M. Ross Pennington

Quantitative assessment of the sulfuric acid contribution to new particle growth.

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.

Ultrafine Particles Near a Roadway Intersection: Origin and Apportionment of Fast Changes in Concentration

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.

Elemental Composition of Nanoparticles with the Nano Aerosol Mass Spectrometer

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.

Apportionment of Motor Vehicle Emissions from Fast Changes in Number Concentration and Chemical Composition of Ultrafine Particles Near a Roadway Intersection

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.

Molecular transformations accompanying the aging of laboratory secondary organic aerosol.

The aging of fresh secondary organic aerosol (SOA), formed in a flow tube reactor by α-pinene ozonolysis, was studied by passing the fresh SOA into a second chamber for reaction with high levels of the hydroxyl radical. Two types of experiments were performed: (1) injection of a short plug of fresh SOA into the second chamber, where the particle mass and average O/C mole ratio were measured as a function of time after injection, and (2) injection of a continuous stream of fresh SOA into the second chamber, where particles were collected on a filter over a period of time for off line analysis by high performance mass spectrometry. These setups allowed the chemistry of SOA aging to be elucidated. The particle mass decreased and average O/C ratio increased with increasing aging time. Aged SOA showed an oligomer distribution shifted to lower molecular weight (fragmentation) and molecular formulas with higher O/C and lower H/C ratios (functionalization). Carbon oxidation states of individual molecules were higher for aged SOA, 0 to +2, than fresh SOA, -1 to 0. Tandem mass spectrometry of oligomers from fresh SOA showed small neutral losses associated with less oxidized functional groups such as aldehydes and ketones, while oligomers from aged SOA showed losses associated with more highly oxidized groups such as acids and peroxyacids. Product ion spectra of fresh SOA showed monomer building blocks with formulas corresponding to primary ozonolysis products such as pinic and pinonic acids, whereas aged SOA monomer building blocks corresponded to extremely oxidized products such as dimethyltricarballylic acid.

Molecular constraints on particle growth during new particle formation

Atmospheric new particle formation (NPF) produces large numbers of nanoparticles which can ultimately impact climate. A firm understanding of the identity and contribution of the inorganic and carbonaceous species to nanoparticle growth is required to assess the climatic importance of NPF. Here, we combine elemental and molecular nanoparticle composition measurements to better define the composition and contribution of carbonaceous matter to nanoparticle growth in a rural/coastal environment. We show that carbonaceous matter can account for more than half of the mass growth of nanoparticles and its composition is consistent with that expected for extremely low volatility organic compounds. An important novel finding is that the carbonaceous matter must contain a substantial amount of nitrogen, whose molecular identity is not fully understood. The results advance our quantitative understanding of the composition and contribution of carbonaceous matter to nanoparticle growth, which is essential to more accurately predict the climatic impacts of NPF.

Silicon is a frequent component of atmospheric nanoparticles

Nanoparticles are the largest fraction of aerosol loading by number. Knowledge of the chemical components present in nanoparticulate matter is needed to understand nanoparticle health and climatic impacts. In this work, we present field measurements using the Nano Aerosol Mass Spectrometer (NAMS), which provides quantitative elemental composition of nanoparticles around 20 nm diameter. NAMS measurements indicate that the element silicon (Si) is a frequent component of nanoparticles. Nanoparticulate Si is most abundant in locations heavily impacted by anthropogenic activities. Wind direction correlations suggest the sources of Si are diffuse, and diurnal trends suggest nanoparticulate Si may result from photochemical processing of gas phase Si-containing compounds, such as cyclic siloxanes. Atmospheric modeling of oxidized cyclic siloxanes is consistent with a diffuse photochemical source of aerosol Si. More broadly, these observations indicate a previously overlooked anthropogenic source of nanoaerosol mass. Further investigation is needed to fully resolve its atmospheric role.

Identification and quantification of particle growth channels during new particle formation

Atmospheric new particle formation (NPF) is a key source of ambient ultrafine particles that may contribute substantially to the global production of cloud condensation nuclei (CCN). While NPF is driven by atmospheric nucle- ation, its impact on CCN concentration depends strongly on atmospheric growth mechanisms since the growth rate must exceed the loss rate due to scavenging in order for the parti- cles to reach the CCN size range. In this work, chemical com- position measurements of 20 nm diameter particles during NPF in Hyytiala, Finland, in March-April 2011 permit iden- tification and quantitative assessment of important growth channels. In this work we show the following: (A) sulfuric acid, a key species associated with atmospheric nucleation, accounts for less than half of particle mass growth during this time period; (B) the sulfate content of a growing parti- cle during NPF is quantitatively explained by condensation of gas-phase sulfuric acid molecules (i.e., sulfuric acid up- take is collision-limited); (C) sulfuric acid condensation sub- stantially impacts the chemical composition of preexisting nanoparticles before new particles have grown to a size suffi- cient to be measured; (D) ammonium and sulfate concentra- tions are highly correlated, indicating that ammonia uptake is driven by sulfuric acid uptake; (E) sulfate neutralization by ammonium does not reach the predicted thermodynamic end point, suggesting that a barrier exists for ammonia up- take; (F) carbonaceous matter accounts for more than half of the particle mass growth, and its oxygen-to-carbon ratio ( 0.5) is characteristic of freshly formed secondary organic aerosol; and (G) differences in the overall growth rate from one formation event to another are caused by variations in the growth rates of all major chemical species, not just one individual species.