Source apportionment of oxidative potential depends on the choice of the assay: insights into 5 protocols comparison and implications for mitigation measures

Abstract

The oxidative potential (OP) of particulate matter (PM) has recently been considered as a viable health-based metric of PM exposure. Several acellular assays have been developed to assess OP, but there is no clear consensus in the methodology or protocols used that allows inter-comparison between studies. This research investigates five different acellular OP assays and their sensitivities with the chemical composition of atmospheric particulate matter (PM₁₀) and its emission sources. We revisited and evaluated PM₁₀ samples collected over one year in the urban Alpine city of Grenoble, France. The OP of PM₁₀ was assessed by integrating assays commonly found in the literature, such as ascorbic acid (AA), dithiothreitol (DTT) and 2,7-dichlorofluorescein (DCFH) but also adding novel and less explored assays such as Ferric-Xylenol Orange (FOX), and a direct ROS-quantification through ˙OH. Detailed source apportionment of PM using positive matrix factorisation (PMF) previously performed was coupled with multiple linear regression (MLR) models to determine the OP contribution of PM₁₀ sources. The results highlight the importance of seasonality in the mass contributions of each source and its corresponding influence on OP. These seasonal differences helped to identify the specific reactivity for each studied OP assay. In winter, a good agreement was found between all the OP assays with anthropogenic sources. However, during warmer months, with a reduction in the share of anthropogenic emissions, a higher impact from biogenic and secondary organic-related aerosols has been found. Our results also show a dissimilar sensitivity of each OP to the PM₁₀ sources, likely associated with the chemical composition and chemical processes involved. Thus, our findings show the importance of combining various OP assays to capture different sensitivities to redox-active species to get a clearer picture of the intrinsic capacity of PM sources to cause damaging oxidative reactions in the lung. Providing the heterogeneity of sources obtained with the different OP assays for a given ambient PM exposure, the choice of a single or a combination of OP method(s) must be rationally evaluated as part of the assessment strategy. Such a choice would offer valuable source-related information as a powerful tool to better understand the nature and the intensity of air pollution and envisage the targeted sources for future mitigation policies.