Reply on RC1

Le résumé fourni par la source

Abstract. Reactive oxygen species (ROS) formation by atmospheric fine particulate matter (PM) is widely implicated in adverse health effects, yet the molecular determinants of particle-level ROS formation remain poorly constrained, largely due to the chemical complexity of organic aerosol (OA). Here, we link ROS formation to molecular-level OA composition by applying a maximum carbonyl ratio-Van Krevelen framework to ambient fine PM collected from four Chinese megacities, a German semi-urban site, a boreal forest, and laboratory-generated secondary organic aerosol (SOA) from multiple precursors. Using ultrahigh-resolution mass spectrometry combined with hydrogen peroxide (H2O2) and radical measurements, we show the intrinsic ROS (H2O2+radicals) formation potential of fine PM is governed by OA composition rather than particle mass. Volume-normalized ROS yields increase with fine PM mass concentration and dominate overall exposure in polluted urban environments. In contrast, mass-normalized ROS yields vary independently of particle loading and are strongly driven by the balance between relative fractions of oxidized organic compounds (RFOOC) and unsaturated organic compounds (RFUOC), i.e., RFOOC/RFUOC. Across urban sites, mass-normalized ROS and H2O2 yields exhibit strong correlations with RFOOC/RFUOC, establishing this metric as a robust predictor of ROS formation. Remote, biogenic-influenced aerosols exhibit higher RFOOC/RFUOC and higher ROS yields per unit mass, whereas urban aerosols exhibit the opposite behavior. Laboratory SOA further supports these composition-reactivity relationships, with OOC-rich biogenic SOA exhibiting the highest ROS yields. This study reveals molecular-level characteristics in governing aerosol ROS formation and introduces a framework for assessing health-relevant aerosol oxidative activity of chemically complex OA.

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