John to Dr. Silver, 26 June 1963

Professional correspondenc

Development and initial evaluation of a dynamic species-resolved model for gas phase chemistry and size-resolved gas/particle partitioning associated with secondary organic aerosol formation

A module for predicting the dynamic evolution of the gas phase species and the aerosol size and composition distribution during formation of secondary organic aerosol (SOA) is presented. The module is based on the inorganic gas-aerosol equilibrium model Simulating the Composition of Atmospheric Particles at Equilibrium 2 (SCAPE2) and updated versions of the lumped Caltech Atmospheric Chemistry Mechanism (CACM) and the Model to Predict the Multiphase Partitioning of Organics (MPMPO). The aerosol phase generally consists of an organic phase and an aqueous phase containing dissolved inorganic and organic components. Simulations are presented in which a single salt (either dry or aqueous), a volatile organic compound, and oxides of nitrogen undergo photo-oxidation to form SOA. Predicted SOA mass yields for classes of aromatic and biogenic hydrocarbons exhibit the proper qualitative behavior when compared to laboratory chamber data. Inasmuch as it is currently not possible to represent explicitly aerosol phase chemistry involving condensed products of gas phase oxidation, the present model can be viewed as the most detailed SOA formation model available yet will undergo continued improvement in the future

Secondary organic aerosol 1. Atmospheric chemical mechanism for production of molecular constituents

This series of three papers addresses the representation of secondary organic aerosol (SOA) in atmospheric models. SOA forms when gas-phase organic species undergo oxidation, leading to products of sufficiently low vapor pressure that can partition between the gas and aerosol phases. The present paper, part 1, is devoted to the development of a gas-phase atmospheric chemical mechanism designed to represent ozone chemistry as well as formation of individual organic oxidation products that are capable of forming SOA. The ozone chemistry in the mechanism draws upon the recent work of Stockwell et al. [1997] and Jenkin et al. [1997] and SAPRC-97 and SAPRC-99 (available from W.P.L. Carter at http://helium.ucr.edu/~carter/). The mechanism is evaluated in the three-dimensional California Institute of Technology (CIT) model [Meng et al., 1998] by simulating gas-phase concentrations in the South Coast Air Basin (SoCAB) of California over the period 27–29 August 1987. Total predicted concentrations of gas-phase SOA compounds are compared with levels of SOA that have been inferred on the basis of ambient organic aerosol measurements during this period. These predicted concentrations indicate that the total gas-phase potential of SOA-forming compounds can account for observed aerosol concentrations. Part 2 develops a thermodynamic gas–aerosol partitioning module, and part 3 presents a full three-dimensional simulation of gas and aerosol levels in the SoCAB during a 1993 episode

Ex Vivo Gene Therapy Using Patient iPSC-Derived NSCs Reverses Pathology in the Brain of a Homologous Mouse Model

SummaryNeural stem cell (NSC) transplantation is a promising strategy for delivering therapeutic proteins in the brain. We evaluated a complete process of ex vivo gene therapy using human induced pluripotent stem cell (iPSC)-derived NSC transplants in a well-characterized mouse model of a human lysosomal storage disease, Sly disease. Human Sly disease fibroblasts were reprogrammed into iPSCs, differentiated into a stable and expandable population of NSCs, genetically corrected with a transposon vector, and assessed for engraftment in NOD/SCID mice. Following neonatal intraventricular transplantation, the NSCs engraft along the rostrocaudal axis of the CNS primarily within white matter tracts and survive for at least 4 months. Genetically corrected iPSC-NSCs transplanted post-symptomatically into the striatum of adult Sly disease mice reversed neuropathology in a zone surrounding the grafts, while control mock-corrected grafts did not. The results demonstrate the potential for ex vivo gene therapy in the brain using human NSCs from autologous, non-neural tissues

Secondary organic aerosol 2. Thermodynamic model for gas/particle partitioning of molecular constituents

