Using Nanoparticle X-ray Spectroscopy to Probe the Formation of Reactive Chemical Gradients in Diffusion-Limited Aerosols.

For aerosol particles that exist in highly viscous, diffusion-limited states, steep chemical gradients are expected to form during photochemical aging in the atmosphere. Under these conditions, species at the aerosol surface are more rapidly transformed than molecules residing in the particle interior. To examine the formation and evolution of chemical gradients at aerosol interfaces, the heterogeneous reaction of hydroxyl radicals (OH) on ∼200 nm particles of pure squalane (a branched, liquid hydrocarbon) and octacosane (a linear, solid hydrocarbon) and binary mixtures of the two are used to understand how diffusion limitations and phase separation impact the particle reactivity. Aerosol mass spectrometry is used to measure the effective heterogeneous OH uptake coefficient (γeff) and oxidation kinetics in the bulk, which are compared with the elemental composition of the surface obtained using X-ray photoemission. When diffusion rates are fast relative to the reaction frequency, as is the case for squalane and low-viscosity squalane-octacosane mixtures, the reaction is efficient (γeff ∼ 0.3) and only limited by the arrival of OH to the interface. However, for cases, where the diffusion rates are slower than reaction rates, as in pure octacosane and higher-viscosity squalane-octacosane mixtures, the heterogeneous reaction occurs in a mixing-limited regime and is ∼10× slower (γeff ∼ 0.03). This is in contrast to carbon and oxygen K edge X-ray absorption measurements that show that the octacosane interface is oxidized much more rapidly than that of pure squalane particles. The O/C ratio of the surface (estimated to be the top 6-8 nm of the interface) is measured to change with rate constants of (3.0 ± 0.9) × 10-13 and (8.6 ± 1.2) × 10-13 cm3 molecule-1 s-1 for squalane and octacosane particles, respectively. The differences in surface oxidation rates are analyzed using a previously published reaction-diffusion model, which suggests that a 1-2 nm highly oxidized crust forms on octacosane particles, whereas in pure squalane, the reaction products are homogeneously mixed within the aerosol. This work illustrates how diffusion limitations can form particles with highly oxidized surfaces even at relatively low oxidant exposures, which is in turn expected to influence their microphysics in the atmosphere

Large and unexpected enrichment in stratospheric ^(16)O^(13)C^(18)O and its meridional variation

The stratospheric CO_2 oxygen isotope budget is thought to be governed primarily by the O(1D)+CO_2 isotope exchange reaction. However, there is increasing evidence that other important physical processes may be occurring that standard isotopic tools have been unable to identify. Measuring the distribution of the exceedingly rare CO_2 isotopologue ^(16)O^(13)C^(18)O, in concert with ^(18)O and ^(17)O abundances, provides sensitivities to these additional processes and, thus, is a valuable test of current models. We identify a large and unexpected meridional variation in stratospheric 16O13C18O, observed as proportions in the polar vortex that are higher than in any naturally derived CO_2 sample to date. We show, through photochemical experiments, that lower ^(16)O^(13)C^(18)O proportions observed in the midlatitudes are determined primarily by the O(1D)+CO_2 isotope exchange reaction, which promotes a stochastic isotopologue distribution. In contrast, higher ^(16)O^(13)C^(18)O proportions in the polar vortex show correlations with long-lived stratospheric tracer and bulk isotope abundances opposite to those observed at midlatitudes and, thus, opposite to those easily explained by O(1D)+CO_2. We believe the most plausible explanation for this meridional variation is either an unrecognized isotopic fractionation associated with the mesospheric photochemistry of CO_2 or temperature-dependent isotopic exchange on polar stratospheric clouds. Unraveling the ultimate source of stratospheric ^(16)O^(13)C^(18)O enrichments may impose additional isotopic constraints on biosphere–atmosphere carbon exchange, biosphere productivity, and their respective responses to climate change

Finishing the euchromatic sequence of the human genome

The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead

Predicting Aerosol Reactivity Across Scales: from the Laboratory to the Atmosphere.

