Alla Zelenyuk

Evaporation kinetics and phase of laboratory and ambient secondary organic aerosol

Field measurements of secondary organic aerosol (SOA) find significantly higher mass loads than predicted by models, sparking intense effort focused on finding additional SOA sources but leaving the fundamental assumptions used by models unchallenged. Current air-quality models use absorptive partitioning theory assuming SOA particles are liquid droplets, forming instantaneous reversible equilibrium with gas phase. Further, they ignore the effects of adsorption of spectator organic species during SOA formation on SOA properties and fate. Using accurate and highly sensitive experimental approach for studying evaporation kinetics of size-selected single SOA particles, we characterized room-temperature evaporation kinetics of laboratory-generated α-pinene SOA and ambient atmospheric SOA. We found that even when gas phase organics are removed, it takes ∼24 h for pure α-pinene SOA particles to evaporate 75% of their mass, which is in sharp contrast to the ∼10 min time scale predicted by current kinetic models. Adsorption of “spectator” organic vapors during SOA formation, and aging of these coated SOA particles, dramatically reduced the evaporation rate, and in some cases nearly stopped it. Ambient SOA was found to exhibit evaporation behavior very similar to that of laboratory-generated coated and aged SOA. For all cases studied in this work, SOA evaporation behavior is nearly size-independent and does not follow the evaporation kinetics of liquid droplets, in sharp contrast with model assumptions. The findings about SOA phase, evaporation rates, and the importance of spectator gases and aging all indicate that there is need to reformulate the way SOA formation and evaporation are treated by models.

Nonequilibrium atmospheric secondary organic aerosol formation and growth

Airborne particles play critical roles in air quality, health effects, visibility, and climate. Secondary organic aerosols (SOA) formed from oxidation of organic gases such as α-pinene account for a significant portion of total airborne particle mass. Current atmospheric models typically incorporate the assumption that SOA mass is a liquid into which semivolatile organic compounds undergo instantaneous equilibrium partitioning to grow the particles into the size range important for light scattering and cloud condensation nuclei activity. We report studies of particles from the oxidation of α-pinene by ozone and NO3 radicals at room temperature. SOA is primarily formed from low-volatility ozonolysis products, with a small contribution from higher volatility organic nitrates from the NO3 reaction. Contrary to expectations, the particulate nitrate concentration is not consistent with equilibrium partitioning between the gas phase and a liquid particle. Rather the fraction of organic nitrates in the particles is only explained by irreversible, kinetically determined uptake of the nitrates on existing particles, with an uptake coefficient that is 1.6% of that for the ozonolysis products. If the nonequilibrium particle formation and growth observed in this atmospherically important system is a general phenomenon in the atmosphere, aerosol models may need to be reformulated. The reformulation of aerosol models could impact the predicted evolution of SOA in the atmosphere both outdoors and indoors, its role in heterogeneous chemistry, its projected impacts on air quality, visibility, and climate, and hence the development of reliable control strategies.

From Agglomerates of Spheres to Irregularly Shaped Particles: Determination of Dynamic Shape Factors from Measurements of Mobility and Vacuum Aerodynamic Diameters

With the advent of advanced real-time aerosol instrumentation, it has become possible to simultaneously measure individual particle mobility and vacuum aerodynamic diameters. This paper presents an experimental exploration of the effect of particle shape on the relationship between mobility and vacuum aerodynamic diameters. We make measurements on systems of three types: (1) Agglomerates of spheres, for which the density and the volume are known; (2) Ammonium sulfate, sodium chloride, succinic acid and lauric acid irregularly shaped particles of known density; and (3) Internally mixed particles, containing organics and ammonium sulfate, of unknown density and shape. For agglomerates of spheres we observe and quantify alignment effects in the Differential Mobility Analyzer (DMA), an important consequence of which is that mobility diameter of aspherical particles can be a function of DMA operating voltages. We report the first measurements of the dynamic shape factors (DSFs) in free molecular regime. We report the first experimental determination of DSF for ammonium sulfate particles, for which we find DSF to increase from 1.03 to 1.07 as particle mobility diameter increases from 160 nm to 500 nm. Three types of NaCl particles were generated and characterized: nearly spherical particles with DSF of ∼ 1.02; cubic with DSF that increases from 1.06 to 1.17 as particle mobility diameter increases; and compact agglomerates with DSF 1.3–1.4. Organic particles were found to be nearly spherical. The data suggest that addition of organics to ammonium sulfate particles lowers their DSF.

