In urban environments, vehicle exhaust and nonexhaust emissions represent important sources of fine particulate matter with an aerodynamic diameter less than 2.5 μm (PM2.5), which plays a central role in adverse health effects and oxidative stress. We collected PM2.5 filter samples from two highway sites (Anaheim and Long Beach, CA) and an urban site (Irvine, CA) to quantify environmentally persistent free radicals (EPFRs) contained in PM2.5 and the generation of radical forms of reactive oxygen species (ROS) in water using electron paramagnetic resonance spectroscopy. The EPFR concentrations were 36 ± 14 pmol m–3 at highway sites, which were about two times higher than those at the urban site. EPFRs correlate positively well with CO, NOx, and elemental and organic carbon, indicating that EPFRs are emitted from vehicular exhaust. Good correlations of EPFRs and Fe and Cu may indicate that EPFRs are stabilized by Fe and Cu emitted from tire and brake wears. EPFRs are negatively correlated with ozone, suggesting that photochemistry does not play a large role in the formation of EPFRs and possibly also indicating that EPFRs are quenched by ozone. Highway PM2.5 are found to generate mainly OH and organic radicals in the aqueous phase. The generated ROS are correlated with PM2.5 mass concentrations and OH radicals show a good correlation with EPFRs, implying the role of EPFRs in aqueous OH radical generation. The PM2.5 oxidative potentials as quantified with the dithiothreitol (DTT) assay are correlated with ROS, OH, and organic radicals for PM2.5 collected in Anaheim, whereas little correlations are observed for Long Beach. These findings highlight the interplay of various PM redox-active chemical components and complex relationship between ROS formation and DTT activity.

Abstract. The photochemical box model CiTTyCAT is used to analyse the absence of oxygen mass-independent anomalies (O-MIF) in volcanic sulfates produced in the troposphere. An aqueous sulfur oxidation module is implemented in the model and coupled to an oxygen isotopic scheme describing the transfer of O-MIF during the oxidation of SO2 by OH in the gas-phase, and by H2O2, O3 and O2 catalysed by TMI in the liquid phase. Multiple model simulations are performed in order to explore the relative importance of the various oxidation pathways for a range of plausible conditions in volcanic plumes. Note that the chemical conditions prevailing in dense volcanic plumes are radically different from those prevailing in the surrounding background air. The first salient finding is that, according to model calculations, OH is expected to carry a very significant O-MIF in sulfur-rich volcanic plumes and, hence, that the volcanic sulfate produced in the gas phase would have a very significant positive isotopic enrichment. The second finding is that, although H2O2 is a major oxidant of SO2 throughout the troposphere, it is very rapidly consumed in sulfur-rich volcanic plumes. As a result, H2O2 is found to be a minor oxidant for volcanic SO2. According to the simulations, oxidation of SO2 by O3 is negligible because volcanic aqueous phases are too acidic. The model predictions of minor or negligible sulfur oxidation by H2O2 and O3, two oxidants carrying large O-MIF, are consistent with the absence of O-MIF seen in most isotopic measurements of volcanic tropospheric sulfate. The third finding is that oxidation by O2∕TMI in volcanic plumes could be very substantial and, in some cases, dominant, notably because the rates of SO2 oxidation by OH, H2O2 and O3 are vastly reduced in a volcanic plume compared to the background air. Only cases where sulfur oxidation by O2∕TMI is very dominant can explain the isotopic composition of volcanic tropospheric sulfate.

The photochemical box-model CiTTyCAT is used to analyse the absence of oxygen mass-independent anomalies (O-MIF) in volcanic sulphates produced in the troposphere. An aqueous sulphur oxidation module is implemented in the model and coupled to an oxygen isotopic scheme describing the transfer of O-MIF during the oxidation of SO 2 by OH in the gas-phase, and by H 2 O 2 , O 3 and O 2 catalysed by TMI in the liquid phase. Multiple model simulations are performed in order to explore the relative importance of the various oxidation pathways for a range of plausible conditions in volcanic 5 plumes. Note that the chemical conditions prevailing in dense volcanic plumes are radically different from those prevailing in the surrounding background air. The first salient finding is that, according to model calculations, OH is expected to carry a very significant O-MIF in sulphur-rich volcanic plumes and, hence, that the volcanic sulphate produced in the gas phase would have a very significant positive isotopic enrichment. The second finding is that, although H 2 O 2 is a major oxidant of SO 2 throughout the troposphere, it is very rapidly consumed in sulphur-rich volcanic plumes. As a result, H 2 O 2 is found to be a 10 minor oxidant for volcanic SO 2 . According to the simulations, oxidation of SO 2 by O 3 is negligible because volcanic aqueous phases are too acidic. The model predictions of minor or negligible sulphur oxidation by H 2 O 2 and O 3 , two oxidants carrying large O-MIF, are consistent with the absence of O-MIF seen in most isotopic measurements of volcanic tropospheric sulphate.The third finding is that oxidation by O 2 /TMI in volcanic plumes could be very substantial and, in some cases, dominant, notably because the rates of SO 2 oxidation by OH, H 2 O 2 , and O 3 are vastly reduced in a volcanic plume compared to the 15 background air. Only cases where sulphur oxidation by O 2 /TMI is very dominant can explain the isotopic composition of volcanic tropospheric sulphate.