Molecular Characterization of the Soluble Fraction of Atmospheric Particulate Matter Obtained by Chemical Oxidation with Nitric Acid
Abstract
The availability of information regarding the composition of ambient atmospheric particulate matter and the transformations that these compounds may undergo upon atmospheric transit is surprisingly absent. Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry is used in this study to rapidly evaluate the composition of the water-soluble organic fraction of standard and collected atmospheric particulate matter samples, and the changes that these mixtures undergo upon oxidation with nitric acid. The standard samples include representative particulate matter collected over the course of a year in Washington, D.C. and particulate matter from the exhaust of a diesel-powered engine. The collected sample is from Norfolk, Virginia during a fire event. The ultra-high resolving power of the instrument allows for the assignment of molecular formulae for detected peaks. Visualization of spectral data for samples was achieved by plotting atomic ratios of 0/C and H/C on van Krevelen diagrams, and the classification of compound types using modified aromaticity index reveals substantial changes in the overall composition of the water-soluble organic fraction of model atmospheric particulate matter samples. The transformations are indicated by significant increases in the number and amount of nitrogen-containing species and condensed aromatic compounds present within the oxidized samples. Possible means, including functionalization of existing compounds, for increase solubilization of these species into the aqueous phase are suggested. The collected atmospheric particulate matter sample shows an unexpected lack of transformation in the overall molecular character upon oxidation, and may be attributed to insufficient sample size and concentration. Fourier transform ion cyclotron resonance mass spectrometry readily provides detailed information about the molecular composition of the water-soluble fraction of atmospheric particulate matter before and after nitric acid oxidation.