The formation of secondary organic aerosol (SOA) by reaction of ozone with monoterpenes (beta-pinene, delta3-carene, limonene, and sabinene) was studied on a short time scale of 3-22 s with a flow tube reactor. Online chemical analysis was performed with the Photoionization Aerosol Mass Spectrometer (PIAMS) to obtain molecular composition and the Nanoaerosol Mass Spectrometer (NAMS) to obtain elemental composition. Molecular composition data showed that dimers and higher order oligomers are formed within seconds after the onset of reaction, indicating that there is no intrinsic kinetic barrier to oligomer formation. Because oligomer formation is fast, it is unlikely that a large number of steps are involved in their formation. Therefore, ion distributions in the PIAMS spectra were interpreted through reactions of intermediates postulated in previous studies with monomer end products or other intermediates. Based on ion signal intensities in the mass spectra, organic peroxides appear to comprise a greater fraction of the aerosol than secondary ozonides. This conclusion is supported by elemental composition data from NAMS that gave C:O ratios in the 2.2-2.7 range.

A flow-tube reactor was used to study the formation of particles from alpha-pinene ozonation. Particle phase products formed within the first 3-22 s of reaction were analyzed online using a scanning mobility particle sizer and two particle mass spectrometers. The first, a photoionization aerosol mass spectrometer (PIAMS), was used to determine the molecular composition of nascent particles between 30 and 50 nm in diameter. The second, a nano-aerosol mass spectrometer (NAMS), was used to determine the elemental composition of individual particles from 50 nm to below 10 nm in diameter. Molecular composition measurements with PIAMS confirm that both the stabilized Criegee intermediate and hydroperoxide channels of alpha-pinene ozonolysis are operative. However, these channels alone cannot explain the high oxygen content of the particles measured with NAMS. The carbon-to-oxygen mole ratios of suspected nucleating agents are in the range of 2.25-4.0, while the measured ratios are from 1.9 for 9 nm particles to 2.5 and 2.7 for 30 and 50 nm particles, respectively. The large oxygen content may arise by cocondensation of small oxygenated molecules such as water or multistep reactions with ozone, water, or other species that produce highly oxygenated macromolecules. In either case, the increasing ratio with increasing particle size suggests that the aerosol becomes less polar with time.

A nanoaerosol mass spectrometer (NAMS) is described for real-time characterization of individual airborne nanoparticles. The NAMS includes an aerodynamic inlet, quadrupole ion guide, quadrupole ion trap, and time-of-flight mass analyzer. Charged particles in the aerosol are drawn through the aerodynamic inlet, focused through the ion guide, and captured in the ion trap. Trapped particles are irradiated with a high-energy laser pulse to reach the "complete ionization limit" where each particle is thought to be completely disintegrated into atomic ions. In this limit, the relative signal intensities of the atomic ions give the atomic composition. The method is first demonstrated with sucrose particles produced with an electrospray generator. Under the conditions used, the particle detection efficiency (fraction of charged particles entering the inlet that are subsequently analyzed) reaches a maximum of 10(-4) at 9.5 nm in diameter and the size distribution of trapped particles has a geometric standard deviation of 1.1 based on a log-normal distribution. A method to deconvolute overlapping multiply charged ions (e.g. C3+ and O4+) is presented. When applied to sucrose spectra, the measured C/O atomic ratio is 1.1, which matches the expected ratio from the molecular formula. The spectra of singly charged bovine serum albumin (BSA) molecules are also presented, and the measured and expected C/N/O atomic ratios are within 15% of the each other. Also observed in the BSA spectra are signals from 13C and 32S which arise from 40 and approximately 34 atoms per molecule (particle), respectively. Potential applications of NAMS to atmospheric chemistry and biotechnology are briefly discussed.

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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.