Time-resolved chemical composition of individual nanoparticles in urban air

Chemical composition measurements of individual ambient nanoparticles were performed with a nanoaerosol mass spectrometer (NAMS) in Wilmington, DE, in May 2006. The atomic composition of each particle was determined from the relative signal intensities of multiply charged atomic ions in the mass spectra. The characteristics of particles with a mass-normalized-diameter of 25 nm analyzed on May 9 and 10, 2006, were studied in detail. Most of these particles contained carbon, nitrogen, oxygen, and sulfur. Almost half of the particles contained silicon, although its contribution to the total atomic composition was usually less than 1%. Alkali and transition metals were observed in a few percent of the particles, also with a contribution to the total atomic composition that was usually less than 1%. A method was developed to infer the amounts of ammonium sulfate, ammonium nitrate, and carbonaceous matter in single particles from the measured atomic compositions. The procedure also permitted estimation of the oxygen to carbon (O:C) atomic ratio of the carbonaceous matter. Two distinct types of particles were found: those having an O:C ratio less than 0.01 and those having a ratio 0.5 or greater. Particles in the low O:C ratio group are consistent with a hydrocarbon composition. Their prevalence during shortterm (1-min) spikes in concentration are consistent with nanoparticle emissions from individual vehicles. Ammonium sulfate was also found in many of these particles. Particles in the high O:C group are consistent with secondary organic aerosol. Most of these particles also contained ammonium sulfate and ammonium nitrate. A steady increase of these particles during the daytime suggested that their formation was photochemically driven.