Ion formation mechanism of cortisone molecules and clusters in charged nanodroplets.

Mass spectrometry measurements coupled with classical molecular dynamics (MD) simulations have been conducted in recent years to understand the final stage of ion formation in electrospray ionization (ESI). Here, to characterize the ion formation mechanism in the recently developed droplet-assisted ionization (DAI) source, MD simulations with various conditions (solute number, temperature, ions, composition) were performed to help explain DAI-based measurements. The specific binding ability of cortisone with preformed ions (ions of sodium, cesium and iodide) in evaporating nanodroplets makes the ion formation process characteristic of both the ion evaporation and charge residue models (IEM and CRM, respectively). Most preformed ions are ejected with dozens of solvent molecules to form gas-phase ions by IEM, while clusters of one or more cortisone molecules with one or more preformed ions remain in the evaporating droplet to form gas-phase ions by CRM. As the ratio of cortisone molecules to preformed ions increases, the number of preformed ions held in the droplet without ejection by the IEM increases. In other words, increasing the molecular solute to preformed ion ratio in the droplet increases the fraction of gas-phase ions formed by CRM relative to IEM. The increase in CRM relative to IEM is accompanied by an increase in the calculated activation energy barrier, which can explain the activation energy measurements by DAI, where droplets without preformed ions exhibit higher activation energies for gas-phase ion formation than droplets containing large numbers of preformed ions.

Online Characterization of Organic Aerosol by Condensational Growth into Aqueous Droplets Coupled with Droplet-Assisted Ionization.

Online analysis of ultrafine (<100 nm diameter) particles was performed by sending the aerosol through a condensation growth chamber (CGC) to create micrometer-size aqueous droplets that were subsequently analyzed by mass spectrometry with droplet-assisted ionization (DAI). Three experiments are reported which illustrate key performance characteristics of the method and give insight into the ion formation process: size-selected cortisone particles, size-selected secondary organic aerosol (SOA) particles, and freshly nucleated SOA under atmospherically relevant conditions. In each case, SOA was produced by α-pinene ozonolysis. For size-selected cortisone particles between 30 and 90 nm diameter and SOA particles between 30 and 70 nm, the ion signal intensity was found to be approximately independent of particle size. This observation is attributed to the formation of aqueous droplets in the CGC whose size distribution is independent of the original particle size. A consequence of this behavior is that the sensitivity of molecular detection increases as the particle size decreases, and the method is particularly well suited for new particle formation studies under atmospherically relevant conditions. This aspect of the CGC-DAI method was illustrated by the online analysis of freshly nucleated SOA samples with median diameters, number concentrations, and mass concentrations on the order of 25 nm, 104 cm-3, 0.2 µg m-3, respectively. Mass spectra of freshly nucleated SOA could be explained by condensation of highly oxidized molecules (HOMs) that subsequently reacted in the particle phase. Size-selected SOA showed increasing oligomerization with increasing particle size, which is consistent with established particle growth mechanisms.

Ion formation in droplet-assisted ionization.

RATIONALE: In droplet-assisted ionization (DAI), intact molecular ions are generated from molecules in aerosol droplets by passing the droplets through a temperature-controlled capillary inlet. Ion formation is explored through the effects of analyte mass flow, droplet solvent composition, and capillary temperature on ion signal intensity. METHODS: A Waters SYNAPT G2-S is adapted for DAI by reconfiguring the inlet with a temperature-controlled capillary. Droplets are generated by atomization of a solution containing analyte and then sampled through the inlet. If desired, solvent can be removed from the droplets prior to analysis by sending the aerosol through a series of diffusion dryers. Size distributions of the dried aerosols allow the mass flow of analyte into the inlet to be determined. RESULTS: Analyte signal intensities are orders of magnitude higher from droplets containing a protic solvent (water) than an aprotic solvent (acetonitrile). The highest signal intensities for DAI are obtained with inlet temperatures above 500°C, though the optimum temperature is analyte dependent. At elevated temperatures, droplets are thought to undergo rapid solvent evaporation and bursting to produce ions. The lowest signal intensities are generally obtained in the 100-350°C range, where slow solvent evaporation is thought to inhibit ion formation. As the temperature decreases from 100°C down to 25°C, the signal intensity increases significantly. When 3-nitrobenzonitrile, a common matrix for solid-state matrix-assisted ionization (MAI), is added to droplets consisting of 50/50 v/v water and acetonitrile, the matrix enhances ion formation to produce a signal intensity comparable to DAI in 100% water. CONCLUSIONS: The results are consistent with other inlet ionization techniques, suggesting that similar ion formation mechanisms are operative. Optimized ion yields (the combined effects of ionization probability and ion transmission) for DAI are currently in the 10-5 to 10-6 range, which is sufficient for many aerosol applications.

