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Urban atmospheric iron speciation

Introduction

Particle bound transition metals are known to associate with generation of reactive oxygen, which may induce toxic effects and the oxidative damage in a cardiopulmonary system. Among those transition metals, Fe is the most significant contributor and a key player in photochemically driven redox reactions of both inorganic and organic contaminants in atmospheric aerosols. Fe-redox cycling may also be a key factor of ROS in macrophage cells exposed to extracts of PM. Fe is present in aerosol PM in two oxidation states Fe (II) and Fe (III). Transformation from Fe (III) to Fe (II) has been shown to be an important pathway of iron in atmospheric environments with hydroxyl radical being generated in the process. Further, Fe (II) species can be oxidized to Fe (III) by molecular oxygen resulting in the formation of the O2- free radical, which may play a significant role in the redox chemistry system.

Previous works on ocean and rain water have focused on photochemically mediated speciation of iron and also the analysis of extracts from PM using atomic absorption spectrometry. So far, there is still little investigation that focuses on iron speciation in atmospheric aerosols. We propose a hypothesis that transformation between Fe (II) and Fe (III) may occur in their aerosol phase in the atmosphere either due to the aqueous phase oxidation in hygroscopic particles, or due to surface reactions directly in aerosol phase. In order to confirm this hypothesis, oxidation states measurement of aerosol phase iron will be carried out. The conventional methods rely mainly on atomic absorption spectroscopy and high performance liquid chromatography- inductively hyphenated plasma mass spectrometry coupled with complex pre-concentration and separation techniques to achieve the analyzing goal. However, the high cost of instrument and low levels of iron species in PM are the drawbacks of these methods and therefore they become the main problem which is the limit of detection in such a trace level. In our work, we will develop an analytical technique combine liquid waveguide capillary cell (LWCC) with spectrophotometry and use ferrozine to achieve the analysis of soluble Fe (II) and Fe (III) in PM. In this method, LWCC can be used to enhance the sensitivity according to the Beer-lambert law that the absorbance of a sample increases with the extension of the optical path length. Fe (II) is determined directly using Ferrozine method to form a complex chelate in which it is detected by spectrometer. Fe (III) can be determined after reduced to Fe (II). ICP-MS in conjunction with ICP-OES will be the best choice for the total iron determination.


Figure 1 CO monitor and interface

Project contact: Ka Lok CHAN