Single Particle Laser Ablation Time-of-Flight Mass Spectrometer: An Introduction to SPLAT

We present our single particle mass spectrometer we call SPLAT. SPLAT was designed to make possible the detection and characterization of particles down to 100 nm, generate reproducible single particle mass spectra, and collect data at a high rate. The final instrument presented here is capable of characterizing individual particles down to 50 nm at the rate of 20 particles per second. In SPLAT mass spectra are generated by a two-step process, thet uses a pulsed infrared laser to heat the particle and a time delayed pulsed UV laser to create ions. We describe a mode of operation that makes it possible to take advantage of the gas phase ionization of the semivolatile components of the particle, while also generating mass spectral signatures of the nonvolatile fraction, thereby proividing complete particle mass spectra. We present some sample results from two field deployments of SPLAT.

Morphology of mixed primary and secondary organic particles and the adsorption of spectator organic gases during aerosol formation

Primary organic aerosol (POA) and associated vapors can play an important role in determining the formation and properties of secondary organic aerosol (SOA). If SOA and POA are miscible, POA will significantly enhance SOA formation and some POA vapor will incorporate into SOA particles. When the two are not miscible, condensation of SOA on POA particles forms particles with complex morphology. In addition, POA vapor can adsorb to the surface of SOA particles increasing their mass and affecting their evaporation rates. To gain insight into SOA/POA interactions we present a detailed experimental investigation of the morphologies of SOA particles formed during ozonolysis of α-pinene in the presence of dioctyl phthalate (DOP) particles, serving as a simplified model of hydrophobic POA, using a single-particle mass spectrometer. Ultraviolet laser depth-profiling experiments were used to characterize two different types of mixed SOA/DOP particles: those formed by condensation of the oxidized α-pinene products on size-selected DOP particles and by condensation of DOP on size-selected α-pinene SOA particles. The results show that the hydrophilic SOA and hydrophobic DOP do not mix but instead form layered phases. In addition, an examination of homogeneously nucleated SOA particles formed in the presence of DOP vapor shows them to have an adsorbed DOP coating layer that is ∼4 nm thick and carries 12% of the particles mass. These results may have implications for SOA formation and behavior in the atmosphere, where numerous organic compounds with various volatilities and different polarities are present.

Effect of hydrophobic primary organic aerosols on secondary organic aerosol formation from ozonolysis of α-pinene

[1] Semi-empirical secondary organic aerosol (SOA) models typically assume a well-mixed organic aerosol phase even in the presence of hydrophobic primary organic aerosols (POA). This assumption significantly enhances the modeled SOA yields as additional organic mass is made available to absorb greater amounts of oxidized secondary organic gases than otherwise. We investigate the applicability of this critical assumption by measuring SOA yields from ozonolysis of a-pinene (a major biogenic SOA precursor) in a smog chamber in the absence and in the presence of dioctyl phthalate (DOP) and lubricating oil seed aerosol. These particles serve as surrogates for urban hydrophobic POA. The results show that these POA did not enhance the SOA yields. If these results are found to apply to other biogenic SOA precursors, then the semiempirical models used in many global models would predict significantly less biogenic SOA mass and display reduced sensitivity to anthropogenic POA emissions than previously thought. Citation: Song, C., R. A. Zaveri, M. L. Alexander, J. A. Thornton, S. Madronich, J. V. Ortega, A. Zelenyuk, X.-Y. Yu, A. Laskin, and D. A. Maughan (2007), Effect of hydrophobic primary organic aerosols on secondary organic aerosol formation from ozonolysis of a-pinene, Geophys. Res. Lett., 34, L20803,

Comparison of FTIR and Particle Mass Spectrometry for the Measurement of Particulate Organic Nitrates

While multifunctional organic nitrates are formed during the atmospheric oxidation of volatile organic compounds, relatively little is known about their signatures in particle mass spectrometers. High resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS) and FTIR spectroscopy on particles impacted on ZnSe windows were applied to NH(4)NO(3), NaNO(3), and isosorbide 5-mononitrate (IMN) particles, and to secondary organic aerosol (SOA) from NO(3) radical reactions at 22 degrees C and 1 atm in air with alpha- and beta-pinene, 3-carene, limonene, and isoprene. For comparison, single particle laser ablation mass spectra (SPLAT II) were also obtained for IMN and SOA from the alpha-pinene reaction. The mass spectra of all particles exhibit significant intensity at m/z 30, and for the SOA, weak peaks corresponding to various organic fragments containing nitrogen [C(x)H(y)N(z)O(a)](+) were identified using HR-ToF-AMS. The NO(+)/NO(2)(+) ratios from HR-ToF-AMS were 10-15 for IMN and the SOA from the alpha- and beta-pinene, 3-carene, and limonene reactions, approximately 5 for the isoprene reaction, 2.4 for NH(4)NO(3) and 80 for NaNO(3). The N/H ratios from HR-ToF-AMS for the SOA were smaller by a factor of 2 to 4 than the -ONO(2)/C-H ratios measured using FTIR. FTIR has the advantage that it provides identification and quantification of functional groups. The NO(+)/NO(2)(+) ratio from HR-ToF-AMS can indicate organic nitrates if they are present at more than 15-60% of the inorganic nitrate, depending on whether the latter is NH(4)NO(3) or NaNO(3). However, unique identification of specific organic nitrates is not possible with either method.