Ion Formation from Rapidly Heated Aqueous Droplets by Droplet-Assisted Ionization.

When aqueous droplets travel through a temperature-controlled capillary from atmospheric pressure into a vacuum, they undergo aerodynamic and/or thermal breakup to give charged progeny droplets that subsequently produce gas-phase molecular ions from solutes that were in the original droplets. This phenomenon is the basis of droplet-assisted ionization, a method that was recently developed for online characterization of aerosols by mass spectrometry. The conditions allowing initial droplets to break up into progeny droplets were studied by computational fluid dynamics (CFD) with a droplet evaporation model. The CFD results were then used to interpret experimental measurements of ion current vs capillary wall temperature. For capillary wall temperatures below about 150 °C, the abilities of droplets to undergo either aerodynamic or thermal breakup are strongly temperature dependent. Above this temperature, the mode of initial droplet breakup becomes temperature independent, and the temperature dependence of the ion signal intensity can be explained in relation to ion formation from charged progeny droplets. Activation energies for ion formation fall into two main categories: ∼41 kJ mol-1 for droplets containing predominantly nonionic solutes, which matches the enthalpy of vaporization for water and suggests a charge residue process for ion formation, and ∼24 kJ mol-1 for droplets containing salts, which suggests an ion evaporation process where the ion is ejected from the droplet surface within a cluster of solvent molecules.

Mechanisms of Atmospherically Relevant Cluster Growth.

Atmospheric aerosols impact global climate either directly by scattering solar radiation or indirectly by serving as cloud condensation nuclei, which influence cloud albedo and precipitation patterns. Our scientific understanding of these impacts is poor relative to that of, for instance, greenhouse gases, in part because it is difficult to predict particle number concentrations. One important pathway by which particles are added to the atmosphere is new particle formation, where gas phase precursors form molecular clusters that subsequently grow to the climatically relevant size range (50-100 nm diameter). It is predicted that up to 50% of atmospheric particles arise from this process, but the key initial chemical processes are poorly resolved. In general, a combination of inorganic and organic molecules are thought to contribute to new particle formation, but the chemical composition of molecular clusters and pathways by which they grow to larger sizes is unclear. Cluster growth is a key component of new particle formation, as it governs whether molecular clusters will become climatically relevant. This Account discusses our recent work to understand the mechanisms underlying new particle growth. Atmospherically relevant molecular clusters containing the likely key contributors to new particle formation (sulfuric acid, ammonia, amines, and water) were investigated experimentally by Fourier transform mass spectrometry as well as computationally by density functional theory. Our laboratory experiments investigated the molecular composition of charged clusters, the molecular pathways by which these clusters may grow, and the kinetics of base incorporation into them. Computational chemistry allowed confirmation and rationalization of the experimental results for charged clusters and extension of these principles to uncharged and hydrated clusters that are difficult to study by mass spectrometry. This combination of approaches enabled us to establish a framework for cluster growth involving sulfuric acid, ammonia, amines, and water. Charged or uncharged, cluster growth occurs primarily through an ammonium (or aminium) bisulfate coordinate. In these clusters, proton transfer is maximized between acids and bases to produce cations (ammonium, aminium) and anions (bisulfate), whereas additional molecules (water and unneutralized sulfuric acid) remain un-ionized. Experimental measurements suggest the growth of positively charged clusters occurs by successive acidification and neutralization steps. The acidification step is nearly barrierless, whereas the neutralization step exhibits a significant activation barrier in the case of ammonia. Bases are also incorporated into these clusters by displacement of one base for another. Base displacement is barrierless on the cluster surface but not within the cluster core. The favorability of amines relative to ammonia in charged clusters is governed by the trade-off between gas phase basicity and binding energetics. Computational studies indicate that water has a relatively small effect on cluster energetics. In short, amines are effective at assisting the formation and initial growth of clusters but become less important as cluster size increases, especially when hydration is considered. More generally, this work shows how experiment and computation can provide important, complementary information to address problems of environmental interest.

