Environmentally persistent free radicals (EPFRs) are a new type of pollutants with longer half-life ranging from several hours to months than conventional free radicals such as hydroxyl radicals (10-9 s). EPFRs lead to the formation of harmful reactive oxygen species (ROS) which could induce cardiovascular disease, lung cancer and DNA damage. Besides, EPFRs are easily formed in the thermal processes and can be adsorbed on the surface of particulate matter (PM) such as total suspended particle (TSP) and PM2.5, etc. Many researchers have carried out studies on the health effects and formation mechanisms of EPFRs on PM, but less efforts were made on illustrating their ambient concentrations and influencing factors, e.g., the compositions of mental oxides and minerals on PM as well as the ambient atmospheric pollutants and meteorological conditions.
In this study, nine TSP samples were collected in a typical urban site in Ningbo from May to July 2018, their surface morphologies and concentrations of EPFRs were determined by the scanning electron microscopy (SEM) and electron paramagnetic resonance (EPR) spectroscopy, respectively. To analyze the possible influencing factors of EPFRs concentrations, the X-ray diffraction (XRD) and inductively coupled plasma mass spectrometry (ICP-MS) were employed to reveal the surface elemental compositions, surface mineral compositions and bulk metal compositions of the TSP samples, respectively. In addition, during the sampling period, the conventional air pollutants (i.e., PM10, SO2, NO2, CO and O3) and meteorological conditions (i.e., wind speed, wind direction, ambient temperature, relative humidity (RH), atmospheric pressure and ultraviolet (UV) radiation) were recorded in a nearby automatic air quality monitoring station.
The results indicated that the TSP were mostly floccule-like aggregates, and some of them were particles with linear or round shapes. Besides, S, Si, Al, C, O, Na, Mg, K, Cl, Ca, Mn, and Fe were found existing in micro areas of the particle surface. In addition, seven out of nine TSP samples exhibited a single, unstructured organic peak with g-factor ranging from 2.00354 to 2.00456 and Δ Hp-p value of 3-5 Gauss, which is indicative of semiquinone-type or other oxygen-centered EPFRs. The concentration of the EPFRs was in the range of 2.08×1016 to 1.41×1017 spins/g in TSP. Among the seven TSP samples with EPFRs signals, only three showed a perfect match with those published in the literature. Therefore, these three samples (i.e., sample 6, 8 and 9) were chosen for the further analysis of potential factors influencing the concentration of EPFRs in the airborne PM.
Among the meteorological conditions and atmospheric pollutants, PM10 showed the highest positive correlation with the concentration of EPFRs (R2 = 0.99), followed by the wind direction (R2 = 0.90), ozone (R2 = 0.79), wind speed (R2 = = 0.78), SO2 (R2 = 0.72) and NO2 (R2 = 0.71). UV radiation and pressure also have the positive effects on the concentration of EPFRs, with R2 equal to 0.37 and 0.36, respectively. By contrast, RH and temperature showed negative correlations with the concentration of EPFRs, with R2 being 1.00 and 0.39, respectively. The main mineral compositions of TSP were CaCO3, SiO2 and CaSO4•2H2O. Among them, CaSO4•2H2O showed a strong negative correlation (R2 = 0.67) with the concentration of EPFRs, as it is reported that the sulphur may inhibit the formation of EPFRs. Furthermore, in terms of bulk metal composition, Ni showed the strongest positive correlation with the concentration of EPFRs (R2 = 0.99), followed by Pb (R2 = 0.29) and Cu (R2 = 0.14) while Sb exhibited the strongest negative correlation (R2 = 0.44) with the concentration of EPFRs. The correlations of the remaining metals with the concentration of EPFRs, i.e., Cr, Mn, Co, Cs and Ad, were all weak (R2 < 0.10).
It should be acknowledged that due to the limited size of samples used in the statistical analyses, the correlations revealed between the potential influencing factors and the concentration of EPFRs on PM lacks the statistical significance. Therefore, a follow up research that includes more valid samples are needed to confirm the findings of this study. In addition, more efforts are needed to investigate other environmental factors on the concentration and formation of EPFRs on PM, such as the airborne nitrates and chlorides as well as the potential organic precursors of EPFRs (e.g., PAHs and dioxins).
|Date of Award||10 Nov 2018|
- Univerisity of Nottingham
|Supervisor||Mengxia Xu (Supervisor), Tao Wu (Supervisor) & Cheng Heng Pang (Supervisor)|