Application of synchrotron radiation for measurement of iron red-ox speciation in atmospherically processed aerosols

In this study, ambient atmospheric particulate matter samples were collected using a size-resolved impactor sampler from three urban sites. The purpose of this study is to gain a better understanding of transformations of aerosol-bound iron as it is processed in the atmosphere. Thus, the aerosol samples were artificially aged to represent long-term transport ( 10 to 40 days) or short-term transport ( 1 to 10 days) and were measured for iron at several time points. At each time point, iron was measured in each size fraction using three different techniques; 1) inductively coupled plasma-mass spectrometry (ICPMS) for total iron, 2) x-ray absorbance near edge structure (XANES) spectroscopy for the measurement of total Fe(II) and Fe(III), and 3) a wet-chemical method to measure soluble Fe( II) and Fe( III). Prior to aging, the XANES spectroscopy results show that a majority (> 60% for each size fraction) of the total iron in the PM is in the form of Fe( III). Fe( III) was shown to be a significant fraction of the soluble iron ( sometimes > 50%), but the relative significance of Fe( III) was found to vary depending on the site. Overall, the total soluble iron depended on the sampling site, but values ranged from less than 1% up to about 20% of the total iron. Over the course of the 40 day aging period, we found moderate changes in the relative Fe( II)/ Fe( III) content. A slight increase was noted in the coarse (> 2.5 mu m) fraction and a slight decrease in the 0.25 to 0.5 mu m fraction. The soluble fraction generally showed ( excepting one day) a decrease of soluble Fe( II) prior to 10 days of aging, followed by a relatively constant concentration. In the short-term transport condition, we found that the sub-micron fraction of soluble Fe( II) spikes at 1 to 3 days of aging, then decreases to near the initial value at around 6 to 10 days. Very little change in soluble Fe( II) was observed in the super-micron fraction.