Droplet Assisted Inlet Ionization for Online Analysis of Airborne Nanoparticles.

Airborne nanoparticles play a key role in climate effects as well as impacting human health. Their small mass and complex chemical composition represent significant challenges for analysis. This work introduces a new ionization method, droplet assisted inlet ionization (DAII), where aqueous droplets are produced from airborne nanoparticles. When these droplets enter the mass spectrometer through a heated inlet, rapid vaporization leads to the formation of molecular ions. The method is demonstrated with test aerosols consisting of polypropylene glycol (PPG), angiotensin II, bovine serum albumin, and the "thermometer" compound p-methoxybenzylpyridinium chloride. High-quality spectra were obtained from PPG particles down to 13 nm in diameter and sampled masses in the low pictogram range. These correspond to aerosol number and mass concentrations smaller than 1000 particles/cm3 and 100 ng/m3, respectively, and a time resolution on the order of seconds. Fragmentation of the thermometer ion using DAII was inlet temperature dependent and similar in magnitude to that observed with a conventional ESI source on the same instrument. DAII should be applicable to other types of aerosols including workplace aerosols and those produced for drug delivery by inhalation.

Aerosol Formation from OH Oxidation of the Volatile Cyclic Methyl Siloxane (cVMS) Decamethylcyclopentasiloxane.

Aerosol formation from OH oxidation of decamethylcyclopentasiloxane (D5, C10H30O5Si5), a cyclic volatile methyl siloxane (cVMS) found in consumer products, was studied in a flow-through photo-oxidation chamber with and without the presence of ammonium sulfate seed aerosol. For the unseeded experiments, chemical characterization with high-performance mass spectrometry showed that the molecular composition changed substantially with aerosol mass loading in the 1-12 µg/m3 range. Monomers (5 Si atoms/molecule) and dimers (10 Si atoms/molecule) dominated the mass spectra of aerosols at higher mass loadings, while ring-opened species (neither 5 nor 10 Si atoms/molecule) dominated the mass spectra of aerosols at lower mass loadings. Molecular signal intensity dependencies upon the aerosol volume/surface area ratio suggest that non-volatile ring-opened species are formed in the gas phase and assist particle formation through condensation, while dimers are formed by accretion reactions within the particle phase as the particles grow. These conclusions are supported by experiments in the presence of seed aerosol with a similar siloxane aerosol mass loading but higher volume/surface area ratio, where ring-opened species are much less prevalent than monomers or dimers and the aerosol yield is higher. Because of the importance of accretion chemistry, the aerosol yield from D5 oxidation is likely to be strongly dependent upon the particle size and morphology.

Aqueous Reaction of Dicarbonyls with Ammonia as a Potential Source of Organic Nitrogen in Airborne Nanoparticles.

Nitrogen-containing organic species such as imines and imidazoles can be formed by aqueous reactions of carbonyl-containing compounds in the presence of ammonia. In the work described here, these reactions are studied in airborne aqueous nanodroplets containing ammonium sulfate and glyoxal, methylglyoxal, or glycolaldehyde using a combination of online and offline mass spectrometry. N/C ratios attributed to the organic fraction of the particles (N/Corg) produced from glyoxal and methylglyoxal were quantified across a wide relative humidity (RH) range. As the RH was lowered, glyoxal was found to increase N/Corg, attributed to "salting-in" with increasing solute concentration, while methylglyoxal led to a decrease in N/Corg, attributed to "salting-out". Glycolaldehyde was found to evaporate from the droplets rather than react in the aqueous phase and did not form particulate-phase organic matter from aerosol drying under any of the conditions studied. The results are discussed in the context of ambient nanoparticle composition measurements and suggest that aqueous chemistry may significantly impact nanoparticle composition and growth during new particle formation in locations where emissions of water-soluble dicarbonyls are high, such as the eastern United States.

Characterization of Highly Oxidized Molecules in Fresh and Aged Biogenic Secondary Organic Aerosol.

