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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 , SO 2 , O 3 , 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 NH + 4 , NO 2- 3 , 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 NH + 4 and Si + . [ABSTRACT FROM AUTHOR]