SPLAT II: An Aircraft Compatible, Ultra-Sensitive, High Precision Instrument for In-Situ Characterization of the Size and Composition of Fine and Ultrafine Particles

This article describes SPLAT II, a new aircraft compatible single particle mass spectrometer that provides significantly improved performance when compared with SPLAT, or any other existing single particle mass spectrometer. SPLAT II detects and characterizes 100% of spherical particles and ∼30% of aspherical particles with diameters between 125 nm and 600 nm. It also brings significant increase in temporal resolution, sizing over 500 particles per second, while characterizing the composition of up to 100 of them. The increase in sensitivity to small particles makes it possible, under most conditions, to use a differential mobility analyzer upfront SPLAT II in order to simultaneously measure in addition to individual particle size and composition, a number of other particle attributes, such as density or effective density, dynamic shape factor, fractal dimension, and even hygroscopicity. SPLAT II provides sizing precision on the order of a monolayer, and makes it possible to distinguish between spherical and aspherical particles. SPLAT II uses a two-step, two-laser process to generate ions. The mass spectra of the semivolatile fraction are generated by ionization in the gas phase, reducing fragmentation and yielding highly reproducible mass spectra, while the mass spectra of the refractory fraction are simultaneously generated by ablation. The instrument control board generates size-dependent delays for laser triggers to eliminate a size-dependent hit-rate and mass spectra are recorded with 14-bit resolution. Data analysis is facilitated by SpectraMiner and ClusterSculptor; two visually driven software packages specifically designed to analyze the large datasets that this instrument produces.

High Precision Density Measurements of Single Particles: The Density of Metastable Phases

We describe a system designed to measure the size, composition, and density of individual spherical particles in real time. It uses a Differential Mobility Analyzer (DMA) to select a monodisperse particle population and the single particle mass spectrometer to measure individual particle aerodynamic diameter. Together the mobility and aerodynamic diameters yield particle density. The mass spectrometer aerodynamic sizing resolution d ν a /Δ d ν a is ∼ 50 and > 100 for 200 nm and 800 nm particles respectively and together with the DMA the overall system resolution is 20. We demonstrate that the line shape of the aerodynamic size distribution can be used to identify asphericity. We present results from two operational schemes: one suitable for most applications, yielding particle density with a precision of ± 2.5%, and a high precision variant, that uses an internal calibrant to remove any of the systematic errors and significantly improves the measurement quality. The high precision scheme is most suitable for laboratory studies, making it possible to follow slight changes in particle density. An application of the system to measure the density of hygroscopic particles in deep metastable phases near zero relative humidity is presented. The density data presented here are consistent with conclusions reached in a number of other studies, namely, that some particle systems, once deliquesced, persist as droplets down to near zero relative humidity.

Synergy between Secondary Organic Aerosols and Long Range Transport of Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs), known for their harmful health effects, undergo long-range transport (LRT) when adsorbed on and/or absorbed in atmospheric particles. The association between atmospheric particles, PAHs, and their LRT has been the subject of many studies yet remains poorly understood. Current models assume PAHs instantaneously attain reversible gas-particle equilibrium. In this paradigm, as gas-phase PAH concentrations are depleted due to oxidation and dilution during LRT, particle-bound PAHs rapidly evaporate to re-establish equilibrium leading to severe underpredictions of LRT potential of particle-bound PAHs. Here we present a new, experimentally based picture in which PAHs trapped inside highly viscous semisolid secondary organic aerosol (SOA) particles, during particle formation, are prevented from evaporation and shielded from oxidation. In contrast, surface-adsorbed PAHs rapidly evaporate leaving no trace. We find synergetic effects between hydrophobic organics and SOA - the presence of hydrophobic organics inside SOA particles drastically slows SOA evaporation to the point that it can almost be ignored, and the highly viscous SOA prevents PAH evaporation ensuring efficient LRT. The data show the assumptions of instantaneous reversible gas-particle equilibrium for PAHs and SOA are fundamentally flawed, providing an explanation for the persistent discrepancy between observed and predicted particle-bound PAHs.