In this work, highly oxidized multifunctional molecules (HOMs) in fresh and aged secondary organic aerosol (SOA) derived from biogenic precursors are characterized with high-resolution mass spectrometry. Fresh SOA was generated by mixing ozone with a biogenic precursor (ß-pinene, limonene, α-pinene) in a flow tube reactor. Aging was performed by passing the fresh SOA through a photochemical reactor where it reacted with hydroxyl radicals. Although these aerosols were as a whole not highly oxidized, molecular analysis identified a significant number of HOMs embedded within it. HOMs in fresh SOA consisted mostly of monomers and dimers, which is consistent with condensation of extremely low-volatility organic compounds (ELVOCs) that have been detected in the gas phase in previous studies and linked to SOA particle formation. Aging caused an increase in the average number of carbon atoms per molecule of the HOMs, which is consistent with particle phase oxidation of (less oxidized) oligomers already existing in fresh SOA. HOMs having different combinations of oxygen-to-carbon ratio, hydrogen-to-carbon ratio and average carbon oxidation state are discussed and compared to low volatility oxygenated organic aerosol (LVOOA), which has been identified in ambient aerosol based on average elemental composition but not fully understood at a molecular level. For the biogenic precursors and experimental conditions studied, HOMs in fresh biogenic SOA have molecular formulas more closely resembling LVOOA than HOMs in aged SOA, suggesting that aging of biogenic SOA is not a good surrogate for ambient LVOOA.

Ion mobility spectrometry-mass spectrometry examination of the structures, stabilities, and extents of hydration of dimethylamine-sulfuric acid clusters.

We applied an atmospheric pressure differential mobility analyzer (DMA) coupled to a time-of-flight mass spectrometer to examine the stability, mass-mobility relationship, and extent of hydration of dimethylamine-sulfuric acid cluster ions, which are of relevance to nucleation in ambient air. Cluster ions were generated by electrospray ionization and were of the form: [H((CH3)2NH)x(H2SO4)y](+) and [(HSO4)((CH3)2NH)x(H2SO4)y](-), where 4 ≤ x ≤ 8, and 5 ≤ y ≤ 12. Under dry conditions, we find that positively charged cluster ions dissociated via loss of both multiple dimethylamine and sulfuric acid molecules after mobility analysis but prior to mass analysis, and few parent ions were detected in the mass spectrometer. Dissociation also occurred for negative ions, but to a lesser extent than for positive ions for the same mass spectrometer inlet conditions. Under humidified conditions (relative humidities up to 30% in the DMA), positively charged cluster ion dissociation in the mass spectrometer inlet was mitigated and occurred primarily by H2SO4 loss from ions containing excess acid molecules. DMA measurements were used to infer collision cross sections (CCSs) for all identifiable cluster ions. Stokes-Millikan equation and diffuse/inelastic gas molecule scattering predicted CCSs overestimate measured CCSs by more than 15%, while elastic-specular collision model predictions are in good agreement with measurements. Finally, cluster ion hydration was examined by monitoring changes in CCSs with increasing relative humidity. All examined cluster ions showed a modest amount of water molecule adsorption, with percentage increases in CCS smaller than 10%. The extent of hydration correlates directly with cluster ion acidity for positive ions.

Growth of Ammonium Bisulfate Clusters by Adsorption of Oxygenated Organic Molecules.

Quantum chemical calculations were employed to model the interactions of the [(NH4(+))4(HSO4(-))4] ammonium bisulfate cluster with one or more molecular products of monoterpene oxidation. A strong interaction was found between the bisulfate ion of this cluster and a carboxylic acid, aldehyde, or ketone functionality of the organic molecule. Free energies of adsorption for carboxylic acids were in the -70 to -73 kJ/mol range, while those for aldehydes and ketones were in the -46 to -50 kJ/mol range. These values suggest that a small ambient [(NH4(+))4(SO4(-))4]cluster is able to adsorb an oxygenated organic molecule. While adsorption of the first molecule is highly favorable, adsorption of subsequent molecules is less so, suggesting that sustained uptake of organic molecules does not occur, and thus is not a pathway for continuing growth of the cluster. This result is consistent with ambient measurements showing that particles below ∼1 nm grow slowly, while those above 1 nm grow at an increasing rate presumably due to a lower surface energy barrier enabling the uptake of organic molecules. This work provides insight into the molecular level interactions which affect sustained cluster growth by uptake of organic molecules.

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.

Formation and growth of molecular clusters containing sulfuric acid, water, ammonia, and dimethylamine.

