Note: When clicking on a Digital Object Identifier (DOI) number, you will be taken to an external site maintained by the publisher.
Some full text articles may not yet be available without a charge during the embargo (administrative interval).
What is a DOI Number?
Some links on this page may take you to non-federal websites. Their policies may differ from this site.
-
The ability of particle-phase chemistry to alter the molecular composition and enhance the growth rate of nanoparticles in the 2–100 nm diameter range is investigated through the use of a kinetic growth model. The molecular components included are sulfuric acid, ammonia, water, a non-volatile organic compound, and a semi-volatile organic compound. Molecular composition and growth rate are compared for particles that grow by partitioning alone vs. those that grow by a combination of partitioning and an accretion reaction in the particle phase between two organic molecules. Particle-phase chemistry causes a change in molecular composition that is particle diameter dependent, and when the reaction involves semi-volatile molecules, the particles grow faster than by partitioning alone. These effects are most pronounced for particles larger than about 20 nm in diameter. The modeling results provide a fundamental basis for understanding recent experimental measurements of the molecular composition of secondary organic aerosol showing that accretion reaction product formation increases linearly with increasing aerosol volume-to-surface-area. They also allow initial estimates of the reaction rate constants for these systems. For secondary aerosol produced by either OH oxidation of the cyclic dimethylsiloxane (D5) or ozonolysis of β-pinene, oligomerization rate constants on the order of 10−3 to 10−1 M−1 s−1 are needed to explain the experimental results. These values are consistent with previously measured rate constants for reactions of hydroperoxides and/or peroxyacids in the condensed phase.more » « less
-
Abstract. New particle formation (NPF) consists of two steps: nucleation andsubsequent growth. At present, chemical and physical mechanisms that governthese two processes are not well understood. Here, we report initial resultsobtained from the TANGENT (Tandem Aerosol Nucleation and Growth EnvironmentTube) experiments. The TANGENT apparatus enables us to study these twoprocesses independently. The present study focuses on the effects oftemperature on sulfuric acid nucleation and further growth. Our results showthat lower temperatures enhance both the nucleation and growth rate.However, under temperatures below 268 K the effects of temperature on thenucleation rate become less significant and the nucleation rate becomes lessdependent on relative humidity, indicating that particle formation in the conditions of ourflow tube takes place via barrierless nucleation at lower temperatures. Wealso examined the growth of newly formed particles under differingtemperature conditions for nucleation and further growth. Our results showthat newly nucleated clusters formed at low temperatures can indeed surviveevaporation and grow in a warmer environment in the presence of SO2 andozone and potentially other contaminant vapors. These results implythat some heterogeneous reactions involving nanoparticles affect nucleationand growth of newly formed particles.more » « less
-
more » « less
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 10−5to 10−6range, which is sufficient for many aerosol applications.
-
more » « less
Abstract The effect of sulfur dioxide on particle formation and growth by ozonolysis of three monoterpenes (
α ‐pinene,β ‐pinene, and limonene) and isoprene was investigated in the presence of monodisperse ammonium sulfate seed particles and an OH scavenger in a flow tube under dry conditions. Without sulfur dioxide, new particle formation was not observed, and seed particle growth was consistent with condensation of low‐volatility oxidation products produced from each organic precursor. With sulfur dioxide, new particle formation was observed from every precursor studied, consistent with sulfuric acid formation by reaction of sulfur dioxide with stabilized Criegee Intermediates. The presence of sulfur dioxide did not significantly affect seed particle growth rates fromα ‐pinene and limonene ozonolysis, although chemical composition measurements revealed the presence of organosulfates in the particles following SO2exposure. Contrarily, the growth of seeds byβ ‐pinene and isoprene ozonolysis was considerably enhanced by sulfur dioxide, and chemical composition measurements revealed that the enhanced growth was not due to additional organic material, suggesting that inorganic sulfate was likely responsible. The results suggest that a previously unconsidered particle‐phase pathway to growth activated by sulfur dioxide may alter production of cloud condensation nuclei over regions with significant SO2‐alkene interactions.
