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.

The Nano Aerosol Mass Spectrometer (NAMS) was deployed to rural/coastal and urban sites to measure the composition of 20-25 nm diameter nanoparticles during new particle formation (NPF). NAMS provides a quantitative measure of the elemental composition of individual, size-selected nanoparticles. In both environments, particles analyzed during NPF were found to be enhanced in elements associated with inorganic species (nitrogen, sulfur) relative to that associated with organic species (carbon). A molecular apportionment algorithm was applied to the elemental data in order to place the elemental composition into a molecular context. These measurements show that sulfate constitutes a substantial fraction of total particle mass in both environments. The contribution of sulfuric acid to new particle growth was quantitatively determined and the gas-phase sulfuric acid concentration required to incorporate the measured sulfate fraction was calculated. The calculated values were compared to those calculated by a sulfuric acid proxy that considers solar radiation and SO(2) levels. The two values agree within experimental uncertainty. Sulfate accounts for 29-46% of the total mass growth of particles. Other species contributing to growth include ammonium, nitrate, and organics. For each location, the relative amounts of these species do not change significantly with growth rate. However, for the coastal location, sulfate contribution increases with increasing temperature whereas nitrate contribution decreases with increasing temperature.

The nano aerosol mass spectrometer (NAMS) was deployed at a coastal site in Lewes, Delaware, to measure the composition of 21 nm mass normalized (18 nm mobility) diameter nanoparticles during new particle formation (NPF) events. NAMS provides a quantitative measure of the atomic composition of individual nanoparticles. NAMS analysis of ambient particles showed only a small change in particle composition during NPF events in Lewes compared with off-event (before and/or after the event). The N mole fraction increased 15% on-event, whereas the C mole fraction decreased 25%, suggesting an enhanced inorganic component to the aerosol during NPF. The measured changes in atomic composition constrain the possible changes in molecular composition. To explore these constraints, an apportionment algorithm was applied to the atomic composition data. This algorithm partitions the atomic composition into sulfate, nitrate, and ammonium on the basis of the atomic abundance of S, N, and O and into organic matter on the basis of C and residual O after removing contributions to inorganic species. Particles were fully neutralized both on-and off-event. The nitrate to sulfate ratio during NPF ranged from 0.7 to 1.4, suggesting that ammonium nitrate is important to particle growth in this environment. Nonetheless, nanoparticles had a significant organic fraction, and upper limits for cationic amine content were determined. The relatively small changes in total particle composition on-event versus off-event suggest that observed changes in particle hygroscopicity and volatility during NPF at other locations may be linked to subtle changes in particle composition or to shifts in the character of the organic content.Received 11 February 2011; accepted 6 April 2011. The authors acknowledge Katherine M. Mullaugh for assistance during the field campaign, Elizabeth Frey of the Delaware Department of Natural Resources and Environmental Control (DNREC), and the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model and READY website (http://www.arl.noaa.gov/ready.php) used in this publication. Bryan R.

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.

The nano aerosol mass spectrometer (NAMS) irradiates individual, size selected nanoparticles with a high energy laser pulse to generate a mass spectrum consisting of multiply charged atomic ions. The elemental composition of the particle is determined from the ion signal intensities of each element, which requires deconvoluting isobaric ion signals at 4 m/z (O(4+) and C(3+)) and at 8 m/z (O(2+) and S(4+)). A method to deconvolute these ion signals using sucrose and ammonium sulfate as calibrants is presented. The approach is based on the assumption that the charge state distribution of a given element is independent of the chemical form of that element in the particle. Relative to previously reported methodology, the new approach permits accurate and precise determination of sulfur, which is crucial for interpretation of ambient nanoparticle data sets. With this approach, the differences between expected and measured elemental ratios of C, O, N, and S for a variety of test particles were generally much less than 10%, although a difference as high as 16% was observed.

High frequency spikes in ultrafine number concentration near a roadway intersection arise from motor vehicles that accelerate after a red light turns green. The present work describes a method to determine the contribution of motor vehicles to the total ambient ultrafine particle mass by correlating these number concentration spikes with fast changes in ultrafine particle chemical composition measured with the nano aerosol mass spectrometer, NAMS. Measurements were performed at an urban air quality monitoring site in Wilmington, Delaware during the summer and winter of 2009. Motor vehicles were found to contribute 48% of the ultrafine particle mass in the winter measurement period, but only 16% of the ultrafine particle mass in the summer period. Chemical composition profiles and contributions to the ultrafine particle mass of spark vs diesel vehicles were estimated by correlating still camera images, chemical composition and spike contribution at each time interval.. The spark and diesel contributions were roughly equal, but the uncertainty in the split was large. The distribution of emissions from individual vehicles was determined by correlating camera images with the spike contribution to particle number concentration at each time interval. A small percentage of motor vehicles were found to emit a disproportionally large concentration of ultrafine particles, and these high emitters included both spark ignition and diesel vehicles.

A wavelet-based algorithm was implemented to separate the high frequency portion of ambient nanoparticle measurements taken during the summer and winter of 2009 in Wilmington, Delaware. These measurements included both number concentration and size distributions recorded once every second by a condensation particle counter (CPC) and a fast mobility particle sizer (FMPS). The high frequency portion of the signal, consisting of a series of abrupt spikes in number concentration that varied in length from a few seconds to tens of seconds, accounted for 6-35% of the daily ambient number concentration with hourly contributions sometimes greater than 50%. When the data were weighted by particle volume, this portion of the signal contributed an average of 20% to the daily PM(0.1) concentration. Particle concentration spikes were preferentially observed from locations surrounding the measurement site where motor vehicles accelerate after a red traffic light turns green. As the distance or transit time from emission to sampling increased, the size distribution shifted to larger particle diameters.