The structures and thermochemistry of molecular clusters containing sulfuric acid, water, ammonia, and/or dimethylamine ((CH3)2NH or DMA) are explored using a combination of Monte Carlo configuration sampling, semiempirical calculations, and density functional theory (DFT) calculations. Clusters are of the general form [(BH(+))n(HSO4(-))n(H2O)y], where B = NH3 or DMA, 2 ≤ n ≤ 8, and 0 ≤ y ≤ 10. Cluster formulas are written based on the computed structures, which uniformly show proton transfer from each sulfuric acid molecule to a base molecule while the water molecules remain un-ionized. Cluster formation is energetically favorable, owing to strong electrostatic attraction among the ions. Water has a minor effect on the energetics of cluster formation, lowering the free energy of formation by ∼ 10% depending on the cluster size and number of water molecules. Cluster growth (addition of one base molecule and one sulfuric acid molecule to a pre-existing cluster) and base substitution (substituting DMA for ammonia) are also energetically favorable processes for both anhydrous and hydrated clusters. However, the effect of water is different for different bases. Hydrated ammonium bisulfate clusters have a more favorable free energy for growth (i.e., incrementing n with fixed y) than anhydrous clusters, while the reverse is observed for dimethylammonium bisulfate clusters, where the free energy for growth is more favorable for anhydrous clusters. The substitution of DMA for ammonia in bisulfate clusters is favorable but exhibits a complex water dependence. Base substitution in smaller bisulfate clusters is enhanced by the presence of water, while base substitution in larger bisulfate clusters is less favorable for hydrated clusters than that for anhydrous clusters. While DMA substitution can stabilize small clusters containing one or a few sulfuric acid molecules, the free energy advantage of forming amine clusters relative to ammonia clusters becomes less pronounced at larger sizes, especially when the effect of water is considered.

Activation barriers in the growth of molecular clusters derived from sulfuric acid and ammonia.

Unraveling the chemical mechanism of atmospheric new particle formation (NPF) has important implications for the broader understanding of the role of aerosols in global climate. We present computational results of the transition states and activation barriers for growth of atmospherically relevant positively charged molecular clusters containing ammonia and sulfuric acid. Sulfuric acid uptake onto the investigated clusters has a small activation free-energy barrier, consistent with nearly collision-limited uptake. Ammonia uptake requires significant reorganization of ions in the preexisting cluster, which yields an activation barrier on the order of 29-53 kJ/mol for the investigated clusters. For this reason, ammonia uptake onto positively charged clusters may be too slow for cluster growth to proceed by the currently accepted mechanism of stepwise addition of sulfuric acid followed by ammonia. The results presented here may have important implications for modeling atmospheric NPF and nanoparticle growth, which typically does not consider an activation barrier along the growth pathway and usually assumes collision-limited molecular uptake.

Fragmentation energetics of clusters relevant to atmospheric new particle formation.

The exact mechanisms by which small clusters form and grow in the atmosphere are poorly understood, but this process may significantly impact cloud condensation nuclei number concentrations and global climate. Sulfuric acid is the key chemical component to new particle formation (NPF), but basic species such as ammonia are also important. Few laboratory experiments address the kinetics or thermodynamics of acid and base incorporation into small clusters. This work utilizes a Fourier transform ion cyclotron resonance mass spectrometer equipped with surface-induced dissociation to investigate time- and collision-energy-resolved fragmentation of positively charged ammonium bisulfate clusters. Critical energies for dissociation are obtained from Rice-Ramsperger-Kassel-Marcus/quasi-equilibrium theory modeling of the experimental data and are compared to quantum chemical calculations of the thermodynamics of cluster dissociation. Fragmentation of ammonium bisulfate clusters occurs by two pathways: (1) a two-step pathway whereby the cluster sequentially loses ammonia followed by sulfuric acid and (2) a one-step pathway whereby the cluster loses an ammonium bisulfate molecule. Experimental critical energies for loss of an ammonia molecule and loss of an ammonium bisulfate molecule are higher than the thermodynamic values. If cluster growth is considered the reverse of cluster fragmentation, these results require the presence of an activation barrier to describe the incorporation of ammonia into small acidic clusters and suggest that kinetically (i.e., diffusion) limited growth should not be assumed. An important corollary is that models of atmospheric NPF should be revised to consider activation barriers to individual chemical steps along the growth pathway.

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.

Origin and impact of particle-to-particle variations in composition measurements with the nano-aerosol mass spectrometer.

