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 developing efficient air pollution abatement strategies. Traditional methods for analysing PM composition, such as collection on filter substrates and subsequent offline analysis with, for example, 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 characterizes 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) investigation and comparison of 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 1 h 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. Although the PM mass contribution of toxic trace elements (e.g. Zn, Cu, As) was generally very low, they were found to be enriched at the urban site Belvaux and the semi-urban site Remich compared to the more rural site Vianden. 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.