration of major ions.