In the nano-aerosol mass spectrometer, individual particles in the 10-30 nm size range are trapped and irradiated with a high pulse energy laser beam. The laser pulse generates a plasma that disintegrates the particle into atomic ions, from which the elemental composition is determined. Particle-to-particle variations among the mass spectra are shown to arise from plasma energetics: Low ionization energy species are enhanced in some spectra while high ionization energy species are enhanced in others. These variations also limit the accuracy and precision of elemental analysis, with higher deviations generally observed when low ionization energy species are dominant in the mass spectrum. For standard datasets generated from nominally identical particles, it is shown that that the error associated with composition measurement is random and that averaging the spectra from a few tens of particles is sufficient for measuring the mole fractions of common elements to within about 10% of the expected value. Averaging a greater number of particles offers limited improvement of the measurement precision but has the deleterious effect of degrading the measurement time-resolution, which is given by the time needed to obtain the required number of particle spectra for averaging. An internally mixed ambient particle dataset was found to give a similar result to the standard datasets, that is, the measured elemental composition converged to the average value after a few tens of particles were averaged.

Thermodynamics of oligomer formation: implications for secondary organic aerosol formation and reactivity.

Dimers and higher order oligomers, whether in the gas or particle phase, can affect important atmospheric processes such as new particle formation, and gas-particle partitioning. In this study, the thermodynamics of dimer formation from various oxidation products of α-pinene ozonolysis are investigated using a combination of Monte Carlo configuration sampling, semi-empirical and density functional theory (DFT) quantum mechanics, and continuum solvent modeling. Favorable dimer formation pathways are found to exist in both gas and condensed phases. The free energies of dimer formation are used to calculate equilibrium constants and expected dimer concentrations under a variety of conditions. In the gas phase, favorable pathways studied include formation of non-covalent dimers of terpenylic acid and/or cis-pinic acid and a covalently-bound peroxyhemiacetal. Under atmospherically relevant conditions, only terpenylic acid forms a dimer in sufficient quantities to contribute to new particle formation. Under conditions typically used in laboratory experiments, several dimer formation pathways may contribute to particle formation. In the condensed phase, non-covalent dimers of terpenylic acid and/or cis-pinic acid and covalently-bound dimers representing a peroxyhemiacetal and a hydrated aldol are favorably formed. Dimer formation is both solution and temperature dependent. A water-like solution appears to promote dimer formation over methanol- or acetonitrile-like solutions. Heating from 298 K to 373 K causes extensive decomposition back to monomers. Dimers that are not favorably formed in either the gas or condensed phase include hemi-acetal, ester, anhydride, and the di(α-hydroxy) ether.

Selective detection and characterization of nanoparticles from motor vehicles.

