Surface Chemical Characterization of 2.5-μm Particulates (PM2.5) from Air Pollution in Salt Lake City Using TOF-SIMS, XPS, and FTIR

Particulate matter with a diameter of 2.5 μm collected in Salt Lake City (SLC PM2.5) was studied using TOF-SIMS (time-of-flight secondary-ion mass spectrometry), XPS (X-ray photoelectron spectroscopy), and FTIR (Fourier transform infrared spectroscopy). The high spatial resolution and high surface sensitivity of TOF-SIMS allow the surfaces of individual particulates to be analyzed. The high mass-resolution of TOF-SIMS provides good separation of signals from different chemical species at the same nominal mass, and the extremely high detection sensitivity of TOF-SIMS makes the detection of trace elements possible. Metallic elements such as Li, Na, Mg, Al, K, Ca, Cr, Mn, Fe, Cu, Zn, Cs, and Bi were detected by TOF-SIMS on the surface of SLC PM2.5. The uranium ion U+ together with its oxide ions UO+ and UO2+ were also found. Inorganic compounds detected include oxides, hydroxides, nitrates, sulfates, silicates, borates, chlorides, etc. Organic compounds detected include hydrocarbons, alcohols, aldehydes, ethers, carboxylic acids, amines, amides, nitriles, etc. A number of polycyclic aromatic hydrocarbons (PAH) and nitrated polycyclic aromatic hydrocarbons were detected by TOF-SIMS. High-resolution XPS C1s spectrum shows functional groups such as C−O, CO2, C−CO2, C−C, and C−H and aromatic π−π* shake-up transitions. High-resolution XPS O 1s spectrum indicates the coexistence of different oxygen compounds on the surface of PM2.5. FTIR results confirm the presence of various organic compounds in SLC PM2.5 detected by TOF-SIMS and XPS.