Performance evaluation of an online monitor based on X-ray fluorescence for detecting elemental concentrations in ambient particulate matter

Abstract. Knowledge of the chemical composition of particulate matter (PM) is essential for understanding its source distribution, identifying potential health impacts of toxic elements and to develop efficient air pollution abatement strategies. Traditional methods for analysing PM composition, such as collection on filter substrates and subsequent offline analysis with e.g., inductively coupled plasma mass spectrometry (ICP-MS), are time-consuming and prone to measurement errors due to multiple preparation steps. Emerging near-real time techniques based on non-destructive Energy Dispersive X-ray Fluorescence (EDXRF) offer advantages for continuous monitoring and source apportionment. This study characterises the Horiba PX-375 EDXRF monitor by applying a straightforward performance evaluation including (a) limit of detection (LoD), (b) identification and quantification of uncertainty sources, and (c) investigating and comparing measurement results from three contrasting sites in Luxembourg (urban, semi-urban, rural). We used multi-element reference materials (ME-RMs) from UC Davis for calibration and performed measurements during spring and summer 2023. The LoDs for toxic elements like Ni, Cu, Zn, and Pb were below 3 ng m^-3 at one-hour time resolution. Higher LoDs were observed for lighter elements (e.g., Al, Si, S, K, Ca). Expanded uncertainties ranged between 5 and 25 % for elemental concentrations above 20 ng m^-3 and were maximal for concentrations below 10 ng m^-3, reaching 60–85 %. Elemental analysis revealed S and mineral elements (Fe, Si, Ca, Al) as dominant contributors to PM10. Toxic elements (As, Ni, Pb) were often below the LoD, suggesting minimal exposure risk in the sampled areas. Our results explained on average 51–74 % of the gravimetric PM10 mass at the three sites. The study highlights the suitability and importance of the continuous PX-375 particle monitor for future air quality monitoring and source apportionment studies, particularly under changing emission scenarios and air pollution abatement strategies.