To fully utilize the results of laboratory-based studies of the chemistry of model atmospheric aerosol reactions, it is important to understand how to relate them to the conditions found in nature. In this study, we have taken a validated reaction-diffusion mechanism for oxidation of C30H62 aerosol by OH under flow tube conditions and examined its predictions for another experimental regime (continuous flow stirred tank reactor) and for the atmosphere, spanning alkane aerosol viscosities from liquid to semisolid. The results show that under OH-concentration-limited and aerosol-mixing-limited conditions, it should be possible to select laboratory experimental conditions where many aspects of the particle phase and volatile product chemistry under atmospheric conditions can be revealed. If the OH collision and organic diffusion rates are comparable, however, reactivity is highly sensitive to the details of both OH concentration and internal mixing. The characteristics of the transition between limiting conditions provide key insights into which parts of the reaction mechanism dominate in the various kinetic regimes

Changes in Reactivity as Chemistry Becomes Confined to an Interface. The Case of Free Radical Oxidation of C30H62 Alkane by OH.

We examine in a simple organic aerosol the transition between heterogeneous chemistry under well-mixed conditions to chemistry under interfacial confinement. A single reaction mechanism, shown to reproduce observed OH oxidation chemistry for liquid and semisolid C30H62, is used in reaction-diffusion simulations to explore reactivity over a broad viscosity range. The results show that when internal mixing of the aerosol is fast and the particle interface is enriched in C-H groups, ketone and alcohol products, formed via peroxy radical disproportionation, predominate. As viscosity increases the reactions become confined to a shell at the gas-aerosol interface. The confinement is accompanied by emergence of acyloxy reaction pathways that are particularly active when the shell is 1 nm or less. We quantify this trend using a reaction-diffusion index, allowing the parts of the mechanism that control reactivity as viscosity increases to be identified

Changes in Reactivity as Chemistry Becomes Confined to an Interface. The Case of Free Radical Oxidation of C30H62 Alkane by OH.

We examine in a simple organic aerosol the transition between heterogeneous chemistry under well-mixed conditions to chemistry under interfacial confinement. A single reaction mechanism, shown to reproduce observed OH oxidation chemistry for liquid and semisolid C30H62, is used in reaction-diffusion simulations to explore reactivity over a broad viscosity range. The results show that when internal mixing of the aerosol is fast and the particle interface is enriched in C-H groups, ketone and alcohol products, formed via peroxy radical disproportionation, predominate. As viscosity increases the reactions become confined to a shell at the gas-aerosol interface. The confinement is accompanied by emergence of acyloxy reaction pathways that are particularly active when the shell is 1 nm or less. We quantify this trend using a reaction-diffusion index, allowing the parts of the mechanism that control reactivity as viscosity increases to be identified

Predicting Aerosol Reactivity Across Scales: from the Laboratory to the Atmosphere.

To fully utilize the results of laboratory-based studies of the chemistry of model atmospheric aerosol reactions, it is important to understand how to relate them to the conditions found in nature. In this study, we have taken a validated reaction-diffusion mechanism for oxidation of C30H62 aerosol by OH under flow tube conditions and examined its predictions for another experimental regime (continuous flow stirred tank reactor) and for the atmosphere, spanning alkane aerosol viscosities from liquid to semisolid. The results show that under OH-concentration-limited and aerosol-mixing-limited conditions, it should be possible to select laboratory experimental conditions where many aspects of the particle phase and volatile product chemistry under atmospheric conditions can be revealed. If the OH collision and organic diffusion rates are comparable, however, reactivity is highly sensitive to the details of both OH concentration and internal mixing. The characteristics of the transition between limiting conditions provide key insights into which parts of the reaction mechanism dominate in the various kinetic regimes

Changes in Reactivity as Chemistry Becomes Confined to an Interface. The Case of Free Radical Oxidation of C₃₀H₆₂ Alkane by OH

We examine in a simple organic aerosol the transition between heterogeneous chemistry under well-mixed conditions to chemistry under interfacial confinement. A single reaction mechanism, shown to reproduce observed OH oxidation chemistry for liquid and semisolid C₃₀H₆₂, is used in reaction–diffusion simulations to explore reactivity over a broad viscosity range. The results show that when internal mixing of the aerosol is fast and the particle interface is enriched in C–H groups, ketone and alcohol products, formed via peroxy radical disproportionation, predominate. As viscosity increases the reactions become confined to a shell at the gas–aerosol interface. The confinement is accompanied by emergence of acyloxy reaction pathways that are particularly active when the shell is 1 nm or less. We quantify this trend using a reaction–diffusion index, allowing the parts of the mechanism that control reactivity as viscosity increases to be identified