Intercomparison of online and offline XRF spectrometers for determining the PM10 elemental composition of ambient aerosol

Measuring the elemental composition of atmospheric particulate matter (PM) can provide useful information on the adverse effects of PM and facilitate the identification of emission sources. Carrying out these measurements at a high temporal resolution (1 h or less) allows describing the fast processes to which aerosol particles are subjected in the atmosphere, leading to a better characterization of the emissions. Energy dispersive X-ray fluorescence (ED-XRF) spectrometry is one of the most widespread techniques used to determine the elemental composition of PM. In recent years, new systems known as online XRF spectrometers have been developed to provide real-time measurements of the PM elemental concentration at a high temporal resolution. Among these advanced instruments, the Xact® 625i Ambient Metals Monitor by Cooper Environmental (USA) performs in situ automated measurements with a user-selected temporal resolution ranging from 15 to 240 min. In this study, an Xact® 625i monitor equipped with a PM10 inlet was deployed for nearly 6 months (July–December 2023) in Milan (Po Valley, Italy) at a monitoring station of the Lombardy Regional Agency for Environmental Protection (ARPA Lombardia). The instrument was configured to quantify 36 elements, ranging from Al to Bi, with 1 h temporal resolution in the PM10 fraction. The objective of the study was to verify the correct functioning of the instrument and to evaluate the quality and robustness of the data produced. Xact® 625i data were aggregated to 24 h daily means and then compared to 24 h PM10 filter data retrieved by ARPA Lombardia in the same station and analyzed offline for the elemental concentration with a benchtop ED-XRF spectrometer. The intercomparison focused on 16 elements (Al, Si, S, Cl, K, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, Br, Sr, and Pb) whose concentrations were consistently above their minimum detection limits (MDLs) for both online and offline techniques. The results of the intercomparison were satisfying, showing that the Xact® 625i elemental concentrations were highly correlated to the offline ED-XRF analyses (R2 ranging from 0.67 to 0.99 and slopes ranging from 0.79 to 1.3, with only a few elements showing slopes up to 1.70).