In this work, we described and characterized a novel online instrument for oxidative potential (OP) measurement using the ferrous orange xylenol (FOX) assay. This assay, based on the Fenton reaction, allows a good sensitivity for H2O2 with a LOD of 0.2 μg H2O2/m3 and an acceptable CV of 12%. Laboratory tests indicated that the instrument is responding to O3 and tert-butyl hydroperoxide but not to NO2. Its 5-min sampling and analysis cycle is short enough to catch the fast decay of the reactive and unstable oxidants potentially presents in the ambient aerosol. These analytical specifications allowed us to continuously quantify the total OPFOX — gaseous + particulate — during more than 20 days at an urban monitoring station in Lausanne (Switzerland) for three different periods of the year. Random Forest Regression and Multiple Linear Regression were used and compared in the development of a time-independent predictive model for OPFOX. Random Forest Regression performed better than the Multiple Linear regression, particularly at high OPFOX values. These results strongly suggest that this online instrument responds to photochemically generated gaseous oxidants, particularly H2O2 and O3. The developed models also incorporated aerosol and meteorological variables — including ultrafine particle size, temperature, and solar radiation that may exhibit collinearity with these gaseous oxidants. Comparing online with offline (filter) samples showed that filter-based OPFOX measurements were substantially underestimated. Therefore, the online FOX measurement provides a more accurate estimate of reactive oxygen species and may complement existing indicators used in epidemiological studies of photochemical air pollution.
Sauvain et al. (Thu,) studied this question.