Urban PM_2.5 Surface Chemistry and Interactions with Bronchoalveolar Lavage Fluid

Abstract

This study investigated the surface chemistry of urban fine particles (PM_2.5), and quantified the adsorbed and desorbed species after exposure to bronchoalveolar lavage fluid (BALF). Urban background and roadside PM_2.5 samples of different mass concentration and total weight were collected in triplicate in the South Bronx region of New York City. Simultaneously, the concentrations of other atmospheric pollutants (CO, NO_x, SO₂, O₃, elemental carbon) were measured, and weather conditions were recorded. The collected PM_2.5 samples underwent one of three treatments: no treatment, treatment in vitro with BALF, or treatment in a saline solution (control). The surfaces of untreated, saline-treated, and BALF-treated PM_2.5 samples were analyzed using x-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). These results were then compared with ambient air pollutant concentrations, weather variables, selected BALF characteristics, and results from a previous London study conducted using identical preparation methods by XPS analysis only. Both XPS and ToF-SIMS detected PM_2.5 surface species and observed changes in surface concentrations after treatment. XPS analysis showed the surface of untreated urban PM_2.5 consisted of 79 to 87% carbon and 10 to 16% oxygen with smaller contributions of N, S, Si, and P in the samples from both background and roadside locations. A wider variety of other inorganic and organic species (including metals, aliphatic and aromatic hydrocarbons, and nitrogen-containing molecules) was detected with ToF-SIMS. Surface characteristics of particles from the roadside and background sites were very similar, except for higher (p <. 05) nitrate concentrations at the roadside, which were attributable to higher roadside NO_x concentrations. Comparable species and quantities were identified in a previous study of London PM_2.5, where PM_2.5 surface chemistry differed considerably depending on the source, particularly in surface concentrations of oxygen and trace species. After treatment with BALF the N-C signal detected by XPS analysis increased in the average by 372 ± 203%, indicating significant surface adsorption of protein or other N-containing biomolecules. Lower (nonsignificant) N-C signals were observed for smoker BALF, compared to nonsmoker BALF. ToF-SIMS data confirmed protein adsorption after BALF treatment—smoker BALF resulted in lower levels of adsorbed proteins compared to nonsmoker BALF. ToF-SIMS also indicated an adsorption of phospholipid on the treated PM_2.5 surfaces. The primary phospholipid in BALF is dipalmitoylphospatidylcholine (DPPC), although positive identification was not possible due to low concentrations at the PM_2.5 surface. Oxygen content of PM_2.5 surfaces was the most significant determinant of both N-C and phospholipid adsorption. The XPS signal of the soluble species NH₄⁺, NO₃^{2 −}, Si, and S decreased in both saline- and BALF-treated samples, showing that these species may be bioavailable in the lung. Similarly, ToF-SIMS analysis suggests the bioavailability of Na⁺ and Al⁺ as well as NH₄⁺ and Si⁺.

The authors gratefully acknowledge the support of the U.S. EPA PM Center Research Program.