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/C_org) 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/C_org, attributed to “salting-in” with increasing solute concentration, while methylglyoxal led to a decrease in N/C_org, 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 (β-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

Particle size and chemical composition effects on elemental analysis with the nano aerosol mass spectrometer

<p>In the Nano Aerosol Mass Spectrometer (NAMS), particles are irradiated with a high energy laser pulse to produce a plasma that quantitatively disintegrates each particle into positively charged atomic ions. Previous work with this method used electrodynamic focusing and trapping of particles 30 nm dia. and below. In the current work, an aerodynamic focusing inlet was used to study particles between 40 and 150 nm dia. The distribution of atomic ion charge states was found to be particle size dependent, shifting toward lower charges with increasing size. This shift also affected the calibration by which elemental composition was determined from atomic ion signal intensities. Size independent calibration could be achieved by restricting the analysis to particles that gave more than 90% of the total signal intensity as multiply charged ions. This approach worked best for particles smaller than about 100 nm dia. since most spectra met this criterion. For the nanoparticles studied, the elemental mole fractions of Group I and II metals, halogens, and low atomic mass nonmetals could be determined within 10% or less of the expected value when the mole fraction was at the 1% level or greater. Some transition and heavy metals could not be quantified, while others could. Quantification appeared to be dependent on the ability of the element to be vaporized. Elements with high melting and boiling points gave particle mass spectra similar to those obtained by laser desorption ionization—mostly singly charged ions with relative intensities strongly biased toward atoms with low ionization energies.</p> <p>Copyright © 2017 American Association for Aerosol Research</p

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 <i>p</i>-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

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

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⁺)_<i>n</i>(HSO₄^–)_<i>n</i>(H₂O)_<i>y</i>], where B = NH₃ or DMA, 2 ≤ <i>n</i> ≤ 8, and 0 ≤ <i>y</i> ≤ 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 <i>n</i> with fixed <i>y</i>) 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

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

Morphology of Organic/Inorganic Aerosol with Varying Seed Particle Water Content

The morphology of mixed organic/inorganic particles can strongly influence the physicochemical properties of aerosols but remains relatively less examined in particle formation studies. The morphologies of inorganic seed particles grown with either α-pinene or limonene secondary organic aerosol (SOA) generated in a flow tube reactor were found to depend on initial seed particle water content. Effloresced and deliquesced ammonium sulfate seed particles were generated at low relative humidity (<15% RH, dry) and high relative humidity (∼60% RH, wet) and were also coated with secondary organic material under low growth and high-growth conditions. Particles were dried and analyzed using scanning mobility particle size spectrometry and transmission electron microscopy for diameter and substrate-induced diameter changes and for the prevalence of phase separation for organic-coated particles. Effloresced inorganic seed particle diameters generally increased after impaction, whereas deliquesced inorganic seed particles had smaller differences in diameter, although they appeared morphologically similar to the effloresced seed particles. Differences in the changes to diameter for deliquesced seed particles suggest crystal restructuring with RH cycling. SOA-coated particles showed negative diameter changes for low organic growth, although wet-seeded organic particles changed by larger magnitudes compared to dry-seeded organic particles. High organic growth gave wide-ranging diameter percent differences for both dry- and wet-seeded samples. Wet-seeded particles with organic coatings occasionally showed a textured morphology unseen in the coated particles with dry seeds. Using a flow tube reactor with a combination of spectrometry and microscopy techniques allowed for insights into the dependence of aerosol particle morphology on formation parameters for two seed conditions and two secondary organic precursors

Structure and Energetics of Nanometer Size Clusters of Sulfuric Acid with Ammonia and Dimethylamine

The structures of positively and negatively charged clusters of sulfuric acid with ammonia and/or dimethylamine ((CH₃)₂NH or DMA) are investigated using a combination of Monte Carlo configuration sampling, semiempirical calculations, and density functional theory (DFT) calculations. Positively charged clusters of the formula [(NH₄⁺)_<i>x</i>(HSO₄^–)_<i>y</i>]⁺, where <i>x</i> = <i>y</i> + 1, are studied for 1 ≤ <i>y</i> ≤ 10. These clusters exhibit strong cation–anion interactions, with no contribution to the hydrogen-bonding network from the bisulfate ion protons. A similar hydrogen-bonding network is found for the [(DMAH⁺)₅(HSO₄^–)₄]^– cluster. Negatively charged clusters derived from the reaction of DMA with [(H₂SO₄)₃(NH₄⁺)(HSO₄^–)₂]⁻ are also studied, up to the fully reacted cluster [(DMAH⁺)₄(HSO₄^–)₅]⁻. These clusters exhibit anion–anion and ion–molecule interactions in addition to cation–anion interactions. While the hydrogen-bonding network is extensive for both positively and negatively charged clusters, the binding energies of ions and molecules in these clusters are determined mostly by electrostatic interactions. The thermodynamics of amine substitution is explored and compared to experimental thermodynamic and kinetic data. Ammonia binds more strongly than DMA to sulfuric acid due to its greater participation in hydrogen bonding and its ability to form a more compact structure that increases electrostatic attraction between oppositely charged ions. However, the greater gas-phase basicity of DMA is sufficient to overcome the stronger binding of ammonia, making substitution of DMA for ammonia thermodynamically favorable. For small clusters of both polarities, substitutions of surface ammonium ions are facile. As the cluster size increases, an ammonium ion becomes encapsulated in the center of the cluster, making it inaccessible to substitution

Chemical Composition of Ambient Nanoparticles on a Particle-by-Particle Basis

The nano aerosol mass spectrometer provides a quantitative measure of the elemental composition of individual, ambient nanoparticles in the 10–30 nm size range. In this work, carbon mole fraction plots are introduced as an efficient means of visualizing the full range of particle compositions in an ambient data set. These plots are constructed by plotting the composition of each particle in the data set, beginning with the particle having the highest carbon mole fraction and ending with the particle having the lowest carbon mole fraction. The method relies on the observation that the carbon content of an ambient particle is generally anticorrelated with oxygen, nitrogen, and sulfur. Carbon mole fraction plots allow internal vs external mixing of particle compositions to be assessed, and they provide a means of exploring the relationship between the oxidation of carbonaceous matter and the presence of inorganic species in a particle. It is shown that unoxidized carbonaceous matter exists primarily as externally mixed particles, whereas oxidized carbonaceous matter is found only in particles that also contain a significant amount of inorganic species. Particles containing oxidized carbonaceous matter are generally neutralized, whereas particles containing unoxidized carbonaceous matter or no carbon at all are acidic. Carbon mole fraction plots show how factor analysis methods such as the Adaptive Resonance Theory – 2a algorithm (ART-2a) and positive matrix factorization partition a continuum of particle compositions into a few fixed composition profiles, and they provide a simple way to characterize how ambient particle compositions change with season and/or location