Humic-Like Substances in Air Pollution Particulates Correlate with Concentrations of Transition Metals and Oxidant Generation

Abstract

We tested the hypotheses that (1) an incomplete oxidation of carbon-based fossil fuels during their combustion produces humic-like substances (HIS), which can be present in air pollution particulates and confer a capacity to complex metals; (2) air pollution particulates collected on PM10 filters can be associated with concentrations of first-row transition metals; (3) particulates can catalyze the production of free radicals by cycling these transition metals through two stable valence states; and (4) concentrations of transition metals and oxidant generation by air pollution particulates increase with the content of HLS associated with these particles. HLS were isolated by alkali extraction. The content of these substances in combustion products of coal, diesel, oil, and wood was 3.1 ± 0.8%, 4.7 ± 1.0%, 1.0 ± 0.1%, and 8.2 ± 0.6%, respectively. Similarly, filters with sequestered air pollution particulates contained HLS ranging from 0.0 to 7.1%. Elemental analysis of these materials isolated from both products of fuel combustion and sequestered particulate disclosed values of C, H, N, and O consistent with an HLS. There were correlations between HLS content and ionizable concentrations of metals, quantified using inductively coupled plasma emission spectroscopy, associated with particulates sequestered on filters. Similarly, HLS content correlated with the absorbance of oxidized products of deoxyribose, demonstrating an affiliation between these substances and free radical generation by sequestered particulate. We conclude that HLS, a potential organic metal chelator, can be isolated from air pollution particulates. Concentrations of acid-soluble transition metals and in vitro oxidant generation correlated with the content of these substances collected on filters.