Most aerosol chemical characterization studies to date involve bulk particle analysis. The surface chemical and physical properties of aerosol particles have rarely been analyzed, despite the particles’ potential health impacts and interactions with gas in the atmosphere. Aerosol particles ranging from 0.056 to 10 μm in size collected using a 10-stage impactor sampler from a busy walkway in a downtown area of Hong Kong were analyzed using X-ray photoelectron spectroscopy (XPS), a technique providing both elemental and chemical state information about the particle surfaces. Six key elements were detected: nitrogen (N), sulfur (S), calcium (Ca), silicon (Si), oxygen (O), and carbon (C). Carbon was the dominant species on the surfaces of all particles regardless of their sizes. A higher carbon concentration was found on the surfaces of the 0.056–0.32 μm particles. The N, Si, Ca, and O concentrations were higher on the surface of the 3.2–10 μm particles than in the smaller size fractions. Sulfur was mainly found on the surface of the 0.32–1.8 μm particles. High-resolution scans of C, N, and S were obtained to provide chemical state information about these elements. Aromatic C-H and aliphatic C-H were found to be the major carbon chemical states. Fullerenic carbon was detected on the surfaces of the finest (0.056–0.32 μm) particles. Oxygen- and nitrogen-containing organics were found on the surfaces of the 0.32–1.8 μm particles. Sulfur was present in the form of sulfates as expected. Ammonium salts, amide, and nitrate were found to form especially on the surfaces of aerosol particles in the nucleation, accumulation, and coarse modes, respectively. Silicates and carbonates were only discovered on the surfaces of coarse-mode particles (3.2–10 μm). The results suggest that both the chemical elements and their chemical states were significantly dependent on the size of the aerosol particles.Copyright 2013 American Association for Aerosol Research