Experimental determination of reactive oxygen species in Taipei aerosols
Introduction
Oxygen-centered free radicals and their metabolites, collectively described as reactive oxygen species (ROS), originate from various sources and have different lifetimes in the atmosphere (Pryor, 1992). Particles in diesel emissions can produce, in vivo without any biological activating systems, a significant amount of ROS such as superoxide anion and hydroxyl radicals (Sagai, Saito, Ichinose, Kodama, & Mori, 1993). Photochemical reactions in polluted air also produce ROS, such as H2O2, hydroxyl radical, and other reactive chemical species (Seinfeld & Pandis, 1998; Sakugawa, Kaplan, Tsai, & Cohen, 1990). In the atmosphere, ROS appears in both gas phase and particulate phase. Several studies on ROS in the gas phase and in rain and cloud droplets have been reported (Olszyna et al., 1988; Sakugawa & Kaplan, 1990; Hellpointner & Gäb, 1989), but concentration data on ROS in the particulate phase are limited.
Particles in diesel emissions and those generated by photochemical reactions are generally smaller than a few tenths of a micrometer in diameter. Particles in this size range have a relatively high rate, about 20–30%, of deposition in lower lung and, as a result, toxicants in these particles can be efficiently transferred to the lungs. On the other hand, most of ROS in gas phase have high solubility and molecular diffusivity, and therefore tend to be absorbed in upper regions of the respiratory tract (Friedlander & Yeh, 1998). ROS is also produced naturally in biological systems to defend against foreign organisms and other environmental challenges. Excessive ROS, either from exogenous or endogenous sources, can induce cell injury that may trigger a cascade of free radical reactions promoting the disease process. Consequently, a central role has been implicated for ROS in the pathogenesis of many pulmonary diseases (Kehrer, 1993).
Freshness of particles is an important factor that affects the concentration of ROS in particles. It has been well documented that freshly fractured silica dust has a higher concentration of free radicals, which decreases with time by first-order kinetics and exhibits a half-life of (Vallyathan, Shi, Dalal, Irr, & Castranova, 1988; Shi & Dalal, 1988). Compared to aged silica dust, freshly ground silica dust shows a greater biological effect on respiratory systems (Vallyathan et al., 1995). Freshly generated welding fumes also have higher concentrations of ROS and induce greater lung inflammation than aged fumes (Antonini et al., 1998). The concentration of ROS in fumes decreases with time exponentially with a half-life of 10 days.
Rapid quantitation of ROS can be made with a fluorogenic probe. Dichlorofluorescin (DCFH2), a non-fluorescent reagent which becomes fluorescent dichlorofluorescein (DCF) when oxidized in the presence of ROS, has a very high sensitivity that is capable of detecting picomole levels of hydrogen peroxides (Cathcart, Schwiers, & Ames, 1983). This fluorogenic probe has been widely applied in measurements of oxidative activity in biological systems, such as intracellular ROS formation (LeBel, Ischiropoulos, & Bondy, 1990; Zhu, Bannenberg, Moldéus, & Shertzer, 1994; Kuo, Jee, Chou, & Ueng, 1998). Its application in the quantitation of ROS in aerosol particles has been made recently (Antonini et al., 1998).
The objective of this study was to determine the concentration of reactive oxygen species in various size fractions of the Taipei aerosols. The factors affecting the concentration of ROS in particles were also investigated.
Section snippets
Air sampling
A Micro-Orifice Uniform Deposit Impactor® (MOUDITM, MSP, Minneapolis, USA) was used to collect particles on Nuclepore® polycarbonate membrane filters (Whatman, pore size, Clifton, USA) and Teflon membrane filters (Gelman Sciences, Teflo, pore size, Ann Arbor, USA). MOUDITM is an eight-stage cascade impactor with a sampling flow rate of . Because this study was aimed at sampling of coarse, fine, very fine and ultrafine particles, only five stages with cut sizes of 0.18, 1, 3.2, 10
Assay of H2O2
Fig. 1 shows that the fluorescence intensity and the H2O2 concentration had a linear relationship with a good correlation . Least-squares analysis for experimental data gave the following equation for the H2O2 concentration:The equation can be used to convert the fluorescence intensity measurement for a particle suspension to an equivalent H2O2 concentration, which is used as an indicator for the reactivity of the ROS in a particle sample
Conclusions
From the measurements of ROS in particles collected at various sites, the following conclusions can be made:
1. For aerosol particles collected at the sidewalk, especially for very fine and ultrafine particles, the intensity of photochemical reactions was a major factor affecting the ROS concentrations.
2. For an identical mass concentration of particles, the pattern that smaller particles have higher ROS concentrations was observed in particles at the sidewalk, but not in the underpass samples.
Acknowledgements
This study was partially supported by the Taiwan National Science Council Grant No. NSC 89-2320-B-002-110. H.F. Hung was supported by a stipend for graduate study awarded by the same grant of Taiwan National Science Council during part of her Ph.D. study. The authors also wish to thank Professor James H. Vincent for his comments on the manuscript.
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