Online chemical characterisation of atmospheric particulate matter (PM) is a topic of interest for Air Quality and atmospheric research. In the recent years there are networks for atmospheric research implementing a combination of ACSM / XRF online / Aethalometers for a complete online characterisation of PM. Instruments for continuous analysis of metals in PM are based on EDXRF measurement. Nowadays, the Xact625i is the most widely used. However, these measurements have limitations mainly related to the measurement technique and the low concentration of PM sampled during short periods.

This paper evaluates the performance of the Xact625i, measuring PM10 with hourly resolution, by comparing with the offline analysis of PM collected in filters during 24h and analyzed offline by EDXRF. Novelty of the manuscript is the comparison with offline EXRF or a long time period (6 months). Results obtained are quite good for most selected elements. Three groups of elements have been identified base on different regression coefficients and slopes. Reasons of these differences are not properly discussed.

Overall, results obtained are of interest and merit publication. However, a major discussion about limitations of the technique and comparison with previous studies is needed.

Line 26: please, indicate that the Xact was equipped with a PM10 size cut inlet

Line 60. You should also comment about limitations of XRF: detection limits of some tracers of interest can be too high compared with PIXE or ICP-MS analyses. Also. Analysis depends on the matrix; so, it can vary depending on the composition of the aerosol.

Section 2.4. What was the minimum number of hours of valid data considered to calculate daily averages for the intercomparison?

Figure 1. Considering all available measurements or only the simultaneous ones?

Is there any difference in the correlations considering the three periods separately?

The discussion about comparison with previous studies should be improved. Are all the differences attributable to the calibration? Can be related to the local PM composition? What is the explanation for the different elements selected at each study? The MDL / ambient concentrations?

Different sizes may affect the comparison for some elements given the matrix effect. Park et al., used a forerunner version equipped with a PM2.5 inlet. Which inlet size was used by Tremper et al., 2018? And by Furger et al., 2017?

Conclusions: you should add some statements about limitations of the technique. For example, limitations for the analysis of some key tracers such as V, Ba, Sn, As…

Line 370: You should add that this Al measurement limitation is related to the instrument.

Please find the attached review comments.