Numerous studies have shown that exposure to motor vehicle emissions increases the probability of heart attacks, asthma attacks, and hospital visits among at-risk individuals. However, while many studies have focused on measurements of ambient nanoparticles near highways, they have not focused on specific road-level domains, such as intersections near population centers. At these locations, very intense spikes in particle number concentration have been observed. These spikes have been linked to motor vehicle activity and have the potential to increase exposure dramatically. Characterizing both the contribution and composition of these spikes is critical in developing exposure models and abatement strategies. To determine the contribution of the particle spikes to the ambient number concentration, we implemented wavelet-based algorithms to isolate the particle spikes from measurements taken during the summer and winter of 2009 in Wilmington, Delaware, adjacent to a roadway intersection that approximately 28,000 vehicles pass through daily. These measurements included both number concentration and size distributions recorded once every second by a condensation particle counter (CPC*; TSI, Inc., St. Paul, MN) 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 3% to 35% of the daily ambient number concentration, with spike contributions sometimes greater than 50% of hourly number concentrations. When the data were weighted by particle volume, this portion of the signal contributed an average of 10% to 20% to the daily concentration of particulate matter (PM) < or = 0.1 microm in aerodynamic diameter (PM0.1). The preferred locations for observing particle concentration spikes were those surrounding the measurement site at which motor vehicles accelerated after a red traffic light turned green. As the distance or transit time from emission to sampling increased, the size distribution shifted to a larger particle size, which confirmed the source assignments. To determine the distribution of emissions from individual vehicles, we correlated camera images with the spike contribution to particle number concentration at each time point. A small percentage of motor vehicles were found to emit a disproportionally large concentration of nanoparticles, and these high emitters included both spark-ignition (SI) and heavy-duty diesel (HDD) vehicles. In addition to characterizing the contribution of the spikes (local sources) to the ambient number concentration, we developed a method to determine the net contribution of motor vehicles (all sources) to the total mass concentration of ambient nanoparticles. To do this, we correlated the concentration of spikes with measurements of fast changes in the chemical composition of nanoparticles measured with the nano aerosol mass spectrometer (NAMS; built by the Johnston group). The 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 each particle was determined from the ion signal intensities of each element. However, overlapping mass-to-charge ratios (m/z) at 4 m/z (O(+4) and C(+3)) and at 8 m/z (O(+2) and S(+4)) needed to be separated into their component ions to obtain a representative composition. To do this, we developed a method to deconvolute these ion signals using sucrose and ammonium sulfate [(NH4)2SO4] as calibration standards. With this approach, the differences between the expected and measured elemental mole fractions of carbon (C), oxygen (O), nitrogen (N), and sulfur (S) for a variety of test particles were generally much less than 10%. Ambient nanoparticles were found to consist mostly of C, O, N, and S. Many particles also contained silicon (Si). The elemental compositions were apportioned into molecular species that are commonly found in ambient aerosol: sulfate (SO4(2-)), nitrate (NO3-), ammonium (NH4+), carbonaceous matter, and when present, silicon dioxide (SiO2). Correlating NAMS chemical-composition measurements with spike contributions allowed for the development of a chemical profile representing motor vehicle emissions, which could be used to apportion their total contribution to the ambient nanoparticle mass. Particles originating from motor vehicles had compositions dominated by unoxidized carbonaceous matter, whereas non-motor vehicle particles consisted mostly of SO42-, NO3-, and oxidized carbonaceous matter. Motor vehicles were found to contribute up to 48% and 60% of the nanoparticle mass and number concentrations, respectively, in the winter measurement period, but only 16% and 49% of the nanoparticle mass and number concentrations, respectively, in the summer period. Chemical-composition profiles and contributions of SI versus HDD vehicles to the nanoparticle mass concentration were estimated by correlating still camera images, chemical composition, and spike contributions at each time point. The total mass contributions from SI and HDD vehicles were roughly equal, but the uncertainty in the split was large. The results of this study suggest that nanoparticle concentrations will be higher adjacent to an intersection than along the same roadway but further from an intersection. Possible ways to reduce the motor vehicle contribution to ambient nanoparticulate matter include minimizing stop-and-go activity at an intersection (i.e., vehicles accelerating after a red light turns green) and identifying the small fraction of motor vehicles that emit a disproportionally large number of nanoparticles.

Targeted absolute quantification of intact proteins by reversed phase liquid chromatography-mass spectrometry, charge reduced electrospray, and condensation particle counting.

A novel approach involving the use of reversed phase liquid chromatography-mass spectrometry (RPLC-MS), charge reduced electrospray (CRES), and condensation particle counting (CPC) for the absolute quantification of intact proteins in liquid solutions is introduced. Under analysis conditions optimized for the quantification of select proteins within their predetermined linear ranges, a set of at least five protein standards with molecular weights (MW) spanning the dynamic ranges of both a quadrupole time-of-flight (QTOF) MS and a suitably selected RPLC column is used to generate a calibration curve of CPC detection efficiency (DE) as a function of the square root of MW. Next, the sample of interest is analyzed, and from the MS-generated MW data, the DE of each target protein is determined from the calibration curve. On the basis of MW, DE, and number concentration (molecules/unit volume), absolute quantification is achieved for each protein of interest. Application of this approach to the absolute quantification of cytochrome C (as target compound) in a commercial protein mixture is demonstrated with a deviation of 8%, a coefficient of variation (CV) of 5%, and a quantification limit of 432 fmol. For nontarget components of the mixture (ribonuclease A, holotransferrin, and apomyoglobin), the percent deviation from the stated concentrations and the CV varied from 0.20 to 23 and from 4.1 to 18, respectively. Performance of the method was further assessed by analyzing a laboratory quality control mixture comprising 0.33 µM of cytochrome C. The calculated value was 0.34 (CV: 5.1%). Universal in essence, the new technique holds strong promise for the absolute quantification of select proteins in liquid samples under conditions of good peak resolution and stable baseline.