Comparison of techniques and measurement methods for determination of the water-soluble carbon and nitrogen in atmospheric particulate matter

The water-soluble fraction of particulate matter (PM) is the most bio-accessible and potentially harmful component due to its ability to be absorbed through the respiratory tract. This study evaluates the content of water-soluble organic carbon (WSOC) and water-soluble total nitrogen (WSTN) in PM2.5 and PM10 filters collected in the Lecce area (Italy), focusing on monthly samples from different site typologies and daily samples from a single background site. WSOC was quantified using two methods on aqueous extracts with a TOC-L CPH analyser: (1) the total organic carbon (TOC) method, which determines WSOC as the difference between total carbon (TC) and inorganic carbon (IC); and (2) the non-purgeable organic carbon (NPOC) method, which removes inorganic carbon by acidification and air purging before TC measurement. The analytical parameters of the NPOC method were optimized using a Design of Experiment (DoE), while WSTN, equivalent to total soluble nitrogen in the extract, was measured simultaneously using the NPOC method in N-mode. Results showed a strong correlation and high consistency between the two WSOC methods, with overall combined averages of WSOCTOC =(3.3 ±0.5) μg/m3 and WSOCNPOC =(3.2 ±0.4) μg/m3. OC solubility was high and similar in both PM fractions, averaging 65 % in PM2.5 and 66 % in PM10 but demonstrated a clear seasonal variability, with higher WSOC/OC ratios in the cold season. Furthermore, the NPOC approach facilitates the simultaneous quantification of WSTN with a reduced sample volume, and the data confirmed the limited content of water- soluble organic nitrogen (WSON) in this dataset, thus enhancing analytical efficiency for routine monitoring.