A model that predicts secondary organic aerosol (SOA) formation based on the thermodynamic equilibrium partitioning of secondary organic oxidation products has been developed for implementation into atmospheric models. Hydrophobic secondary products are assumed to partition to an absorbing organic aerosol consisting of primary organic aerosol (POA) and other secondary hydrophobic organics according to an equilibrium partitioning coefficient calculated iteratively for each secondary compound present. The hydrophobic module is evaluated by studying the partitioning of octadecanoic acid to surrogate POA species. As expected, the amount of octadecanoic acid predicted to be present in the aerosol phase increases as the total amount of absorbing material increases or as the total amount of acid present increases. Hydrophilic secondary compounds partition to an aqueous phase via Henry's law; the fraction of each compound's mass that partitions is determined by its Henry's law constant and its acid dissociation constant(s). The available liquid water content (LWC) of the aerosol is determined iteratively between an inorganic aerosol module and the hydrophilic module, which is evaluated by studying the partitioning of glyoxalic and malic acids. While glyoxalic acid tends to remain in the gas phase, malic acid partitions strongly to the aqueous phase, with ions being the dominant form in the aqueous phase. As expected, an increase in relative humidity increases the amount of water associated with the organics (ΔLWC), and a lower aerosol pH favors molecular solutes over ionized forms. Increasing pH results in higher effective Henry's law constants for the acids, yielding higher organic aerosol concentrations. Results also indicate that increasing ΔLWC induces additional partitioning of inorganics to the aqueous phase

Estimate of global atmospheric organic aerosol from oxidation of biogenic hydrocarbons

The results from a series of outdoor chamber experiments establishing the atmospheric aerosol-forming potential of fourteen terpenoid hydrocarbons have been used to estimate the annual amount of secondary organic aerosol formed globally from compounds emitted by vegetation. Hydroxyl radical, ozone, and nitrate radical oxidation each contribute to aerosol formation in full-photooxidation experiments; because oxidation by nitrate radical under ambient, remote conditions is likely to be negligible, parameters describing aerosol formation from hydroxyl radical and ozone reaction only are developed. Chamber results, temporally and spatially resolved, compound-specific estimates of biogenic hydrocarbon emissions, and hydroxyl radical and ozone fields are combined to lead to an estimate for atmospheric secondary organic aerosol formed annually from biogenic precursors of 18.5 Tg, a number smaller than the previously published estimate of 30–270 Tg [Andreae and Crutzen, 1997]

Recent Decisions

Comments on recent decisions by John F. Murray, William C. Rindone, Eugene G. Griffin, John G. Curran, John P. Callahan, and William J. Ragan

Organic aerosol formation from the oxidation of biogenic hydrocarbons

A series of outdoor chamber experiments has been used to establish and characterize the significant atmospheric aerosol-forming potentials of the most prevalent biogenic hydrocarbons emitted by vegetation. These compounds were also studied to elucidate the effect of structure on aerosol yield for these types of compounds. Because oxidation products partition between the gas and aerosol phases, the aerosol yields of the parent biogenic hydrocarbons depend on the concentration of organic aerosol into which these products can be absorbed. For organic mass concentrations between 5 and 40 µg m^(-3), mass-based yields in photooxidation experiments range from 17 to 67% for sesquiterpenes, from 2 to 23% for cyclic diolefins, from 2 to 15% for bicyclic olefins, and from 2 to 6% for the acyclic triolefin ocimene. In these photooxidation experiments, hydroxyl and nitrate radicals and ozone can contribute to consumption of the parent hydrocarbon. For bicyclic olefins (α-pinene, β-pinene, Δ^3-carene, and sabinene), experiments were also carried out at daytime temperatures in a dark system in the presence of ozone or nitrate radicals alone. For ozonolysis experiments, resulting aerosol yields are less dependent on organic mass concentration, when compared to full, sunlight-driven photooxidation. Nitrate radical experiments exhibit extremely high conversion to aerosol for β-pinene, sabinene, and Δ^3-carene. The relative importance of aerosol formation from each type of reaction for bicyclic olefin photooxidation is elucidated

Space Applications of Mass Spectrometry

Mass spectrometers have been involved in essentially all aspects of space exploration. This chapter outlines some of these many uses. Mass spectrometers have not only helped to expand our knowledge and understanding of the world and solar system around us, they have helped to put man safely in space and expand our frontier. Mass spectrometry continues to prove to be a very reliable, robust, and flexible analytical instrument, ensuring that its use will continue to help aid our investigation of the universe and this small planet that we call home